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