94 lines
4.2 KiB
Plaintext
94 lines
4.2 KiB
Plaintext
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.
|