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.\" This page Copyright (C) 2010 - 2015 Len Brown <>
.\" Distributed under the GPL, Copyleft 1994.
x86_energy_perf_policy \- Manage Energy vs. Performance Policy via x86 Model Specific Registers
.B x86_energy_perf_policy
.RB "[ options ] [ scope ] [field \ value]"
.RB "scope: \-\-cpu\ cpu-list | \-\-pkg\ pkg-list"
.RB "cpu-list, pkg-list: # | #,# | #-# | all"
.RB "field: \-\-all | \-\-epb | \-\-hwp-epp | \-\-hwp-min | \-\-hwp-max | \-\-hwp-desired"
.RB "other: (\-\-force | \-\-hwp-enable | \-\-turbo-enable) value)"
.RB "value: # | default | performance | balance-performance | balance-power | power"
displays and updates energy-performance policy settings specific to
Intel Architecture Processors. Settings are accessed via Model Specific Register (MSR)
updates, no matter if the Linux cpufreq sub-system is enabled or not.
may affect a wide range of hardware decisions,
such as how aggressively the hardware enters and exits CPU idle states (C-states)
and Processor Performance States (P-states).
This policy hint does not replace explicit OS C-state and P-state selection.
Rather, it tells the hardware how aggressively to implement those selections.
Further, it allows the OS to influence energy/performance trade-offs where there
is no software interface, such as in the opportunistic "turbo-mode" P-state range.
Note that MSR_IA32_ENERGY_PERF_BIAS is defined per CPU,
but some implementations
share a single MSR among all CPUs in each processor package.
On those systems, a write to EPB on one processor will
be visible, and will have an effect, on all CPUs
in the same processor package.
Hardware P-States (HWP) are effectively an expansion of hardware
P-state control from the opportunistic turbo-mode P-state range
to include the entire range of available P-states.
On Broadwell Xeon, the initial HWP implementation, EPB influenced HWP.
That influence was removed in subsequent generations,
where it was moved to the
Energy_Performance_Preference (EPP) field in
a pair of dedicated MSRs -- MSR_IA32_HWP_REQUEST and MSR_IA32_HWP_REQUEST_PKG.
EPP is the most commonly managed knob in HWP mode,
but MSR_IA32_HWP_REQUEST also allows the user to specify
minimum-frequency for Quality-of-Service,
and maximum-frequency for power-capping.
MSR_IA32_HWP_REQUEST is defined per-CPU.
MSR_IA32_HWP_REQUEST_PKG has the same capability as MSR_IA32_HWP_REQUEST,
but it can simultaneously set the default policy for all CPUs within a package.
A bit in per-CPU MSR_IA32_HWP_REQUEST indicates whether it is
over-ruled-by or exempt-from MSR_IA32_HWP_REQUEST_PKG.
MSR_HWP_CAPABILITIES shows the default values for the fields
in MSR_IA32_HWP_REQUEST. It is displayed when no values
are being written.
\fB-c, --cpu\fP Operate on the MSR_IA32_HWP_REQUEST for each CPU in a CPU-list.
The CPU-list may be comma-separated CPU numbers, with dash for range
or the string "all". Eg. '--cpu 1,4,6-8' or '--cpu all'.
When --cpu is used, \fB--hwp-use-pkg\fP is available, which specifies whether the per-cpu
MSR_IA32_HWP_REQUEST should be over-ruled by MSR_IA32_HWP_REQUEST_PKG (1),
or exempt from MSR_IA32_HWP_REQUEST_PKG (0).
\fB-p, --pkg\fP Operate on the MSR_IA32_HWP_REQUEST_PKG for each package in the package-list.
The list is a string of individual package numbers separated
by commas, and or ranges of package numbers separated by a dash,
or the string "all".
For example '--pkg 1,3' or '--pkg all'
.I normal | default
Set a policy with a normal balance between performance and energy efficiency.
The processor will tolerate minor performance compromise
for potentially significant energy savings.
This is a reasonable default for most desktops and servers.
"default" is a synonym for "normal".
.I performance
Set a policy for maximum performance,
accepting no performance sacrifice for the benefit of energy efficiency.
.I balance-performance
Set a policy with a high priority on performance,
but allowing some performance loss to benefit energy efficiency.
.I balance-power
Set a policy where the performance and power are balanced.
This is the default.
.I power
Set a policy where the processor can accept
a measurable performance impact to maximize energy efficiency.
The following table shows the mapping from the value strings above to actual MSR values.
This mapping is defined in the Linux-kernel header, msr-index.h.
performance 0 0
balance-performance 4 128
normal, default 6 128
balance-power 8 192
power 15 255
For MSR_IA32_HWP_REQUEST performance fields
(--hwp-min, --hwp-max, --hwp-desired), the value option
is in units of 100 MHz, Eg. 12 signifies 1200 MHz.
\fB-a, --all value-string\fP Sets all EPB and EPP and HWP limit fields to the value associated with
the value-string. In addition, enables turbo-mode and HWP-mode, if they were previous disabled.
Thus "--all normal" will set a system without cpufreq into a well known configuration.
\fB-B, --epb\fP set EPB per-core or per-package.
See value strings in the table above.
\fB-d, --debug\fP debug increases verbosity. By default
x86_energy_perf_policy is silent for updates,
and verbose for read-only mode.
\fB-P, --hwp-epp\fP set HWP.EPP per-core or per-package.
See value strings in the table above.
\fB-m, --hwp-min\fP request HWP to not go below the specified core/bus ratio.
The "default" is the value found in IA32_HWP_CAPABILITIES.min.
\fB-M, --hwp-max\fP request HWP not exceed a the specified core/bus ratio.
The "default" is the value found in IA32_HWP_CAPABILITIES.max.
\fB-D, --hwp-desired\fP request HWP 'desired' frequency.
The "normal" setting is 0, which
corresponds to 'full autonomous' HWP control.
Non-zero performance values request a specific performance
level on this processor, specified in multiples of 100 MHz.
\fB-w, --hwp-window\fP specify integer number of microsec
in the sliding window that HWP uses to maintain average frequency.
This parameter is meaningful only when the "desired" field above is non-zero.
Default is 0, allowing the HW to choose.
\fB-f, --force\fP writes the specified values without bounds checking.
\fB-U, --hwp-use-pkg\fP (0 | 1), when used in conjunction with --cpu,
indicates whether the per-CPU MSR_IA32_HWP_REQUEST should be overruled (1)
or exempt (0) from per-Package MSR_IA32_HWP_REQUEST_PKG settings.
The default is exempt.
\fB-H, --hwp-enable\fP enable HardWare-P-state (HWP) mode. Once enabled, system RESET is required to disable HWP mode.
\fB-t, --turbo-enable\fP enable (1) or disable (0) turbo mode.
\fB-v, --version\fP print version and exit.
If no request to change policy is made,
the default behavior is to read
and display the current system state,
including the default capabilities.
This utility writes directly to Model Specific Registers.
There is no locking or coordination should this utility
be used to modify HWP limit fields at the same time that
intel_pstate's sysfs attributes access the same MSRs.
Note that --hwp-desired and --hwp-window are considered experimental.
Future versions of Linux reserve the right to access these
fields internally -- potentially conflicting with user-space access.
# sudo x86_energy_perf_policy
cpu0: EPB 6
cpu0: HWP_REQ: min 6 max 35 des 0 epp 128 window 0x0 (0*10^0us) use_pkg 0
cpu0: HWP_CAP: low 1 eff 8 guar 27 high 35
cpu1: EPB 6
cpu1: HWP_REQ: min 6 max 35 des 0 epp 128 window 0x0 (0*10^0us) use_pkg 0
cpu1: HWP_CAP: low 1 eff 8 guar 27 high 35
cpu2: EPB 6
cpu2: HWP_REQ: min 6 max 35 des 0 epp 128 window 0x0 (0*10^0us) use_pkg 0
cpu2: HWP_CAP: low 1 eff 8 guar 27 high 35
cpu3: EPB 6
cpu3: HWP_REQ: min 6 max 35 des 0 epp 128 window 0x0 (0*10^0us) use_pkg 0
cpu3: HWP_CAP: low 1 eff 8 guar 27 high 35
.B "x86_energy_perf_policy"
runs only as root.
Intel(R) 64 and IA-32 Architectures Software Developer's Manual
Len Brown