x86_energy_perf_policy man page

x86_energy_perf_policy — Manage Energy vs. Performance Policy via x86 Model Specific Registers


x86_energy_perf_policy [ options ] [ scope ] [field  value]
scope: --cpu cpu-list | --pkg pkg-list
cpu-list, pkg-list: # | #,# | #-# | all
field: --all | --epb | --hwp-epp | --hwp-min | --hwp-max | --hwp-desired
other: (--force | --hwp-enable | --turbo-enable)  value)
value: # | default | performance | balance-performance | balance-power | power


x86_energy_perf_policy 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.

Policy in MSR_IA32_ENERGY_PERF_BIAS (EPB) 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, EBP 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.

Scope Options

-c, --cpu 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, --hwp-use-pkg 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).

-p, --pkg 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'

Value Options

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".

performance Set a policy for maximum performance, accepting no performance sacrifice for the benefit of energy efficiency.

balance-performance Set a policy with a high priority on performance, but allowing some performance loss to benefit energy efficiency.

balance-power Set a policy where the performance and power are balanced. This is the default.

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.

Field Options

-a, --all value-string 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.

-B, --epb set EPB per-core or per-package. See value strings in the table above.

-d, --debug debug increases verbosity.  By default x86_energy_perf_policy is silent for updates, and verbose for read-only mode.

-P, --hwp-epp set HWP.EPP per-core or per-package. See value strings in the table above.

-m, --hwp-min request HWP to not go below the specified core/bus ratio. The "default" is the value found in IA32_HWP_CAPABILITIES.min.

-M, --hwp-max request HWP not exceed a the specified core/bus ratio. The "default" is the value found in IA32_HWP_CAPABILITIES.max.

-D, --hwp-desired 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.

-w, --hwp-window 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.

Other Options

-f, --force
writes the specified values without bounds checking.
-U, --hwp-use-pkg
(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.
-H, --hwp-enable
enable HardWare-P-state (HWP) mode.  Once enabled, system RESET is required to disable HWP mode.
-t, --turbo-enable
enable (1) or disable (0) turbo mode.
-v, --version
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


x86_energy_perf_policy runs only as root.



See Also

Intel(R) 64 and IA-32 Architectures Software Developer's Manual


Len Brown