X86_ENERGY_PERF_POLICY(8) System Manager's Manual X86_ENERGY_PERF_POLICY(8)
NAME
x86_energy_perf_policy - Manage Energy vs. Performance Policy via x86
Model Specific Registers
SYNOPSIS
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-de-
sired
other: (--force | --hwp-enable | --turbo-enable) value)
value: # | default | performance | balance-performance | balance-power
| power
DESCRIPTION
x86_energy_perf_policy displays and updates energy-performance policy
settings specific to Intel Architecture Processors. Settings are ac-
cessed 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 im-
plement those selections. Further, it allows the OS to influence en-
ergy/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 in-
clude the entire range of available P-states. On Broadwell Xeon, the
initial HWP implementation, EPB influenced HWP. That influence was re-
moved in subsequent generations, where it was moved to the Energy_Per-
formance_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_RE-
QUEST 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_RE-
QUEST, 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 writ-
ten.
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_RE-
QUEST_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 rea-
sonable default for most desktops and servers. "default" is a synonym
for "normal".
performance Set a policy for maximum performance, accepting no perfor-
mance 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 bal-
anced. This is the default.
power Set a policy where the processor can accept a measurable perfor-
mance 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 signi-
fies 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_pol-
icy 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" set-
ting 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 win-
dow 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, indi-
cates 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 capabili-
ties.
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 in-
ternally -- 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
/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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