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-Processor boosting control
-
- - information for users -
-
-Quick guide for the impatient:
---------------------
-/sys/devices/system/cpu/cpufreq/boost
-controls the boost setting for the whole system. You can read and write
-that file with either "0" (boosting disabled) or "1" (boosting allowed).
-Reading or writing 1 does not mean that the system is boosting at this
-very moment, but only that the CPU _may_ raise the frequency at it's
-discretion.
---------------------
-
-Introduction
--------------
-Some CPUs support a functionality to raise the operating frequency of
-some cores in a multi-core package if certain conditions apply, mostly
-if the whole chip is not fully utilized and below it's intended thermal
-budget. The decision about boost disable/enable is made either at hardware
-(e.g. x86) or software (e.g ARM).
-On Intel CPUs this is called "Turbo Boost", AMD calls it "Turbo-Core",
-in technical documentation "Core performance boost". In Linux we use
-the term "boost" for convenience.
-
-Rationale for disable switch
-----------------------------
-
-Though the idea is to just give better performance without any user
-intervention, sometimes the need arises to disable this functionality.
-Most systems offer a switch in the (BIOS) firmware to disable the
-functionality at all, but a more fine-grained and dynamic control would
-be desirable:
-1. While running benchmarks, reproducible results are important. Since
- the boosting functionality depends on the load of the whole package,
- single thread performance can vary. By explicitly disabling the boost
- functionality at least for the benchmark's run-time the system will run
- at a fixed frequency and results are reproducible again.
-2. To examine the impact of the boosting functionality it is helpful
- to do tests with and without boosting.
-3. Boosting means overclocking the processor, though under controlled
- conditions. By raising the frequency and the voltage the processor
- will consume more power than without the boosting, which may be
- undesirable for instance for mobile users. Disabling boosting may
- save power here, though this depends on the workload.
-
-
-User controlled switch
-----------------------
-
-To allow the user to toggle the boosting functionality, the cpufreq core
-driver exports a sysfs knob to enable or disable it. There is a file:
-/sys/devices/system/cpu/cpufreq/boost
-which can either read "0" (boosting disabled) or "1" (boosting enabled).
-The file is exported only when cpufreq driver supports boosting.
-Explicitly changing the permissions and writing to that file anyway will
-return EINVAL.
-
-On supported CPUs one can write either a "0" or a "1" into this file.
-This will either disable the boost functionality on all cores in the
-whole system (0) or will allow the software or hardware to boost at will
-(1).
-
-Writing a "1" does not explicitly boost the system, but just allows the
-CPU to boost at their discretion. Some implementations take external
-factors like the chip's temperature into account, so boosting once does
-not necessarily mean that it will occur every time even using the exact
-same software setup.
-
-
-AMD legacy cpb switch
----------------------
-The AMD powernow-k8 driver used to support a very similar switch to
-disable or enable the "Core Performance Boost" feature of some AMD CPUs.
-This switch was instantiated in each CPU's cpufreq directory
-(/sys/devices/system/cpu[0-9]*/cpufreq) and was called "cpb".
-Though the per CPU existence hints at a more fine grained control, the
-actual implementation only supported a system-global switch semantics,
-which was simply reflected into each CPU's file. Writing a 0 or 1 into it
-would pull the other CPUs to the same state.
-For compatibility reasons this file and its behavior is still supported
-on AMD CPUs, though it is now protected by a config switch
-(X86_ACPI_CPUFREQ_CPB). On Intel CPUs this file will never be created,
-even with the config option set.
-This functionality is considered legacy and will be removed in some future
-kernel version.
-
-More fine grained boosting control
-----------------------------------
-
-Technically it is possible to switch the boosting functionality at least
-on a per package basis, for some CPUs even per core. Currently the driver
-does not support it, but this may be implemented in the future.