Manual: x86_energy_perf_policy

X86_ENERGY_PERF_POLICY(8) System Manager's Manual X86_ENERGY_PERF_POLICY(8)

NAME
x86_energy_perf_policy - Manage Energy vs. Performance Policy

DESCRIPTION
x86_energy_perf_policy displays and updates energy-performance policy settings specific to Intel Architecture Proces‐
sors. It summarizes settings available in standard Linux interfaces (eg. cpufreq), and also decodes underlying Model
Specific Register (MSRs). While x86_energy_perf_policy can manage energy-performance policy using only MSR access,
it prefers standard Linux kernel interfaces, when they are available.

BACKGROUND
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 imple‐
ment those selections. Further, it allows the OS to influence energy/performance trade-offs where there is no soft‐
ware interface, such as in the opportunistic "turbo-mode" P-state range. Note that MSR_IA32_ENERGY_PERF_BIAS is de‐
fined per CPU, but some implementations share a single MSR among all CPUs in each processor package. On those sys‐
tems, 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_Perfor‐
mance_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 in‐
dividual 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 effi‐
ciency.

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.

VALUE STRING EPB EPP
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 multi‐
ples of 100 MHz.

-w, --hwp-window specify integer number of microsec in the sliding window that HWP uses to maintain average fre‐
quency. 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, includ‐
ing the default capabilities.

WARNING
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.

EXAMPLE
# 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

NOTES
x86_energy_perf_policy runs only as root.

FILES
EPB: /sys/devices/system/cpu/cpu*/power/energy_perf_bias
EPP: /sys/devices/system/cpu/cpu*/cpufreq/energy_performance_preference
MSR: /dev/cpu/*/msr

SEE ALSO
msr(4)
Intel(R) 64 and IA-32 Architectures Software Developer's Manual

AUTHORS
Len Brown

X86_ENERGY_PERF_POLICY(8)

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Manual: x86_energy_perf_policy

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