Documentation/cpu-freq/governors.txt - pub/scm/linux/kernel/git/mcgrof/linux-next - Git at Google

      CPU frequency and voltage scaling code in the Linux(TM) kernel


 		         L i n u x    C P U F r e q

 		      C P U F r e q   G o v e r n o r s

 		   - information for users and developers -


 		    Dominik Brodowski  <linux@brodo.de>
             some additions and corrections by Nico Golde <nico@ngolde.de>
 		Rafael J. Wysocki <rafael.j.wysocki@intel.com>
 		   Viresh Kumar <viresh.kumar@linaro.org>


    Clock scaling allows you to change the clock speed of the CPUs on the
     fly. This is a nice method to save battery power, because the lower
             the clock speed, the less power the CPU consumes.


 Contents:
 ---------
 1.   What is a CPUFreq Governor?

 2.   Governors In the Linux Kernel
 2.1  Performance
 2.2  Powersave
 2.3  Userspace
 2.4  Ondemand
 2.5  Conservative
 2.6  Schedutil

 3.   The Governor Interface in the CPUfreq Core

 4.   References


 1. What Is A CPUFreq Governor?
 ==============================

 Most cpufreq drivers (except the intel_pstate and longrun) or even most
 cpu frequency scaling algorithms only allow the CPU frequency to be set
 to predefined fixed values.  In order to offer dynamic frequency
 scaling, the cpufreq core must be able to tell these drivers of a
 "target frequency". So these specific drivers will be transformed to
 offer a "->target/target_index/fast_switch()" call instead of the
 "->setpolicy()" call. For set_policy drivers, all stays the same,
 though.

 How to decide what frequency within the CPUfreq policy should be used?
 That's done using "cpufreq governors".

 Basically, it's the following flow graph:

 CPU can be set to switch independently	 |	   CPU can only be set
       within specific "limits"		 |       to specific frequencies

                                  "CPUfreq policy"
 		consists of frequency limits (policy->{min,max})
   		     and CPUfreq governor to be used
 			 /		      \
 			/		       \
 		       /		       the cpufreq governor decides
 		      /			       (dynamically or statically)
 		     /			       what target_freq to set within
 		    /			       the limits of policy->{min,max}
 		   /			            \
 		  /				     \
 	Using the ->setpolicy call,		 Using the ->target/target_index/fast_switch call,
 	    the limits and the			  the frequency closest
 	     "policy" is set.			  to target_freq is set.
 						  It is assured that it
 						  is within policy->{min,max}


 2. Governors In the Linux Kernel
 ================================

 2.1 Performance
 ---------------

 The CPUfreq governor "performance" sets the CPU statically to the
 highest frequency within the borders of scaling_min_freq and
 scaling_max_freq.


 2.2 Powersave
 -------------

 The CPUfreq governor "powersave" sets the CPU statically to the
 lowest frequency within the borders of scaling_min_freq and
 scaling_max_freq.


 2.3 Userspace
 -------------

 The CPUfreq governor "userspace" allows the user, or any userspace
 program running with UID "root", to set the CPU to a specific frequency
 by making a sysfs file "scaling_setspeed" available in the CPU-device
 directory.


 2.4 Ondemand
 ------------

 The CPUfreq governor "ondemand" sets the CPU frequency depending on the
 current system load. Load estimation is triggered by the scheduler
 through the update_util_data->func hook; when triggered, cpufreq checks
 the CPU-usage statistics over the last period and the governor sets the
 CPU accordingly.  The CPU must have the capability to switch the
 frequency very quickly.

 Sysfs files:

 * sampling_rate:

   Measured in uS (10^-6 seconds), this is how often you want the kernel
   to look at the CPU usage and to make decisions on what to do about the
   frequency.  Typically this is set to values of around '10000' or more.
   It's default value is (cmp. with users-guide.txt): transition_latency
   * 1000.  Be aware that transition latency is in ns and sampling_rate
   is in us, so you get the same sysfs value by default.  Sampling rate
   should always get adjusted considering the transition latency to set
   the sampling rate 750 times as high as the transition latency in the
   bash (as said, 1000 is default), do:

   $ echo `$(($(cat cpuinfo_transition_latency) * 750 / 1000)) > ondemand/sampling_rate

 * sampling_rate_min:

   The sampling rate is limited by the HW transition latency:
   transition_latency * 100

   Or by kernel restrictions:
   - If CONFIG_NO_HZ_COMMON is set, the limit is 10ms fixed.
   - If CONFIG_NO_HZ_COMMON is not set or nohz=off boot parameter is
     used, the limits depend on the CONFIG_HZ option:
     HZ=1000: min=20000us  (20ms)
     HZ=250:  min=80000us  (80ms)
     HZ=100:  min=200000us (200ms)

   The highest value of kernel and HW latency restrictions is shown and
   used as the minimum sampling rate.

 * up_threshold:

   This defines what the average CPU usage between the samplings of
   'sampling_rate' needs to be for the kernel to make a decision on
   whether it should increase the frequency.  For example when it is set
   to its default value of '95' it means that between the checking
   intervals the CPU needs to be on average more than 95% in use to then
   decide that the CPU frequency needs to be increased.

 * ignore_nice_load:

   This parameter takes a value of '0' or '1'. When set to '0' (its
   default), all processes are counted towards the 'cpu utilisation'
   value.  When set to '1', the processes that are run with a 'nice'
   value will not count (and thus be ignored) in the overall usage
   calculation.  This is useful if you are running a CPU intensive
   calculation on your laptop that you do not care how long it takes to
   complete as you can 'nice' it and prevent it from taking part in the
   deciding process of whether to increase your CPU frequency.

 * sampling_down_factor:

   This parameter controls the rate at which the kernel makes a decision
   on when to decrease the frequency while running at top speed. When set
   to 1 (the default) decisions to reevaluate load are made at the same
   interval regardless of current clock speed. But when set to greater
   than 1 (e.g. 100) it acts as a multiplier for the scheduling interval
   for reevaluating load when the CPU is at its top speed due to high
   load. This improves performance by reducing the overhead of load
   evaluation and helping the CPU stay at its top speed when truly busy,
   rather than shifting back and forth in speed. This tunable has no
   effect on behavior at lower speeds/lower CPU loads.

 * powersave_bias:

   This parameter takes a value between 0 to 1000. It defines the
   percentage (times 10) value of the target frequency that will be
   shaved off of the target. For example, when set to 100 -- 10%, when
   ondemand governor would have targeted 1000 MHz, it will target
   1000 MHz - (10% of 1000 MHz) = 900 MHz instead. This is set to 0
   (disabled) by default.

   When AMD frequency sensitivity powersave bias driver --
   drivers/cpufreq/amd_freq_sensitivity.c is loaded, this parameter
   defines the workload frequency sensitivity threshold in which a lower
   frequency is chosen instead of ondemand governor's original target.
   The frequency sensitivity is a hardware reported (on AMD Family 16h
   Processors and above) value between 0 to 100% that tells software how
   the performance of the workload running on a CPU will change when
   frequency changes. A workload with sensitivity of 0% (memory/IO-bound)
   will not perform any better on higher core frequency, whereas a
   workload with sensitivity of 100% (CPU-bound) will perform better
   higher the frequency. When the driver is loaded, this is set to 400 by
   default -- for CPUs running workloads with sensitivity value below
   40%, a lower frequency is chosen. Unloading the driver or writing 0
   will disable this feature.


 2.5 Conservative
 ----------------

 The CPUfreq governor "conservative", much like the "ondemand"
 governor, sets the CPU frequency depending on the current usage.  It
 differs in behaviour in that it gracefully increases and decreases the
 CPU speed rather than jumping to max speed the moment there is any load
 on the CPU. This behaviour is more suitable in a battery powered
 environment.  The governor is tweaked in the same manner as the
 "ondemand" governor through sysfs with the addition of:

 * freq_step:

   This describes what percentage steps the cpu freq should be increased
   and decreased smoothly by.  By default the cpu frequency will increase
   in 5% chunks of your maximum cpu frequency.  You can change this value
   to anywhere between 0 and 100 where '0' will effectively lock your CPU
   at a speed regardless of its load whilst '100' will, in theory, make
   it behave identically to the "ondemand" governor.

 * down_threshold:

   Same as the 'up_threshold' found for the "ondemand" governor but for
   the opposite direction.  For example when set to its default value of
   '20' it means that if the CPU usage needs to be below 20% between
   samples to have the frequency decreased.

 * sampling_down_factor:

   Similar functionality as in "ondemand" governor.  But in
   "conservative", it controls the rate at which the kernel makes a
   decision on when to decrease the frequency while running in any speed.
   Load for frequency increase is still evaluated every sampling rate.


 2.6 Schedutil
 -------------

 The "schedutil" governor aims at better integration with the Linux
 kernel scheduler.  Load estimation is achieved through the scheduler's
 Per-Entity Load Tracking (PELT) mechanism, which also provides
 information about the recent load [1].  This governor currently does
 load based DVFS only for tasks managed by CFS. RT and DL scheduler tasks
 are always run at the highest frequency.  Unlike all the other
 governors, the code is located under the kernel/sched/ directory.

 Sysfs files:

 * rate_limit_us:

   This contains a value in microseconds. The governor waits for
   rate_limit_us time before reevaluating the load again, after it has
   evaluated the load once.

 For an in-depth comparison with the other governors refer to [2].


 3. The Governor Interface in the CPUfreq Core
 =============================================

 A new governor must register itself with the CPUfreq core using
 "cpufreq_register_governor". The struct cpufreq_governor, which has to
 be passed to that function, must contain the following values:

 governor->name - A unique name for this governor.
 governor->owner - .THIS_MODULE for the governor module (if appropriate).

 plus a set of hooks to the functions implementing the governor's logic.

 The CPUfreq governor may call the CPU processor driver using one of
 these two functions:

 int cpufreq_driver_target(struct cpufreq_policy *policy,
                                  unsigned int target_freq,
                                  unsigned int relation);

 int __cpufreq_driver_target(struct cpufreq_policy *policy,
                                    unsigned int target_freq,
                                    unsigned int relation);

 target_freq must be within policy->min and policy->max, of course.
 What's the difference between these two functions? When your governor is
 in a direct code path of a call to governor callbacks, like
 governor->start(), the policy->rwsem is still held in the cpufreq core,
 and there's no need to lock it again (in fact, this would cause a
 deadlock). So use __cpufreq_driver_target only in these cases. In all
 other cases (for example, when there's a "daemonized" function that
 wakes up every second), use cpufreq_driver_target to take policy->rwsem
 before the command is passed to the cpufreq driver.

 4. References
 =============

 [1] Per-entity load tracking: https://lwn.net/Articles/531853/
 [2] Improvements in CPU frequency management: https://lwn.net/Articles/682391/
	CPU frequency and voltage scaling code in the Linux(TM) kernel


	L i n u x C P U F r e q

	C P U F r e q G o v e r n o r s

	- information for users and developers -


	Dominik Brodowski <linux@brodo.de>
	some additions and corrections by Nico Golde <nico@ngolde.de>
	Rafael J. Wysocki <rafael.j.wysocki@intel.com>
	Viresh Kumar <viresh.kumar@linaro.org>



	Clock scaling allows you to change the clock speed of the CPUs on the
	fly. This is a nice method to save battery power, because the lower
	the clock speed, the less power the CPU consumes.


	Contents:
	---------
	1. What is a CPUFreq Governor?

	2. Governors In the Linux Kernel
	2.1 Performance
	2.2 Powersave
	2.3 Userspace
	2.4 Ondemand
	2.5 Conservative
	2.6 Schedutil

	3. The Governor Interface in the CPUfreq Core

	4. References


	1. What Is A CPUFreq Governor?
	==============================

	Most cpufreq drivers (except the intel_pstate and longrun) or even most
	cpu frequency scaling algorithms only allow the CPU frequency to be set
	to predefined fixed values. In order to offer dynamic frequency
	scaling, the cpufreq core must be able to tell these drivers of a
	"target frequency". So these specific drivers will be transformed to
	offer a "->target/target_index/fast_switch()" call instead of the
	"->setpolicy()" call. For set_policy drivers, all stays the same,
	though.

	How to decide what frequency within the CPUfreq policy should be used?
	That's done using "cpufreq governors".

	Basically, it's the following flow graph:

	CPU can be set to switch independently \| CPU can only be set
	within specific "limits" \| to specific frequencies

	"CPUfreq policy"
	consists of frequency limits (policy->{min,max})
	and CPUfreq governor to be used
	/ \
	/ \
	/ the cpufreq governor decides
	/ (dynamically or statically)
	/ what target_freq to set within
	/ the limits of policy->{min,max}
	/ \
	/ \
	Using the ->setpolicy call, Using the ->target/target_index/fast_switch call,
	the limits and the the frequency closest
	"policy" is set. to target_freq is set.
	It is assured that it
	is within policy->{min,max}


	2. Governors In the Linux Kernel
	================================

	2.1 Performance
	---------------

	The CPUfreq governor "performance" sets the CPU statically to the
	highest frequency within the borders of scaling_min_freq and
	scaling_max_freq.


	2.2 Powersave
	-------------

	The CPUfreq governor "powersave" sets the CPU statically to the
	lowest frequency within the borders of scaling_min_freq and
	scaling_max_freq.


	2.3 Userspace
	-------------

	The CPUfreq governor "userspace" allows the user, or any userspace
	program running with UID "root", to set the CPU to a specific frequency
	by making a sysfs file "scaling_setspeed" available in the CPU-device
	directory.


	2.4 Ondemand
	------------

	The CPUfreq governor "ondemand" sets the CPU frequency depending on the
	current system load. Load estimation is triggered by the scheduler
	through the update_util_data->func hook; when triggered, cpufreq checks
	the CPU-usage statistics over the last period and the governor sets the
	CPU accordingly. The CPU must have the capability to switch the
	frequency very quickly.

	Sysfs files:

	* sampling_rate:

	Measured in uS (10^-6 seconds), this is how often you want the kernel
	to look at the CPU usage and to make decisions on what to do about the
	frequency. Typically this is set to values of around '10000' or more.
	It's default value is (cmp. with users-guide.txt): transition_latency
	* 1000. Be aware that transition latency is in ns and sampling_rate
	is in us, so you get the same sysfs value by default. Sampling rate
	should always get adjusted considering the transition latency to set
	the sampling rate 750 times as high as the transition latency in the
	bash (as said, 1000 is default), do:

	$ echo `$(($(cat cpuinfo_transition_latency) * 750 / 1000)) > ondemand/sampling_rate

	* sampling_rate_min:

	The sampling rate is limited by the HW transition latency:
	transition_latency * 100

	Or by kernel restrictions:
	- If CONFIG_NO_HZ_COMMON is set, the limit is 10ms fixed.
	- If CONFIG_NO_HZ_COMMON is not set or nohz=off boot parameter is
	used, the limits depend on the CONFIG_HZ option:
	HZ=1000: min=20000us (20ms)
	HZ=250: min=80000us (80ms)
	HZ=100: min=200000us (200ms)

	The highest value of kernel and HW latency restrictions is shown and
	used as the minimum sampling rate.

	* up_threshold:

	This defines what the average CPU usage between the samplings of
	'sampling_rate' needs to be for the kernel to make a decision on
	whether it should increase the frequency. For example when it is set
	to its default value of '95' it means that between the checking
	intervals the CPU needs to be on average more than 95% in use to then
	decide that the CPU frequency needs to be increased.

	* ignore_nice_load:

	This parameter takes a value of '0' or '1'. When set to '0' (its
	default), all processes are counted towards the 'cpu utilisation'
	value. When set to '1', the processes that are run with a 'nice'
	value will not count (and thus be ignored) in the overall usage
	calculation. This is useful if you are running a CPU intensive
	calculation on your laptop that you do not care how long it takes to
	complete as you can 'nice' it and prevent it from taking part in the
	deciding process of whether to increase your CPU frequency.

	* sampling_down_factor:

	This parameter controls the rate at which the kernel makes a decision
	on when to decrease the frequency while running at top speed. When set
	to 1 (the default) decisions to reevaluate load are made at the same
	interval regardless of current clock speed. But when set to greater
	than 1 (e.g. 100) it acts as a multiplier for the scheduling interval
	for reevaluating load when the CPU is at its top speed due to high
	load. This improves performance by reducing the overhead of load
	evaluation and helping the CPU stay at its top speed when truly busy,
	rather than shifting back and forth in speed. This tunable has no
	effect on behavior at lower speeds/lower CPU loads.

	* powersave_bias:

	This parameter takes a value between 0 to 1000. It defines the
	percentage (times 10) value of the target frequency that will be
	shaved off of the target. For example, when set to 100 -- 10%, when
	ondemand governor would have targeted 1000 MHz, it will target
	1000 MHz - (10% of 1000 MHz) = 900 MHz instead. This is set to 0
	(disabled) by default.

	When AMD frequency sensitivity powersave bias driver --
	drivers/cpufreq/amd_freq_sensitivity.c is loaded, this parameter
	defines the workload frequency sensitivity threshold in which a lower
	frequency is chosen instead of ondemand governor's original target.
	The frequency sensitivity is a hardware reported (on AMD Family 16h
	Processors and above) value between 0 to 100% that tells software how
	the performance of the workload running on a CPU will change when
	frequency changes. A workload with sensitivity of 0% (memory/IO-bound)
	will not perform any better on higher core frequency, whereas a
	workload with sensitivity of 100% (CPU-bound) will perform better
	higher the frequency. When the driver is loaded, this is set to 400 by
	default -- for CPUs running workloads with sensitivity value below
	40%, a lower frequency is chosen. Unloading the driver or writing 0
	will disable this feature.


	2.5 Conservative
	----------------

	The CPUfreq governor "conservative", much like the "ondemand"
	governor, sets the CPU frequency depending on the current usage. It
	differs in behaviour in that it gracefully increases and decreases the
	CPU speed rather than jumping to max speed the moment there is any load
	on the CPU. This behaviour is more suitable in a battery powered
	environment. The governor is tweaked in the same manner as the
	"ondemand" governor through sysfs with the addition of:

	* freq_step:

	This describes what percentage steps the cpu freq should be increased
	and decreased smoothly by. By default the cpu frequency will increase
	in 5% chunks of your maximum cpu frequency. You can change this value
	to anywhere between 0 and 100 where '0' will effectively lock your CPU
	at a speed regardless of its load whilst '100' will, in theory, make
	it behave identically to the "ondemand" governor.

	* down_threshold:

	Same as the 'up_threshold' found for the "ondemand" governor but for
	the opposite direction. For example when set to its default value of
	'20' it means that if the CPU usage needs to be below 20% between
	samples to have the frequency decreased.

	* sampling_down_factor:

	Similar functionality as in "ondemand" governor. But in
	"conservative", it controls the rate at which the kernel makes a
	decision on when to decrease the frequency while running in any speed.
	Load for frequency increase is still evaluated every sampling rate.


	2.6 Schedutil
	-------------

	The "schedutil" governor aims at better integration with the Linux
	kernel scheduler. Load estimation is achieved through the scheduler's
	Per-Entity Load Tracking (PELT) mechanism, which also provides
	information about the recent load [1]. This governor currently does
	load based DVFS only for tasks managed by CFS. RT and DL scheduler tasks
	are always run at the highest frequency. Unlike all the other
	governors, the code is located under the kernel/sched/ directory.

	Sysfs files:

	* rate_limit_us:

	This contains a value in microseconds. The governor waits for
	rate_limit_us time before reevaluating the load again, after it has
	evaluated the load once.

	For an in-depth comparison with the other governors refer to [2].


	3. The Governor Interface in the CPUfreq Core
	=============================================

	A new governor must register itself with the CPUfreq core using
	"cpufreq_register_governor". The struct cpufreq_governor, which has to
	be passed to that function, must contain the following values:

	governor->name - A unique name for this governor.
	governor->owner - .THIS_MODULE for the governor module (if appropriate).

	plus a set of hooks to the functions implementing the governor's logic.

	The CPUfreq governor may call the CPU processor driver using one of
	these two functions:

	int cpufreq_driver_target(struct cpufreq_policy *policy,
	unsigned int target_freq,
	unsigned int relation);

	int __cpufreq_driver_target(struct cpufreq_policy *policy,
	unsigned int target_freq,
	unsigned int relation);

	target_freq must be within policy->min and policy->max, of course.
	What's the difference between these two functions? When your governor is
	in a direct code path of a call to governor callbacks, like
	governor->start(), the policy->rwsem is still held in the cpufreq core,
	and there's no need to lock it again (in fact, this would cause a
	deadlock). So use __cpufreq_driver_target only in these cases. In all
	other cases (for example, when there's a "daemonized" function that
	wakes up every second), use cpufreq_driver_target to take policy->rwsem
	before the command is passed to the cpufreq driver.

	4. References
	=============

	[1] Per-entity load tracking: https://lwn.net/Articles/531853/
	[2] Improvements in CPU frequency management: https://lwn.net/Articles/682391/