------------------------------------------------------------------------------
                       T H E  /proc   F I L E S Y S T E M
------------------------------------------------------------------------------
/proc/sys         Terrehon Bowden <terrehon@pacbell.net>        October 7 1999
                  Bodo Bauer <bb@ricochet.net>

2.4.x update	  Jorge Nerin <comandante@zaralinux.com>      November 14 2000
move /proc/sys	  Shen Feng <shen@cn.fujitsu.com>		  April 1 2009
------------------------------------------------------------------------------
Version 1.3                                              Kernel version 2.2.12
					      Kernel version 2.4.0-test11-pre4
------------------------------------------------------------------------------
fixes/update part 1.1  Stefani Seibold <stefani@seibold.net>       June 9 2009

Table of Contents
-----------------

  0     Preface
  0.1	Introduction/Credits
  0.2	Legal Stuff

  1	Collecting System Information
  1.1	Process-Specific Subdirectories
  1.2	Kernel data
  1.3	IDE devices in /proc/ide
  1.4	Networking info in /proc/net
  1.5	SCSI info
  1.6	Parallel port info in /proc/parport
  1.7	TTY info in /proc/tty
  1.8	Miscellaneous kernel statistics in /proc/stat
  1.9	Ext4 file system parameters

  2	Modifying System Parameters

  3	Per-Process Parameters
  3.1	/proc/<pid>/oom_adj & /proc/<pid>/oom_score_adj - Adjust the oom-killer
								score
  3.2	/proc/<pid>/oom_score - Display current oom-killer score
  3.3	/proc/<pid>/io - Display the IO accounting fields
  3.4	/proc/<pid>/coredump_filter - Core dump filtering settings
  3.5	/proc/<pid>/mountinfo - Information about mounts
  3.6	/proc/<pid>/comm  & /proc/<pid>/task/<tid>/comm
  3.7   /proc/<pid>/task/<tid>/children - Information about task children
  3.8   /proc/<pid>/fdinfo/<fd> - Information about opened file
  3.9   /proc/<pid>/map_files - Information about memory mapped files

  4	Configuring procfs
  4.1	Mount options

------------------------------------------------------------------------------
Preface
------------------------------------------------------------------------------

0.1 Introduction/Credits
------------------------

This documentation is  part of a soon (or  so we hope) to be  released book on
the SuSE  Linux distribution. As  there is  no complete documentation  for the
/proc file system and we've used  many freely available sources to write these
chapters, it  seems only fair  to give the work  back to the  Linux community.
This work is  based on the 2.2.*  kernel version and the  upcoming 2.4.*. I'm
afraid it's still far from complete, but we  hope it will be useful. As far as
we know, it is the first 'all-in-one' document about the /proc file system. It
is focused  on the Intel  x86 hardware,  so if you  are looking for  PPC, ARM,
SPARC, AXP, etc., features, you probably  won't find what you are looking for.
It also only covers IPv4 networking, not IPv6 nor other protocols - sorry. But
additions and patches  are welcome and will  be added to this  document if you
mail them to Bodo.

We'd like  to  thank Alan Cox, Rik van Riel, and Alexey Kuznetsov and a lot of
other people for help compiling this documentation. We'd also like to extend a
special thank  you to Andi Kleen for documentation, which we relied on heavily
to create  this  document,  as well as the additional information he provided.
Thanks to  everybody  else  who contributed source or docs to the Linux kernel
and helped create a great piece of software... :)

If you  have  any comments, corrections or additions, please don't hesitate to
contact Bodo  Bauer  at  bb@ricochet.net.  We'll  be happy to add them to this
document.

The   latest   version    of   this   document   is    available   online   at
http://tldp.org/LDP/Linux-Filesystem-Hierarchy/html/proc.html

If  the above  direction does  not works  for you,  you could  try the  kernel
mailing  list  at  linux-kernel@vger.kernel.org  and/or try  to  reach  me  at
comandante@zaralinux.com.

0.2 Legal Stuff
---------------

We don't  guarantee  the  correctness  of this document, and if you come to us
complaining about  how  you  screwed  up  your  system  because  of  incorrect
documentation, we won't feel responsible...

------------------------------------------------------------------------------
CHAPTER 1: COLLECTING SYSTEM INFORMATION
------------------------------------------------------------------------------

------------------------------------------------------------------------------
In This Chapter
------------------------------------------------------------------------------
* Investigating  the  properties  of  the  pseudo  file  system  /proc and its
  ability to provide information on the running Linux system
* Examining /proc's structure
* Uncovering  various  information  about the kernel and the processes running
  on the system
------------------------------------------------------------------------------


The proc  file  system acts as an interface to internal data structures in the
kernel. It  can  be  used to obtain information about the system and to change
certain kernel parameters at runtime (sysctl).

First, we'll  take  a  look  at the read-only parts of /proc. In Chapter 2, we
show you how you can use /proc/sys to change settings.

1.1 Process-Specific Subdirectories
-----------------------------------

The directory  /proc  contains  (among other things) one subdirectory for each
process running on the system, which is named after the process ID (PID).

The link  self  points  to  the  process reading the file system. Each process
subdirectory has the entries listed in Table 1-1.


Table 1-1: Process specific entries in /proc
..............................................................................
 File		Content
 clear_refs	Clears page referenced bits shown in smaps output
 cmdline	Command line arguments
 cpu		Current and last cpu in which it was executed	(2.4)(smp)
 cwd		Link to the current working directory
 environ	Values of environment variables
 exe		Link to the executable of this process
 fd		Directory, which contains all file descriptors
 maps		Memory maps to executables and library files	(2.4)
 mem		Memory held by this process
 root		Link to the root directory of this process
 stat		Process status
 statm		Process memory status information
 status		Process status in human readable form
 wchan		If CONFIG_KALLSYMS is set, a pre-decoded wchan
 pagemap	Page table
 stack		Report full stack trace, enable via CONFIG_STACKTRACE
 smaps		a extension based on maps, showing the memory consumption of
		each mapping and flags associated with it
 numa_maps	an extension based on maps, showing the memory locality and
		binding policy as well as mem usage (in pages) of each mapping.
..............................................................................

For example, to get the status information of a process, all you have to do is
read the file /proc/PID/status:

  >cat /proc/self/status
  Name:   cat
  State:  R (running)
  Tgid:   5452
  Pid:    5452
  PPid:   743
  TracerPid:      0						(2.4)
  Uid:    501     501     501     501
  Gid:    100     100     100     100
  FDSize: 256
  Groups: 100 14 16
  VmPeak:     5004 kB
  VmSize:     5004 kB
  VmLck:         0 kB
  VmHWM:       476 kB
  VmRSS:       476 kB
  VmData:      156 kB
  VmStk:        88 kB
  VmExe:        68 kB
  VmLib:      1412 kB
  VmPTE:        20 kb
  VmSwap:        0 kB
  Threads:        1
  SigQ:   0/28578
  SigPnd: 0000000000000000
  ShdPnd: 0000000000000000
  SigBlk: 0000000000000000
  SigIgn: 0000000000000000
  SigCgt: 0000000000000000
  CapInh: 00000000fffffeff
  CapPrm: 0000000000000000
  CapEff: 0000000000000000
  CapBnd: ffffffffffffffff
  Seccomp:        0
  voluntary_ctxt_switches:        0
  nonvoluntary_ctxt_switches:     1

This shows you nearly the same information you would get if you viewed it with
the ps  command.  In  fact,  ps  uses  the  proc  file  system  to  obtain its
information.  But you get a more detailed  view of the  process by reading the
file /proc/PID/status. It fields are described in table 1-2.

The  statm  file  contains  more  detailed  information about the process
memory usage. Its seven fields are explained in Table 1-3.  The stat file
contains details information about the process itself.  Its fields are
explained in Table 1-4.

(for SMP CONFIG users)
For making accounting scalable, RSS related information are handled in
asynchronous manner and the vaule may not be very precise. To see a precise
snapshot of a moment, you can see /proc/<pid>/smaps file and scan page table.
It's slow but very precise.

Table 1-2: Contents of the status files (as of 2.6.30-rc7)
..............................................................................
 Field                       Content
 Name                        filename of the executable
 State                       state (R is running, S is sleeping, D is sleeping
                             in an uninterruptible wait, Z is zombie,
			     T is traced or stopped)
 Tgid                        thread group ID
 Pid                         process id
 PPid                        process id of the parent process
 TracerPid                   PID of process tracing this process (0 if not)
 Uid                         Real, effective, saved set, and  file system UIDs
 Gid                         Real, effective, saved set, and  file system GIDs
 FDSize                      number of file descriptor slots currently allocated
 Groups                      supplementary group list
 VmPeak                      peak virtual memory size
 VmSize                      total program size
 VmLck                       locked memory size
 VmHWM                       peak resident set size ("high water mark")
 VmRSS                       size of memory portions
 VmData                      size of data, stack, and text segments
 VmStk                       size of data, stack, and text segments
 VmExe                       size of text segment
 VmLib                       size of shared library code
 VmPTE                       size of page table entries
 VmSwap                      size of swap usage (the number of referred swapents)
 Threads                     number of threads
 SigQ                        number of signals queued/max. number for queue
 SigPnd                      bitmap of pending signals for the thread
 ShdPnd                      bitmap of shared pending signals for the process
 SigBlk                      bitmap of blocked signals
 SigIgn                      bitmap of ignored signals
 SigCgt                      bitmap of caught signals
 CapInh                      bitmap of inheritable capabilities
 CapPrm                      bitmap of permitted capabilities
 CapEff                      bitmap of effective capabilities
 CapBnd                      bitmap of capabilities bounding set
 Seccomp                     seccomp mode, like prctl(PR_GET_SECCOMP, ...)
 Cpus_allowed                mask of CPUs on which this process may run
 Cpus_allowed_list           Same as previous, but in "list format"
 Mems_allowed                mask of memory nodes allowed to this process
 Mems_allowed_list           Same as previous, but in "list format"
 voluntary_ctxt_switches     number of voluntary context switches
 nonvoluntary_ctxt_switches  number of non voluntary context switches
..............................................................................

Table 1-3: Contents of the statm files (as of 2.6.8-rc3)
..............................................................................
 Field    Content
 size     total program size (pages)		(same as VmSize in status)
 resident size of memory portions (pages)	(same as VmRSS in status)
 shared   number of pages that are shared	(i.e. backed by a file)
 trs      number of pages that are 'code'	(not including libs; broken,
							includes data segment)
 lrs      number of pages of library		(always 0 on 2.6)
 drs      number of pages of data/stack		(including libs; broken,
							includes library text)
 dt       number of dirty pages			(always 0 on 2.6)
..............................................................................


Table 1-4: Contents of the stat files (as of 2.6.30-rc7)
..............................................................................
 Field          Content
  pid           process id
  tcomm         filename of the executable
  state         state (R is running, S is sleeping, D is sleeping in an
                uninterruptible wait, Z is zombie, T is traced or stopped)
  ppid          process id of the parent process
  pgrp          pgrp of the process
  sid           session id
  tty_nr        tty the process uses
  tty_pgrp      pgrp of the tty
  flags         task flags
  min_flt       number of minor faults
  cmin_flt      number of minor faults with child's
  maj_flt       number of major faults
  cmaj_flt      number of major faults with child's
  utime         user mode jiffies
  stime         kernel mode jiffies
  cutime        user mode jiffies with child's
  cstime        kernel mode jiffies with child's
  priority      priority level
  nice          nice level
  num_threads   number of threads
  it_real_value	(obsolete, always 0)
  start_time    time the process started after system boot
  vsize         virtual memory size
  rss           resident set memory size
  rsslim        current limit in bytes on the rss
  start_code    address above which program text can run
  end_code      address below which program text can run
  start_stack   address of the start of the main process stack
  esp           current value of ESP
  eip           current value of EIP
  pending       bitmap of pending signals
  blocked       bitmap of blocked signals
  sigign        bitmap of ignored signals
  sigcatch      bitmap of caught signals
  wchan         address where process went to sleep
  0             (place holder)
  0             (place holder)
  exit_signal   signal to send to parent thread on exit
  task_cpu      which CPU the task is scheduled on
  rt_priority   realtime priority
  policy        scheduling policy (man sched_setscheduler)
  blkio_ticks   time spent waiting for block IO
  gtime         guest time of the task in jiffies
  cgtime        guest time of the task children in jiffies
  start_data    address above which program data+bss is placed
  end_data      address below which program data+bss is placed
  start_brk     address above which program heap can be expanded with brk()
  arg_start     address above which program command line is placed
  arg_end       address below which program command line is placed
  env_start     address above which program environment is placed
  env_end       address below which program environment is placed
  exit_code     the thread's exit_code in the form reported by the waitpid system call
..............................................................................

The /proc/PID/maps file containing the currently mapped memory regions and
their access permissions.

The format is:

address           perms offset  dev   inode      pathname

08048000-08049000 r-xp 00000000 03:00 8312       /opt/test
08049000-0804a000 rw-p 00001000 03:00 8312       /opt/test
0804a000-0806b000 rw-p 00000000 00:00 0          [heap]
a7cb1000-a7cb2000 ---p 00000000 00:00 0
a7cb2000-a7eb2000 rw-p 00000000 00:00 0
a7eb2000-a7eb3000 ---p 00000000 00:00 0
a7eb3000-a7ed5000 rw-p 00000000 00:00 0          [stack:1001]
a7ed5000-a8008000 r-xp 00000000 03:00 4222       /lib/libc.so.6
a8008000-a800a000 r--p 00133000 03:00 4222       /lib/libc.so.6
a800a000-a800b000 rw-p 00135000 03:00 4222       /lib/libc.so.6
a800b000-a800e000 rw-p 00000000 00:00 0
a800e000-a8022000 r-xp 00000000 03:00 14462      /lib/libpthread.so.0
a8022000-a8023000 r--p 00013000 03:00 14462      /lib/libpthread.so.0
a8023000-a8024000 rw-p 00014000 03:00 14462      /lib/libpthread.so.0
a8024000-a8027000 rw-p 00000000 00:00 0
a8027000-a8043000 r-xp 00000000 03:00 8317       /lib/ld-linux.so.2
a8043000-a8044000 r--p 0001b000 03:00 8317       /lib/ld-linux.so.2
a8044000-a8045000 rw-p 0001c000 03:00 8317       /lib/ld-linux.so.2
aff35000-aff4a000 rw-p 00000000 00:00 0          [stack]
ffffe000-fffff000 r-xp 00000000 00:00 0          [vdso]

where "address" is the address space in the process that it occupies, "perms"
is a set of permissions:

 r = read
 w = write
 x = execute
 s = shared
 p = private (copy on write)

"offset" is the offset into the mapping, "dev" is the device (major:minor), and
"inode" is the inode  on that device.  0 indicates that  no inode is associated
with the memory region, as the case would be with BSS (uninitialized data).
The "pathname" shows the name associated file for this mapping.  If the mapping
is not associated with a file:

 [heap]                   = the heap of the program
 [stack]                  = the stack of the main process
 [stack:1001]             = the stack of the thread with tid 1001
 [vdso]                   = the "virtual dynamic shared object",
                            the kernel system call handler

 or if empty, the mapping is anonymous.

The /proc/PID/task/TID/maps is a view of the virtual memory from the viewpoint
of the individual tasks of a process. In this file you will see a mapping marked
as [stack] if that task sees it as a stack. This is a key difference from the
content of /proc/PID/maps, where you will see all mappings that are being used
as stack by all of those tasks. Hence, for the example above, the task-level
map, i.e. /proc/PID/task/TID/maps for thread 1001 will look like this:

08048000-08049000 r-xp 00000000 03:00 8312       /opt/test
08049000-0804a000 rw-p 00001000 03:00 8312       /opt/test
0804a000-0806b000 rw-p 00000000 00:00 0          [heap]
a7cb1000-a7cb2000 ---p 00000000 00:00 0
a7cb2000-a7eb2000 rw-p 00000000 00:00 0
a7eb2000-a7eb3000 ---p 00000000 00:00 0
a7eb3000-a7ed5000 rw-p 00000000 00:00 0          [stack]
a7ed5000-a8008000 r-xp 00000000 03:00 4222       /lib/libc.so.6
a8008000-a800a000 r--p 00133000 03:00 4222       /lib/libc.so.6
a800a000-a800b000 rw-p 00135000 03:00 4222       /lib/libc.so.6
a800b000-a800e000 rw-p 00000000 00:00 0
a800e000-a8022000 r-xp 00000000 03:00 14462      /lib/libpthread.so.0
a8022000-a8023000 r--p 00013000 03:00 14462      /lib/libpthread.so.0
a8023000-a8024000 rw-p 00014000 03:00 14462      /lib/libpthread.so.0
a8024000-a8027000 rw-p 00000000 00:00 0
a8027000-a8043000 r-xp 00000000 03:00 8317       /lib/ld-linux.so.2
a8043000-a8044000 r--p 0001b000 03:00 8317       /lib/ld-linux.so.2
a8044000-a8045000 rw-p 0001c000 03:00 8317       /lib/ld-linux.so.2
aff35000-aff4a000 rw-p 00000000 00:00 0
ffffe000-fffff000 r-xp 00000000 00:00 0          [vdso]

The /proc/PID/smaps is an extension based on maps, showing the memory
consumption for each of the process's mappings. For each of mappings there
is a series of lines such as the following:

08048000-080bc000 r-xp 00000000 03:02 13130      /bin/bash
Size:               1084 kB
Rss:                 892 kB
Pss:                 374 kB
Shared_Clean:        892 kB
Shared_Dirty:          0 kB
Private_Clean:         0 kB
Private_Dirty:         0 kB
Referenced:          892 kB
Anonymous:             0 kB
Swap:                  0 kB
KernelPageSize:        4 kB
MMUPageSize:           4 kB
Locked:              374 kB
VmFlags: rd ex mr mw me de

the first of these lines shows the same information as is displayed for the
mapping in /proc/PID/maps.  The remaining lines show the size of the mapping
(size), the amount of the mapping that is currently resident in RAM (RSS), the
process' proportional share of this mapping (PSS), the number of clean and
dirty private pages in the mapping.  Note that even a page which is part of a
MAP_SHARED mapping, but has only a single pte mapped, i.e.  is currently used
by only one process, is accounted as private and not as shared.  "Referenced"
indicates the amount of memory currently marked as referenced or accessed.
"Anonymous" shows the amount of memory that does not belong to any file.  Even
a mapping associated with a file may contain anonymous pages: when MAP_PRIVATE
and a page is modified, the file page is replaced by a private anonymous copy.
"Swap" shows how much would-be-anonymous memory is also used, but out on
swap.

"VmFlags" field deserves a separate description. This member represents the kernel
flags associated with the particular virtual memory area in two letter encoded
manner. The codes are the following:
    rd  - readable
    wr  - writeable
    ex  - executable
    sh  - shared
    mr  - may read
    mw  - may write
    me  - may execute
    ms  - may share
    gd  - stack segment growns down
    pf  - pure PFN range
    dw  - disabled write to the mapped file
    lo  - pages are locked in memory
    io  - memory mapped I/O area
    sr  - sequential read advise provided
    rr  - random read advise provided
    dc  - do not copy area on fork
    de  - do not expand area on remapping
    ac  - area is accountable
    nr  - swap space is not reserved for the area
    ht  - area uses huge tlb pages
    nl  - non-linear mapping
    ar  - architecture specific flag
    dd  - do not include area into core dump
    sd  - soft-dirty flag
    mm  - mixed map area
    hg  - huge page advise flag
    nh  - no-huge page advise flag
    mg  - mergable advise flag

Note that there is no guarantee that every flag and associated mnemonic will
be present in all further kernel releases. Things get changed, the flags may
be vanished or the reverse -- new added.

This file is only present if the CONFIG_MMU kernel configuration option is
enabled.

The /proc/PID/clear_refs is used to reset the PG_Referenced and ACCESSED/YOUNG
bits on both physical and virtual pages associated with a process, and the
soft-dirty bit on pte (see Documentation/vm/soft-dirty.txt for details).
To clear the bits for all the pages associated with the process
    > echo 1 > /proc/PID/clear_refs

To clear the bits for the anonymous pages associated with the process
    > echo 2 > /proc/PID/clear_refs

To clear the bits for the file mapped pages associated with the process
    > echo 3 > /proc/PID/clear_refs

To clear the soft-dirty bit
    > echo 4 > /proc/PID/clear_refs

To reset the peak resident set size ("high water mark") to the process's
current value:
    > echo 5 > /proc/PID/clear_refs

Any other value written to /proc/PID/clear_refs will have no effect.

The /proc/pid/pagemap gives the PFN, which can be used to find the pageflags
using /proc/kpageflags and number of times a page is mapped using
/proc/kpagecount. For detailed explanation, see Documentation/vm/pagemap.txt.

The /proc/pid/numa_maps is an extension based on maps, showing the memory
locality and binding policy, as well as the memory usage (in pages) of
each mapping. The output follows a general format where mapping details get
summarized separated by blank spaces, one mapping per each file line:

address   policy    mapping details

00400000 default file=/usr/local/bin/app mapped=1 active=0 N3=1 kernelpagesize_kB=4
00600000 default file=/usr/local/bin/app anon=1 dirty=1 N3=1 kernelpagesize_kB=4
3206000000 default file=/lib64/ld-2.12.so mapped=26 mapmax=6 N0=24 N3=2 kernelpagesize_kB=4
320621f000 default file=/lib64/ld-2.12.so anon=1 dirty=1 N3=1 kernelpagesize_kB=4
3206220000 default file=/lib64/ld-2.12.so anon=1 dirty=1 N3=1 kernelpagesize_kB=4
3206221000 default anon=1 dirty=1 N3=1 kernelpagesize_kB=4
3206800000 default file=/lib64/libc-2.12.so mapped=59 mapmax=21 active=55 N0=41 N3=18 kernelpagesize_kB=4
320698b000 default file=/lib64/libc-2.12.so
3206b8a000 default file=/lib64/libc-2.12.so anon=2 dirty=2 N3=2 kernelpagesize_kB=4
3206b8e000 default file=/lib64/libc-2.12.so anon=1 dirty=1 N3=1 kernelpagesize_kB=4
3206b8f000 default anon=3 dirty=3 active=1 N3=3 kernelpagesize_kB=4
7f4dc10a2000 default anon=3 dirty=3 N3=3 kernelpagesize_kB=4
7f4dc10b4000 default anon=2 dirty=2 active=1 N3=2 kernelpagesize_kB=4
7f4dc1200000 default file=/anon_hugepage\040(deleted) huge anon=1 dirty=1 N3=1 kernelpagesize_kB=2048
7fff335f0000 default stack anon=3 dirty=3 N3=3 kernelpagesize_kB=4
7fff3369d000 default mapped=1 mapmax=35 active=0 N3=1 kernelpagesize_kB=4

Where:
"address" is the starting address for the mapping;
"policy" reports the NUMA memory policy set for the mapping (see vm/numa_memory_policy.txt);
"mapping details" summarizes mapping data such as mapping type, page usage counters,
node locality page counters (N0 == node0, N1 == node1, ...) and the kernel page
size, in KB, that is backing the mapping up.

1.2 Kernel data
---------------

Similar to  the  process entries, the kernel data files give information about
the running kernel. The files used to obtain this information are contained in
/proc and  are  listed  in Table 1-5. Not all of these will be present in your
system. It  depends  on the kernel configuration and the loaded modules, which
files are there, and which are missing.

Table 1-5: Kernel info in /proc
..............................................................................
 File        Content                                           
 apm         Advanced power management info                    
 buddyinfo   Kernel memory allocator information (see text)	(2.5)
 bus         Directory containing bus specific information     
 cmdline     Kernel command line                               
 cpuinfo     Info about the CPU                                
 devices     Available devices (block and character)           
 dma         Used DMS channels                                 
 filesystems Supported filesystems                             
 driver	     Various drivers grouped here, currently rtc (2.4)
 execdomains Execdomains, related to security			(2.4)
 fb	     Frame Buffer devices				(2.4)
 fs	     File system parameters, currently nfs/exports	(2.4)
 ide         Directory containing info about the IDE subsystem 
 interrupts  Interrupt usage                                   
 iomem	     Memory map						(2.4)
 ioports     I/O port usage                                    
 irq	     Masks for irq to cpu affinity			(2.4)(smp?)
 isapnp	     ISA PnP (Plug&Play) Info				(2.4)
 kcore       Kernel core image (can be ELF or A.OUT(deprecated in 2.4))   
 kmsg        Kernel messages                                   
 ksyms       Kernel symbol table                               
 loadavg     Load average of last 1, 5 & 15 minutes                
 locks       Kernel locks                                      
 meminfo     Memory info                                       
 misc        Miscellaneous                                     
 modules     List of loaded modules                            
 mounts      Mounted filesystems                               
 net         Networking info (see text)                        
 pagetypeinfo Additional page allocator information (see text)  (2.5)
 partitions  Table of partitions known to the system           
 pci	     Deprecated info of PCI bus (new way -> /proc/bus/pci/,
             decoupled by lspci					(2.4)
 rtc         Real time clock                                   
 scsi        SCSI info (see text)                              
 slabinfo    Slab pool info                                    
 softirqs    softirq usage
 stat        Overall statistics                                
 swaps       Swap space utilization                            
 sys         See chapter 2                                     
 sysvipc     Info of SysVIPC Resources (msg, sem, shm)		(2.4)
 tty	     Info of tty drivers
 uptime      Wall clock since boot, combined idle time of all cpus
 version     Kernel version                                    
 video	     bttv info of video resources			(2.4)
 vmallocinfo Show vmalloced areas
..............................................................................

You can,  for  example,  check  which interrupts are currently in use and what
they are used for by looking in the file /proc/interrupts:

  > cat /proc/interrupts 
             CPU0        
    0:    8728810          XT-PIC  timer 
    1:        895          XT-PIC  keyboard 
    2:          0          XT-PIC  cascade 
    3:     531695          XT-PIC  aha152x 
    4:    2014133          XT-PIC  serial 
    5:      44401          XT-PIC  pcnet_cs 
    8:          2          XT-PIC  rtc 
   11:          8          XT-PIC  i82365 
   12:     182918          XT-PIC  PS/2 Mouse 
   13:          1          XT-PIC  fpu 
   14:    1232265          XT-PIC  ide0 
   15:          7          XT-PIC  ide1 
  NMI:          0 

In 2.4.* a couple of lines where added to this file LOC & ERR (this time is the
output of a SMP machine):

  > cat /proc/interrupts 

             CPU0       CPU1       
    0:    1243498    1214548    IO-APIC-edge  timer
    1:       8949       8958    IO-APIC-edge  keyboard
    2:          0          0          XT-PIC  cascade
    5:      11286      10161    IO-APIC-edge  soundblaster
    8:          1          0    IO-APIC-edge  rtc
    9:      27422      27407    IO-APIC-edge  3c503
   12:     113645     113873    IO-APIC-edge  PS/2 Mouse
   13:          0          0          XT-PIC  fpu
   14:      22491      24012    IO-APIC-edge  ide0
   15:       2183       2415    IO-APIC-edge  ide1
   17:      30564      30414   IO-APIC-level  eth0
   18:        177        164   IO-APIC-level  bttv
  NMI:    2457961    2457959 
  LOC:    2457882    2457881 
  ERR:       2155

NMI is incremented in this case because every timer interrupt generates a NMI
(Non Maskable Interrupt) which is used by the NMI Watchdog to detect lockups.

LOC is the local interrupt counter of the internal APIC of every CPU.

ERR is incremented in the case of errors in the IO-APIC bus (the bus that
connects the CPUs in a SMP system. This means that an error has been detected,
the IO-APIC automatically retry the transmission, so it should not be a big
problem, but you should read the SMP-FAQ.

In 2.6.2* /proc/interrupts was expanded again.  This time the goal was for
/proc/interrupts to display every IRQ vector in use by the system, not
just those considered 'most important'.  The new vectors are:

  THR -- interrupt raised when a machine check threshold counter
  (typically counting ECC corrected errors of memory or cache) exceeds
  a configurable threshold.  Only available on some systems.

  TRM -- a thermal event interrupt occurs when a temperature threshold
  has been exceeded for the CPU.  This interrupt may also be generated
  when the temperature drops back to normal.

  SPU -- a spurious interrupt is some interrupt that was raised then lowered
  by some IO device before it could be fully processed by the APIC.  Hence
  the APIC sees the interrupt but does not know what device it came from.
  For this case the APIC will generate the interrupt with a IRQ vector
  of 0xff. This might also be generated by chipset bugs.

  RES, CAL, TLB -- rescheduling, call and TLB flush interrupts are
  sent from one CPU to another per the needs of the OS.  Typically,
  their statistics are used by kernel developers and interested users to
  determine the occurrence of interrupts of the given type.

The above IRQ vectors are displayed only when relevant.  For example,
the threshold vector does not exist on x86_64 platforms.  Others are
suppressed when the system is a uniprocessor.  As of this writing, only
i386 and x86_64 platforms support the new IRQ vector displays.

Of some interest is the introduction of the /proc/irq directory to 2.4.
It could be used to set IRQ to CPU affinity, this means that you can "hook" an
IRQ to only one CPU, or to exclude a CPU of handling IRQs. The contents of the
irq subdir is one subdir for each IRQ, and two files; default_smp_affinity and
prof_cpu_mask.

For example 
  > ls /proc/irq/
  0  10  12  14  16  18  2  4  6  8  prof_cpu_mask
  1  11  13  15  17  19  3  5  7  9  default_smp_affinity
  > ls /proc/irq/0/
  smp_affinity

smp_affinity is a bitmask, in which you can specify which CPUs can handle the
IRQ, you can set it by doing:

  > echo 1 > /proc/irq/10/smp_affinity

This means that only the first CPU will handle the IRQ, but you can also echo
5 which means that only the first and fourth CPU can handle the IRQ.

The contents of each smp_affinity file is the same by default:

  > cat /proc/irq/0/smp_affinity
  ffffffff

There is an alternate interface, smp_affinity_list which allows specifying
a cpu range instead of a bitmask:

  > cat /proc/irq/0/smp_affinity_list
  1024-1031

The default_smp_affinity mask applies to all non-active IRQs, which are the
IRQs which have not yet been allocated/activated, and hence which lack a
/proc/irq/[0-9]* directory.

The node file on an SMP system shows the node to which the device using the IRQ
reports itself as being attached. This hardware locality information does not
include information about any possible driver locality preference.

prof_cpu_mask specifies which CPUs are to be profiled by the system wide
profiler. Default value is ffffffff (all cpus if there are only 32 of them).

The way IRQs are routed is handled by the IO-APIC, and it's Round Robin
between all the CPUs which are allowed to handle it. As usual the kernel has
more info than you and does a better job than you, so the defaults are the
best choice for almost everyone.  [Note this applies only to those IO-APIC's
that support "Round Robin" interrupt distribution.]

There are  three  more  important subdirectories in /proc: net, scsi, and sys.
The general  rule  is  that  the  contents,  or  even  the  existence of these
directories, depend  on your kernel configuration. If SCSI is not enabled, the
directory scsi  may  not  exist. The same is true with the net, which is there
only when networking support is present in the running kernel.

The slabinfo  file  gives  information  about  memory usage at the slab level.
Linux uses  slab  pools for memory management above page level in version 2.2.
Commonly used  objects  have  their  own  slab  pool (such as network buffers,
directory cache, and so on).

..............................................................................

> cat /proc/buddyinfo

Node 0, zone      DMA      0      4      5      4      4      3 ...
Node 0, zone   Normal      1      0      0      1    101      8 ...
Node 0, zone  HighMem      2      0      0      1      1      0 ...

External fragmentation is a problem under some workloads, and buddyinfo is a
useful tool for helping diagnose these problems.  Buddyinfo will give you a 
clue as to how big an area you can safely allocate, or why a previous
allocation failed.

Each column represents the number of pages of a certain order which are 
available.  In this case, there are 0 chunks of 2^0*PAGE_SIZE available in 
ZONE_DMA, 4 chunks of 2^1*PAGE_SIZE in ZONE_DMA, 101 chunks of 2^4*PAGE_SIZE 
available in ZONE_NORMAL, etc... 

More information relevant to external fragmentation can be found in
pagetypeinfo.

> cat /proc/pagetypeinfo
Page block order: 9
Pages per block:  512

Free pages count per migrate type at order       0      1      2      3      4      5      6      7      8      9     10
Node    0, zone      DMA, type    Unmovable      0      0      0      1      1      1      1      1      1      1      0
Node    0, zone      DMA, type  Reclaimable      0      0      0      0      0      0      0      0      0      0      0
Node    0, zone      DMA, type      Movable      1      1      2      1      2      1      1      0      1      0      2
Node    0, zone      DMA, type      Reserve      0      0      0      0      0      0      0      0      0      1      0
Node    0, zone      DMA, type      Isolate      0      0      0      0      0      0      0      0      0      0      0
Node    0, zone    DMA32, type    Unmovable    103     54     77      1      1      1     11      8      7      1      9
Node    0, zone    DMA32, type  Reclaimable      0      0      2      1      0      0      0      0      1      0      0
Node    0, zone    DMA32, type      Movable    169    152    113     91     77     54     39     13      6      1    452
Node    0, zone    DMA32, type      Reserve      1      2      2      2      2      0      1      1      1      1      0
Node    0, zone    DMA32, type      Isolate      0      0      0      0      0      0      0      0      0      0      0

Number of blocks type     Unmovable  Reclaimable      Movable      Reserve      Isolate
Node 0, zone      DMA            2            0            5            1            0
Node 0, zone    DMA32           41            6          967            2            0

Fragmentation avoidance in the kernel works by grouping pages of different
migrate types into the same contiguous regions of memory called page blocks.
A page block is typically the size of the default hugepage size e.g. 2MB on
X86-64. By keeping pages grouped based on their ability to move, the kernel
can reclaim pages within a page block to satisfy a high-order allocation.

The pagetypinfo begins with information on the size of a page block. It
then gives the same type of information as buddyinfo except broken down
by migrate-type and finishes with details on how many page blocks of each
type exist.

If min_free_kbytes has been tuned correctly (recommendations made by hugeadm
from libhugetlbfs http://sourceforge.net/projects/libhugetlbfs/), one can
make an estimate of the likely number of huge pages that can be allocated
at a given point in time. All the "Movable" blocks should be allocatable
unless memory has been mlock()'d. Some of the Reclaimable blocks should
also be allocatable although a lot of filesystem metadata may have to be
reclaimed to achieve this.

..............................................................................

meminfo:

Provides information about distribution and utilization of memory.  This
varies by architecture and compile options.  The following is from a
16GB PIII, which has highmem enabled.  You may not have all of these fields.

> cat /proc/meminfo

The "Locked" indicates whether the mapping is locked in memory or not.


MemTotal:     16344972 kB
MemFree:      13634064 kB
MemAvailable: 14836172 kB
Buffers:          3656 kB
Cached:        1195708 kB
SwapCached:          0 kB
Active:         891636 kB
Inactive:      1077224 kB
HighTotal:    15597528 kB
HighFree:     13629632 kB
LowTotal:       747444 kB
LowFree:          4432 kB
SwapTotal:           0 kB
SwapFree:            0 kB
Dirty:             968 kB
Writeback:           0 kB
AnonPages:      861800 kB
Mapped:         280372 kB
Slab:           284364 kB
SReclaimable:   159856 kB
SUnreclaim:     124508 kB
PageTables:      24448 kB
NFS_Unstable:        0 kB
Bounce:              0 kB
WritebackTmp:        0 kB
CommitLimit:   7669796 kB
Committed_AS:   100056 kB
VmallocTotal:   112216 kB
VmallocUsed:       428 kB
VmallocChunk:   111088 kB
AnonHugePages:   49152 kB

    MemTotal: Total usable ram (i.e. physical ram minus a few reserved
              bits and the kernel binary code)
     MemFree: The sum of LowFree+HighFree
MemAvailable: An estimate of how much memory is available for starting new
              applications, without swapping. Calculated from MemFree,
              SReclaimable, the size of the file LRU lists, and the low
              watermarks in each zone.
              The estimate takes into account that the system needs some
              page cache to function well, and that not all reclaimable
              slab will be reclaimable, due to items being in use. The
              impact of those factors will vary from system to system.
     Buffers: Relatively temporary storage for raw disk blocks
              shouldn't get tremendously large (20MB or so)
      Cached: in-memory cache for files read from the disk (the
              pagecache).  Doesn't include SwapCached
  SwapCached: Memory that once was swapped out, is swapped back in but
              still also is in the swapfile (if memory is needed it
              doesn't need to be swapped out AGAIN because it is already
              in the swapfile. This saves I/O)
      Active: Memory that has been used more recently and usually not
              reclaimed unless absolutely necessary.
    Inactive: Memory which has been less recently used.  It is more
              eligible to be reclaimed for other purposes
   HighTotal:
    HighFree: Highmem is all memory above ~860MB of physical memory
              Highmem areas are for use by userspace programs, or
              for the pagecache.  The kernel must use tricks to access
              this memory, making it slower to access than lowmem.
    LowTotal:
     LowFree: Lowmem is memory which can be used for everything that
              highmem can be used for, but it is also available for the
              kernel's use for its own data structures.  Among many
              other things, it is where everything from the Slab is
              allocated.  Bad things happen when you're out of lowmem.
   SwapTotal: total amount of swap space available
    SwapFree: Memory which has been evicted from RAM, and is temporarily
              on the disk
       Dirty: Memory which is waiting to get written back to the disk
   Writeback: Memory which is actively being written back to the disk
   AnonPages: Non-file backed pages mapped into userspace page tables
AnonHugePages: Non-file backed huge pages mapped into userspace page tables
      Mapped: files which have been mmaped, such as libraries
        Slab: in-kernel data structures cache
SReclaimable: Part of Slab, that might be reclaimed, such as caches
  SUnreclaim: Part of Slab, that cannot be reclaimed on memory pressure
  PageTables: amount of memory dedicated to the lowest level of page
              tables.
NFS_Unstable: NFS pages sent to the server, but not yet committed to stable
	      storage
      Bounce: Memory used for block device "bounce buffers"
WritebackTmp: Memory used by FUSE for temporary writeback buffers
 CommitLimit: Based on the overcommit ratio ('vm.overcommit_ratio'),
              this is the total amount of  memory currently available to
              be allocated on the system. This limit is only adhered to
              if strict overcommit accounting is enabled (mode 2 in
              'vm.overcommit_memory').
              The CommitLimit is calculated with the following formula:
              CommitLimit = ([total RAM pages] - [total huge TLB pages]) *
                             overcommit_ratio / 100 + [total swap pages]
              For example, on a system with 1G of physical RAM and 7G
              of swap with a `vm.overcommit_ratio` of 30 it would
              yield a CommitLimit of 7.3G.
              For more details, see the memory overcommit documentation
              in vm/overcommit-accounting.
Committed_AS: The amount of memory presently allocated on the system.
              The committed memory is a sum of all of the memory which
              has been allocated by processes, even if it has not been
              "used" by them as of yet. A process which malloc()'s 1G
              of memory, but only touches 300M of it will show up as
	      using 1G. This 1G is memory which has been "committed" to
              by the VM and can be used at any time by the allocating
              application. With strict overcommit enabled on the system
              (mode 2 in 'vm.overcommit_memory'),allocations which would
              exceed the CommitLimit (detailed above) will not be permitted.
              This is useful if one needs to guarantee that processes will
              not fail due to lack of memory once that memory has been
              successfully allocated.
VmallocTotal: total size of vmalloc memory area
 VmallocUsed: amount of vmalloc area which is used
VmallocChunk: largest contiguous block of vmalloc area which is free

..............................................................................

vmallocinfo:

Provides information about vmalloced/vmaped areas. One line per area,
containing the virtual address range of the area, size in bytes,
caller information of the creator, and optional information depending
on the kind of area :

 pages=nr    number of pages
 phys=addr   if a physical address was specified
 ioremap     I/O mapping (ioremap() and friends)
 vmalloc     vmalloc() area
 vmap        vmap()ed pages
 user        VM_USERMAP area
 vpages      buffer for pages pointers was vmalloced (huge area)
 N<node>=nr  (Only on NUMA kernels)
             Number of pages allocated on memory node <node>

> cat /proc/vmallocinfo
0xffffc20000000000-0xffffc20000201000 2101248 alloc_large_system_hash+0x204 ...
  /0x2c0 pages=512 vmalloc N0=128 N1=128 N2=128 N3=128
0xffffc20000201000-0xffffc20000302000 1052672 alloc_large_system_hash+0x204 ...
  /0x2c0 pages=256 vmalloc N0=64 N1=64 N2=64 N3=64
0xffffc20000302000-0xffffc20000304000    8192 acpi_tb_verify_table+0x21/0x4f...
  phys=7fee8000 ioremap
0xffffc20000304000-0xffffc20000307000   12288 acpi_tb_verify_table+0x21/0x4f...
  phys=7fee7000 ioremap
0xffffc2000031d000-0xffffc2000031f000    8192 init_vdso_vars+0x112/0x210
0xffffc2000031f000-0xffffc2000032b000   49152 cramfs_uncompress_init+0x2e ...
  /0x80 pages=11 vmalloc N0=3 N1=3 N2=2 N3=3
0xffffc2000033a000-0xffffc2000033d000   12288 sys_swapon+0x640/0xac0      ...
  pages=2 vmalloc N1=2
0xffffc20000347000-0xffffc2000034c000   20480 xt_alloc_table_info+0xfe ...
  /0x130 [x_tables] pages=4 vmalloc N0=4
0xffffffffa0000000-0xffffffffa000f000   61440 sys_init_module+0xc27/0x1d00 ...
   pages=14 vmalloc N2=14
0xffffffffa000f000-0xffffffffa0014000   20480 sys_init_module+0xc27/0x1d00 ...
   pages=4 vmalloc N1=4
0xffffffffa0014000-0xffffffffa0017000   12288 sys_init_module+0xc27/0x1d00 ...
   pages=2 vmalloc N1=2
0xffffffffa0017000-0xffffffffa0022000   45056 sys_init_module+0xc27/0x1d00 ...
   pages=10 vmalloc N0=10

..............................................................................

softirqs:

Provides counts of softirq handlers serviced since boot time, for each cpu.

> cat /proc/softirqs
                CPU0       CPU1       CPU2       CPU3
      HI:          0          0          0          0
   TIMER:      27166      27120      27097      27034
  NET_TX:          0          0          0         17
  NET_RX:         42          0          0         39
   BLOCK:          0          0        107       1121
 TASKLET:          0          0          0        290
   SCHED:      27035      26983      26971      26746
 HRTIMER:          0          0          0          0
     RCU:       1678       1769       2178       2250


1.3 IDE devices in /proc/ide
----------------------------

The subdirectory /proc/ide contains information about all IDE devices of which
the kernel  is  aware.  There is one subdirectory for each IDE controller, the
file drivers  and a link for each IDE device, pointing to the device directory
in the controller specific subtree.

The file  drivers  contains general information about the drivers used for the
IDE devices:

  > cat /proc/ide/drivers
  ide-cdrom version 4.53
  ide-disk version 1.08

More detailed  information  can  be  found  in  the  controller  specific
subdirectories. These  are  named  ide0,  ide1  and  so  on.  Each  of  these
directories contains the files shown in table 1-6.


Table 1-6: IDE controller info in  /proc/ide/ide?
..............................................................................
 File    Content                                 
 channel IDE channel (0 or 1)                    
 config  Configuration (only for PCI/IDE bridge) 
 mate    Mate name                               
 model   Type/Chipset of IDE controller          
..............................................................................

Each device  connected  to  a  controller  has  a separate subdirectory in the
controllers directory.  The  files  listed in table 1-7 are contained in these
directories.


Table 1-7: IDE device information
..............................................................................
 File             Content                                    
 cache            The cache                                  
 capacity         Capacity of the medium (in 512Byte blocks) 
 driver           driver and version                         
 geometry         physical and logical geometry              
 identify         device identify block                      
 media            media type                                 
 model            device identifier                          
 settings         device setup                               
 smart_thresholds IDE disk management thresholds             
 smart_values     IDE disk management values                 
..............................................................................

The most  interesting  file is settings. This file contains a nice overview of
the drive parameters:

  # cat /proc/ide/ide0/hda/settings 
  name                    value           min             max             mode 
  ----                    -----           ---             ---             ---- 
  bios_cyl                526             0               65535           rw 
  bios_head               255             0               255             rw 
  bios_sect               63              0               63              rw 
  breada_readahead        4               0               127             rw 
  bswap                   0               0               1               r 
  file_readahead          72              0               2097151         rw 
  io_32bit                0               0               3               rw 
  keepsettings            0               0               1               rw 
  max_kb_per_request      122             1               127             rw 
  multcount               0               0               8               rw 
  nice1                   1               0               1               rw 
  nowerr                  0               0               1               rw 
  pio_mode                write-only      0               255             w 
  slow                    0               0               1               rw 
  unmaskirq               0               0               1               rw 
  using_dma               0               0               1               rw 


1.4 Networking info in /proc/net
--------------------------------

The subdirectory  /proc/net  follows  the  usual  pattern. Table 1-8 shows the
additional values  you  get  for  IP  version 6 if you configure the kernel to
support this. Table 1-9 lists the files and their meaning.


Table 1-8: IPv6 info in /proc/net
..............................................................................
 File       Content                                               
 udp6       UDP sockets (IPv6)                                    
 tcp6       TCP sockets (IPv6)                                    
 raw6       Raw device statistics (IPv6)                          
 igmp6      IP multicast addresses, which this host joined (IPv6) 
 if_inet6   List of IPv6 interface addresses                      
 ipv6_route Kernel routing table for IPv6                         
 rt6_stats  Global IPv6 routing tables statistics                 
 sockstat6  Socket statistics (IPv6)                              
 snmp6      Snmp data (IPv6)                                      
..............................................................................


Table 1-9: Network info in /proc/net
..............................................................................
 File          Content                                                         
 arp           Kernel  ARP table                                               
 dev           network devices with statistics                                 
 dev_mcast     the Layer2 multicast groups a device is listening too
               (interface index, label, number of references, number of bound
               addresses). 
 dev_stat      network device status                                           
 ip_fwchains   Firewall chain linkage                                          
 ip_fwnames    Firewall chain names                                            
 ip_masq       Directory containing the masquerading tables                    
 ip_masquerade Major masquerading table                                        
 netstat       Network statistics                                              
 raw           raw device statistics                                           
 route         Kernel routing table                                            
 rpc           Directory containing rpc info                                   
 rt_cache      Routing cache                                                   
 snmp          SNMP data                                                       
 sockstat      Socket statistics                                               
 tcp           TCP  sockets                                                    
 udp           UDP sockets                                                     
 unix          UNIX domain sockets                                             
 wireless      Wireless interface data (Wavelan etc)                           
 igmp          IP multicast addresses, which this host joined                  
 psched        Global packet scheduler parameters.                             
 netlink       List of PF_NETLINK sockets                                      
 ip_mr_vifs    List of multicast virtual interfaces                            
 ip_mr_cache   List of multicast routing cache                                 
..............................................................................

You can  use  this  information  to see which network devices are available in
your system and how much traffic was routed over those devices:

  > cat /proc/net/dev 
  Inter-|Receive                                                   |[... 
   face |bytes    packets errs drop fifo frame compressed multicast|[... 
      lo:  908188   5596     0    0    0     0          0         0 [...         
    ppp0:15475140  20721   410    0    0   410          0         0 [...  
    eth0:  614530   7085     0    0    0     0          0         1 [... 
   
  ...] Transmit 
  ...] bytes    packets errs drop fifo colls carrier compressed 
  ...]  908188     5596    0    0    0     0       0          0 
  ...] 1375103    17405    0    0    0     0       0          0 
  ...] 1703981     5535    0    0    0     3       0          0 

In addition, each Channel Bond interface has its own directory.  For
example, the bond0 device will have a directory called /proc/net/bond0/.
It will contain information that is specific to that bond, such as the
current slaves of the bond, the link status of the slaves, and how
many times the slaves link has failed.

1.5 SCSI info
-------------

If you  have  a  SCSI  host adapter in your system, you'll find a subdirectory
named after  the driver for this adapter in /proc/scsi. You'll also see a list
of all recognized SCSI devices in /proc/scsi:

  >cat /proc/scsi/scsi 
  Attached devices: 
  Host: scsi0 Channel: 00 Id: 00 Lun: 00 
    Vendor: IBM      Model: DGHS09U          Rev: 03E0 
    Type:   Direct-Access                    ANSI SCSI revision: 03 
  Host: scsi0 Channel: 00 Id: 06 Lun: 00 
    Vendor: PIONEER  Model: CD-ROM DR-U06S   Rev: 1.04 
    Type:   CD-ROM                           ANSI SCSI revision: 02 


The directory  named  after  the driver has one file for each adapter found in
the system.  These  files  contain information about the controller, including
the used  IRQ  and  the  IO  address range. The amount of information shown is
dependent on  the adapter you use. The example shows the output for an Adaptec
AHA-2940 SCSI adapter:

  > cat /proc/scsi/aic7xxx/0 
   
  Adaptec AIC7xxx driver version: 5.1.19/3.2.4 
  Compile Options: 
    TCQ Enabled By Default : Disabled 
    AIC7XXX_PROC_STATS     : Disabled 
    AIC7XXX_RESET_DELAY    : 5 
  Adapter Configuration: 
             SCSI Adapter: Adaptec AHA-294X Ultra SCSI host adapter 
                             Ultra Wide Controller 
      PCI MMAPed I/O Base: 0xeb001000 
   Adapter SEEPROM Config: SEEPROM found and used. 
        Adaptec SCSI BIOS: Enabled 
                      IRQ: 10 
                     SCBs: Active 0, Max Active 2, 
                           Allocated 15, HW 16, Page 255 
               Interrupts: 160328 
        BIOS Control Word: 0x18b6 
     Adapter Control Word: 0x005b 
     Extended Translation: Enabled 
  Disconnect Enable Flags: 0xffff 
       Ultra Enable Flags: 0x0001 
   Tag Queue Enable Flags: 0x0000 
  Ordered Queue Tag Flags: 0x0000 
  Default Tag Queue Depth: 8 
      Tagged Queue By Device array for aic7xxx host instance 0: 
        {255,255,255,255,255,255,255,255,255,255,255,255,255,255,255,255} 
      Actual queue depth per device for aic7xxx host instance 0: 
        {1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1} 
  Statistics: 
  (scsi0:0:0:0) 
    Device using Wide/Sync transfers at 40.0 MByte/sec, offset 8 
    Transinfo settings: current(12/8/1/0), goal(12/8/1/0), user(12/15/1/0) 
    Total transfers 160151 (74577 reads and 85574 writes) 
  (scsi0:0:6:0) 
    Device using Narrow/Sync transfers at 5.0 MByte/sec, offset 15 
    Transinfo settings: current(50/15/0/0), goal(50/15/0/0), user(50/15/0/0) 
    Total transfers 0 (0 reads and 0 writes) 


1.6 Parallel port info in /proc/parport
---------------------------------------

The directory  /proc/parport  contains information about the parallel ports of
your system.  It  has  one  subdirectory  for  each port, named after the port
number (0,1,2,...).

These directories contain the four files shown in Table 1-10.


Table 1-10: Files in /proc/parport
..............................................................................
 File      Content                                                             
 autoprobe Any IEEE-1284 device ID information that has been acquired.         
 devices   list of the device drivers using that port. A + will appear by the
           name of the device currently using the port (it might not appear
           against any). 
 hardware  Parallel port's base address, IRQ line and DMA channel.             
 irq       IRQ that parport is using for that port. This is in a separate
           file to allow you to alter it by writing a new value in (IRQ
           number or none). 
..............................................................................

1.7 TTY info in /proc/tty
-------------------------

Information about  the  available  and actually used tty's can be found in the
directory /proc/tty.You'll  find  entries  for drivers and line disciplines in
this directory, as shown in Table 1-11.


Table 1-11: Files in /proc/tty
..............................................................................
 File          Content                                        
 drivers       list of drivers and their usage                
 ldiscs        registered line disciplines                    
 driver/serial usage statistic and status of single tty lines 
..............................................................................

To see  which  tty's  are  currently in use, you can simply look into the file
/proc/tty/drivers:

  > cat /proc/tty/drivers 
  pty_slave            /dev/pts      136   0-255 pty:slave 
  pty_master           /dev/ptm      128   0-255 pty:master 
  pty_slave            /dev/ttyp       3   0-255 pty:slave 
  pty_master           /dev/pty        2   0-255 pty:master 
  serial               /dev/cua        5   64-67 serial:callout 
  serial               /dev/ttyS       4   64-67 serial 
  /dev/tty0            /dev/tty0       4       0 system:vtmaster 
  /dev/ptmx            /dev/ptmx       5       2 system 
  /dev/console         /dev/console    5       1 system:console 
  /dev/tty             /dev/tty        5       0 system:/dev/tty 
  unknown              /dev/tty        4    1-63 console 


1.8 Miscellaneous kernel statistics in /proc/stat
-------------------------------------------------

Various pieces   of  information about  kernel activity  are  available in the
/proc/stat file.  All  of  the numbers reported  in  this file are  aggregates
since the system first booted.  For a quick look, simply cat the file:

  > cat /proc/stat
  cpu  2255 34 2290 22625563 6290 127 456 0 0
  cpu0 1132 34 1441 11311718 3675 127 438 0 0
  cpu1 1123 0 849 11313845 2614 0 18 0 0
  intr 114930548 113199788 3 0 5 263 0 4 [... lots more numbers ...]
  ctxt 1990473
  btime 1062191376
  processes 2915
  procs_running 1
  procs_blocked 0
  softirq 183433 0 21755 12 39 1137 231 21459 2263

The very first  "cpu" line aggregates the  numbers in all  of the other "cpuN"
lines.  These numbers identify the amount of time the CPU has spent performing
different kinds of work.  Time units are in USER_HZ (typically hundredths of a
second).  The meanings of the columns are as follows, from left to right:

- user: normal processes executing in user mode
- nice: niced processes executing in user mode
- system: processes executing in kernel mode
- idle: twiddling thumbs
- iowait: waiting for I/O to complete
- irq: servicing interrupts
- softirq: servicing softirqs
- steal: involuntary wait
- guest: running a normal guest
- guest_nice: running a niced guest

The "intr" line gives counts of interrupts  serviced since boot time, for each
of the  possible system interrupts.   The first  column  is the  total of  all
interrupts serviced  including  unnumbered  architecture specific  interrupts;
each  subsequent column is the  total for that particular numbered interrupt.
Unnumbered interrupts are not shown, only summed into the total.

The "ctxt" line gives the total number of context switches across all CPUs.

The "btime" line gives  the time at which the  system booted, in seconds since
the Unix epoch.

The "processes" line gives the number  of processes and threads created, which
includes (but  is not limited  to) those  created by  calls to the  fork() and
clone() system calls.

The "procs_running" line gives the total number of threads that are
running or ready to run (i.e., the total number of runnable threads).

The   "procs_blocked" line gives  the  number of  processes currently blocked,
waiting for I/O to complete.

The "softirq" line gives counts of softirqs serviced since boot time, for each
of the possible system softirqs. The first column is the total of all
softirqs serviced; each subsequent column is the total for that particular
softirq.


1.9 Ext4 file system parameters
-------------------------------

Information about mounted ext4 file systems can be found in
/proc/fs/ext4.  Each mounted filesystem will have a directory in
/proc/fs/ext4 based on its device name (i.e., /proc/fs/ext4/hdc or
/proc/fs/ext4/dm-0).   The files in each per-device directory are shown
in Table 1-12, below.

Table 1-12: Files in /proc/fs/ext4/<devname>
..............................................................................
 File            Content                                        
 mb_groups       details of multiblock allocator buddy cache of free blocks
..............................................................................

2.0 /proc/consoles
------------------
Shows registered system console lines.

To see which character device lines are currently used for the system console
/dev/console, you may simply look into the file /proc/consoles:

  > cat /proc/consoles
  tty0                 -WU (ECp)       4:7
  ttyS0                -W- (Ep)        4:64

The columns are:

  device               name of the device
  operations           R = can do read operations
                       W = can do write operations
                       U = can do unblank
  flags                E = it is enabled
                       C = it is preferred console
                       B = it is primary boot console
                       p = it is used for printk buffer
                       b = it is not a TTY but a Braille device
                       a = it is safe to use when cpu is offline
  major:minor          major and minor number of the device separated by a colon

------------------------------------------------------------------------------
Summary
------------------------------------------------------------------------------
The /proc file system serves information about the running system. It not only
allows access to process data but also allows you to request the kernel status
by reading files in the hierarchy.

The directory  structure  of /proc reflects the types of information and makes
it easy, if not obvious, where to look for specific data.
------------------------------------------------------------------------------

------------------------------------------------------------------------------
CHAPTER 2: MODIFYING SYSTEM PARAMETERS
------------------------------------------------------------------------------

------------------------------------------------------------------------------
In This Chapter
------------------------------------------------------------------------------
* Modifying kernel parameters by writing into files found in /proc/sys
* Exploring the files which modify certain parameters
* Review of the /proc/sys file tree
------------------------------------------------------------------------------


A very  interesting part of /proc is the directory /proc/sys. This is not only
a source  of  information,  it also allows you to change parameters within the
kernel. Be  very  careful  when attempting this. You can optimize your system,
but you  can  also  cause  it  to  crash.  Never  alter kernel parameters on a
production system.  Set  up  a  development machine and test to make sure that
everything works  the  way  you want it to. You may have no alternative but to
reboot the machine once an error has been made.

To change  a  value,  simply  echo  the new value into the file. An example is
given below  in the section on the file system data. You need to be root to do
this. You  can  create  your  own  boot script to perform this every time your
system boots.

The files  in /proc/sys can be used to fine tune and monitor miscellaneous and
general things  in  the operation of the Linux kernel. Since some of the files
can inadvertently  disrupt  your  system,  it  is  advisable  to  read  both
documentation and  source  before actually making adjustments. In any case, be
very careful  when  writing  to  any  of these files. The entries in /proc may
change slightly between the 2.1.* and the 2.2 kernel, so if there is any doubt
review the kernel documentation in the directory /usr/src/linux/Documentation.
This chapter  is  heavily  based  on the documentation included in the pre 2.2
kernels, and became part of it in version 2.2.1 of the Linux kernel.

Please see: Documentation/sysctl/ directory for descriptions of these
entries.

------------------------------------------------------------------------------
Summary
------------------------------------------------------------------------------
Certain aspects  of  kernel  behavior  can be modified at runtime, without the
need to  recompile  the kernel, or even to reboot the system. The files in the
/proc/sys tree  can  not only be read, but also modified. You can use the echo
command to write value into these files, thereby changing the default settings
of the kernel.
------------------------------------------------------------------------------

------------------------------------------------------------------------------
CHAPTER 3: PER-PROCESS PARAMETERS
------------------------------------------------------------------------------

3.1 /proc/<pid>/oom_adj & /proc/<pid>/oom_score_adj- Adjust the oom-killer score
--------------------------------------------------------------------------------

These file can be used to adjust the badness heuristic used to select which
process gets killed in out of memory conditions.

The badness heuristic assigns a value to each candidate task ranging from 0
(never kill) to 1000 (always kill) to determine which process is targeted.  The
units are roughly a proportion along that range of allowed memory the process
may allocate from based on an estimation of its current memory and swap use.
For example, if a task is using all allowed memory, its badness score will be
1000.  If it is using half of its allowed memory, its score will be 500.

There is an additional factor included in the badness score: the current memory
and swap usage is discounted by 3% for root processes.

The amount of "allowed" memory depends on the context in which the oom killer
was called.  If it is due to the memory assigned to the allocating task's cpuset
being exhausted, the allowed memory represents the set of mems assigned to that
cpuset.  If it is due to a mempolicy's node(s) being exhausted, the allowed
memory represents the set of mempolicy nodes.  If it is due to a memory
limit (or swap limit) being reached, the allowed memory is that configured
limit.  Finally, if it is due to the entire system being out of memory, the
allowed memory represents all allocatable resources.

The value of /proc/<pid>/oom_score_adj is added to the badness score before it
is used to determine which task to kill.  Acceptable values range from -1000
(OOM_SCORE_ADJ_MIN) to +1000 (OOM_SCORE_ADJ_MAX).  This allows userspace to
polarize the preference for oom killing either by always preferring a certain
task or completely disabling it.  The lowest possible value, -1000, is
equivalent to disabling oom killing entirely for that task since it will always
report a badness score of 0.

Consequently, it is very simple for userspace to define the amount of memory to
consider for each task.  Setting a /proc/<pid>/oom_score_adj value of +500, for
example, is roughly equivalent to allowing the remainder of tasks sharing the
same system, cpuset, mempolicy, or memory controller resources to use at least
50% more memory.  A value of -500, on the other hand, would be roughly
equivalent to discounting 50% of the task's allowed memory from being considered
as scoring against the task.

For backwards compatibility with previous kernels, /proc/<pid>/oom_adj may also
be used to tune the badness score.  Its acceptable values range from -16
(OOM_ADJUST_MIN) to +15 (OOM_ADJUST_MAX) and a special value of -17
(OOM_DISABLE) to disable oom killing entirely for that task.  Its value is
scaled linearly with /proc/<pid>/oom_score_adj.

The value of /proc/<pid>/oom_score_adj may be reduced no lower than the last
value set by a CAP_SYS_RESOURCE process. To reduce the value any lower
requires CAP_SYS_RESOURCE.

Caveat: when a parent task is selected, the oom killer will sacrifice any first
generation children with separate address spaces instead, if possible.  This
avoids servers and important system daemons from being killed and loses the
minimal amount of work.


3.2 /proc/<pid>/oom_score - Display current oom-killer score
-------------------------------------------------------------

This file can be used to check the current score used by the oom-killer is for
any given <pid>. Use it together with /proc/<pid>/oom_score_adj to tune which
process should be killed in an out-of-memory situation.


3.3  /proc/<pid>/io - Display the IO accounting fields
-------------------------------------------------------

This file contains IO statistics for each running process

Example
-------

test:/tmp # dd if=/dev/zero of=/tmp/test.dat &
[1] 3828

test:/tmp # cat /proc/3828/io
rchar: 323934931
wchar: 323929600
syscr: 632687
syscw: 632675
read_bytes: 0
write_bytes: 323932160
cancelled_write_bytes: 0


Description
-----------

rchar
-----

I/O counter: chars read
The number of bytes which this task has caused to be read from storage. This
is simply the sum of bytes which this process passed to read() and pread().
It includes things like tty IO and it is unaffected by whether or not actual
physical disk IO was required (the read might have been satisfied from
pagecache)


wchar
-----

I/O counter: chars written
The number of bytes which this task has caused, or shall cause to be written
to disk. Similar caveats apply here as with rchar.


syscr
-----

I/O counter: read syscalls
Attempt to count the number of read I/O operations, i.e. syscalls like read()
and pread().


syscw
-----

I/O counter: write syscalls
Attempt to count the number of write I/O operations, i.e. syscalls like
write() and pwrite().


read_bytes
----------

I/O counter: bytes read
Attempt to count the number of bytes which this process really did cause to
be fetched from the storage layer. Done at the submit_bio() level, so it is
accurate for block-backed filesystems. <please add status regarding NFS and
CIFS at a later time>


write_bytes
-----------

I/O counter: bytes written
Attempt to count the number of bytes which this process caused to be sent to
the storage layer. This is done at page-dirtying time.


cancelled_write_bytes
---------------------

The big inaccuracy here is truncate. If a process writes 1MB to a file and
then deletes the file, it will in fact perform no writeout. But it will have
been accounted as having caused 1MB of write.
In other words: The number of bytes which this process caused to not happen,
by truncating pagecache. A task can cause "negative" IO too. If this task
truncates some dirty pagecache, some IO which another task has been accounted
for (in its write_bytes) will not be happening. We _could_ just subtract that
from the truncating task's write_bytes, but there is information loss in doing
that.


Note
----

At its current implementation state, this is a bit racy on 32-bit machines: if
process A reads process B's /proc/pid/io while process B is updating one of
those 64-bit counters, process A could see an intermediate result.


More information about this can be found within the taskstats documentation in
Documentation/accounting.

3.4 /proc/<pid>/coredump_filter - Core dump filtering settings
---------------------------------------------------------------
When a process is dumped, all anonymous memory is written to a core file as
long as the size of the core file isn't limited. But sometimes we don't want
to dump some memory segments, for example, huge shared memory. Conversely,
sometimes we want to save file-backed memory segments into a core file, not
only the individual files.

/proc/<pid>/coredump_filter allows you to customize which memory segments
will be dumped when the <pid> process is dumped. coredump_filter is a bitmask
of memory types. If a bit of the bitmask is set, memory segments of the
corresponding memory type are dumped, otherwise they are not dumped.

The following 7 memory types are supported:
  - (bit 0) anonymous private memory
  - (bit 1) anonymous shared memory
  - (bit 2) file-backed private memory
  - (bit 3) file-backed shared memory
  - (bit 4) ELF header pages in file-backed private memory areas (it is
            effective only if the bit 2 is cleared)
  - (bit 5) hugetlb private memory
  - (bit 6) hugetlb shared memory

  Note that MMIO pages such as frame buffer are never dumped and vDSO pages
  are always dumped regardless of the bitmask status.

  Note bit 0-4 doesn't effect any hugetlb memory. hugetlb memory are only
  effected by bit 5-6.

Default value of coredump_filter is 0x23; this means all anonymous memory
segments and hugetlb private memory are dumped.

If you don't want to dump all shared memory segments attached to pid 1234,
write 0x21 to the process's proc file.

  $ echo 0x21 > /proc/1234/coredump_filter

When a new process is created, the process inherits the bitmask status from its
parent. It is useful to set up coredump_filter before the program runs.
For example:

  $ echo 0x7 > /proc/self/coredump_filter
  $ ./some_program

3.5	/proc/<pid>/mountinfo - Information about mounts
--------------------------------------------------------

This file contains lines of the form:

36 35 98:0 /mnt1 /mnt2 rw,noatime master:1 - ext3 /dev/root rw,errors=continue
(1)(2)(3)   (4)   (5)      (6)      (7)   (8) (9)   (10)         (11)

(1) mount ID:  unique identifier of the mount (may be reused after umount)
(2) parent ID:  ID of parent (or of self for the top of the mount tree)
(3) major:minor:  value of st_dev for files on filesystem
(4) root:  root of the mount within the filesystem
(5) mount point:  mount point relative to the process's root
(6) mount options:  per mount options
(7) optional fields:  zero or more fields of the form "tag[:value]"
(8) separator:  marks the end of the optional fields
(9) filesystem type:  name of filesystem of the form "type[.subtype]"
(10) mount source:  filesystem specific information or "none"
(11) super options:  per super block options

Parsers should ignore all unrecognised optional fields.  Currently the
possible optional fields are:

shared:X  mount is shared in peer group X
master:X  mount is slave to peer group X
propagate_from:X  mount is slave and receives propagation from peer group X (*)
unbindable  mount is unbindable

(*) X is the closest dominant peer group under the process's root.  If
X is the immediate master of the mount, or if there's no dominant peer
group under the same root, then only the "master:X" field is present
and not the "propagate_from:X" field.

For more information on mount propagation see:

  Documentation/filesystems/sharedsubtree.txt


3.6	/proc/<pid>/comm  & /proc/<pid>/task/<tid>/comm
--------------------------------------------------------
These files provide a method to access a tasks comm value. It also allows for
a task to set its own or one of its thread siblings comm value. The comm value
is limited in size compared to the cmdline value, so writing anything longer
then the kernel's TASK_COMM_LEN (currently 16 chars) will result in a truncated
comm value.


3.7	/proc/<pid>/task/<tid>/children - Information about task children
-------------------------------------------------------------------------
This file provides a fast way to retrieve first level children pids
of a task pointed by <pid>/<tid> pair. The format is a space separated
stream of pids.

Note the "first level" here -- if a child has own children they will
not be listed here, one needs to read /proc/<children-pid>/task/<tid>/children
to obtain the descendants.

Since this interface is intended to be fast and cheap it doesn't
guarantee to provide precise results and some children might be
skipped, especially if they've exited right after we printed their
pids, so one need to either stop or freeze processes being inspected
if precise results are needed.


3.8	/proc/<pid>/fdinfo/<fd> - Information about opened file
---------------------------------------------------------------
This file provides information associated with an opened file. The regular
files have at least three fields -- 'pos', 'flags' and mnt_id. The 'pos'
represents the current offset of the opened file in decimal form [see lseek(2)
for details], 'flags' denotes the octal O_xxx mask the file has been
created with [see open(2) for details] and 'mnt_id' represents mount ID of
the file system containing the opened file [see 3.5 /proc/<pid>/mountinfo
for details].

A typical output is

	pos:	0
	flags:	0100002
	mnt_id:	19

The files such as eventfd, fsnotify, signalfd, epoll among the regular pos/flags
pair provide additional information particular to the objects they represent.

	Eventfd files
	~~~~~~~~~~~~~
	pos:	0
	flags:	04002
	mnt_id:	9
	eventfd-count:	5a

	where 'eventfd-count' is hex value of a counter.

	Signalfd files
	~~~~~~~~~~~~~~
	pos:	0
	flags:	04002
	mnt_id:	9
	sigmask:	0000000000000200

	where 'sigmask' is hex value of the signal mask associated
	with a file.

	Epoll files
	~~~~~~~~~~~
	pos:	0
	flags:	02
	mnt_id:	9
	tfd:        5 events:       1d data: ffffffffffffffff

	where 'tfd' is a target file descriptor number in decimal form,
	'events' is events mask being watched and the 'data' is data
	associated with a target [see epoll(7) for more details].

	Fsnotify files
	~~~~~~~~~~~~~~
	For inotify files the format is the following

	pos:	0
	flags:	02000000
	inotify wd:3 ino:9e7e sdev:800013 mask:800afce ignored_mask:0 fhandle-bytes:8 fhandle-type:1 f_handle:7e9e0000640d1b6d

	where 'wd' is a watch descriptor in decimal form, ie a target file
	descriptor number, 'ino' and 'sdev' are inode and device where the
	target file resides and the 'mask' is the mask of events, all in hex
	form [see inotify(7) for more details].

	If the kernel was built with exportfs support, the path to the target
	file is encoded as a file handle.  The file handle is provided by three
	fields 'fhandle-bytes', 'fhandle-type' and 'f_handle', all in hex
	format.

	If the kernel is built without exportfs support the file handle won't be
	printed out.

	If there is no inotify mark attached yet the 'inotify' line will be omitted.

	For fanotify files the format is

	pos:	0
	flags:	02
	mnt_id:	9
	fanotify flags:10 event-flags:0
	fanotify mnt_id:12 mflags:40 mask:38 ignored_mask:40000003
	fanotify ino:4f969 sdev:800013 mflags:0 mask:3b ignored_mask:40000000 fhandle-bytes:8 fhandle-type:1 f_handle:69f90400c275b5b4

	where fanotify 'flags' and 'event-flags' are values used in fanotify_init
	call, 'mnt_id' is the mount point identifier, 'mflags' is the value of
	flags associated with mark which are tracked separately from events
	mask. 'ino', 'sdev' are target inode and device, 'mask' is the events
	mask and 'ignored_mask' is the mask of events which are to be ignored.
	All in hex format. Incorporation of 'mflags', 'mask' and 'ignored_mask'
	does provide information about flags and mask used in fanotify_mark
	call [see fsnotify manpage for details].

	While the first three lines are mandatory and always printed, the rest is
	optional and may be omitted if no marks created yet.

	Timerfd files
	~~~~~~~~~~~~~

	pos:	0
	flags:	02
	mnt_id:	9
	clockid: 0
	ticks: 0
	settime flags: 01
	it_value: (0, 49406829)
	it_interval: (1, 0)

	where 'clockid' is the clock type and 'ticks' is the number of the timer expirations
	that have occurred [see timerfd_create(2) for details]. 'settime flags' are
	flags in octal form been used to setup the timer [see timerfd_settime(2) for
	details]. 'it_value' is remaining time until the timer exiration.
	'it_interval' is the interval for the timer. Note the timer might be set up
	with TIMER_ABSTIME option which will be shown in 'settime flags', but 'it_value'
	still exhibits timer's remaining time.

3.9	/proc/<pid>/map_files - Information about memory mapped files
---------------------------------------------------------------------
This directory contains symbolic links which represent memory mapped files
the process is maintaining.  Example output:

     | lr-------- 1 root root 64 Jan 27 11:24 333c600000-333c620000 -> /usr/lib64/ld-2.18.so
     | lr-------- 1 root root 64 Jan 27 11:24 333c81f000-333c820000 -> /usr/lib64/ld-2.18.so
     | lr-------- 1 root root 64 Jan 27 11:24 333c820000-333c821000 -> /usr/lib64/ld-2.18.so
     | ...
     | lr-------- 1 root root 64 Jan 27 11:24 35d0421000-35d0422000 -> /usr/lib64/libselinux.so.1
     | lr-------- 1 root root 64 Jan 27 11:24 400000-41a000 -> /usr/bin/ls

The name of a link represents the virtual memory bounds of a mapping, i.e.
vm_area_struct::vm_start-vm_area_struct::vm_end.

The main purpose of the map_files is to retrieve a set of memory mapped
files in a fast way instead of parsing /proc/<pid>/maps or
/proc/<pid>/smaps, both of which contain many more records.  At the same
time one can open(2) mappings from the listings of two processes and
comparing their inode numbers to figure out which anonymous memory areas
are actually shared.

------------------------------------------------------------------------------
Configuring procfs
------------------------------------------------------------------------------

4.1	Mount options
---------------------

The following mount options are supported:

	hidepid=	Set /proc/<pid>/ access mode.
	gid=		Set the group authorized to learn processes information.

hidepid=0 means classic mode - everybody may access all /proc/<pid>/ directories
(default).

hidepid=1 means users may not access any /proc/<pid>/ directories but their
own.  Sensitive files like cmdline, sched*, status are now protected against
other users.  This makes it impossible to learn whether any user runs
specific program (given the program doesn't reveal itself by its behaviour).
As an additional bonus, as /proc/<pid>/cmdline is unaccessible for other users,
poorly written programs passing sensitive information via program arguments are
now protected against local eavesdroppers.

hidepid=2 means hidepid=1 plus all /proc/<pid>/ will be fully invisible to other
users.  It doesn't mean that it hides a fact whether a process with a specific
pid value exists (it can be learned by other means, e.g. by "kill -0 $PID"),
but it hides process' uid and gid, which may be learned by stat()'ing
/proc/<pid>/ otherwise.  It greatly complicates an intruder's task of gathering
information about running processes, whether some daemon runs with elevated
privileges, whether other user runs some sensitive program, whether other users
run any program at all, etc.

gid= defines a group authorized to learn processes information otherwise
prohibited by hidepid=.  If you use some daemon like identd which needs to learn
information about processes information, just add identd to this group.