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Documentation / cpu-freq / boost.txt

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Based on kernel version 4.10.8. Page generated on 2017-04-01 14:42 EST.

1	Processor boosting control
3		- information for users -
5	Quick guide for the impatient:
6	--------------------
7	/sys/devices/system/cpu/cpufreq/boost
8	controls the boost setting for the whole system. You can read and write
9	that file with either "0" (boosting disabled) or "1" (boosting allowed).
10	Reading or writing 1 does not mean that the system is boosting at this
11	very moment, but only that the CPU _may_ raise the frequency at it's
12	discretion.
13	--------------------
15	Introduction
16	-------------
17	Some CPUs support a functionality to raise the operating frequency of
18	some cores in a multi-core package if certain conditions apply, mostly
19	if the whole chip is not fully utilized and below it's intended thermal
20	budget. The decision about boost disable/enable is made either at hardware
21	(e.g. x86) or software (e.g ARM).
22	On Intel CPUs this is called "Turbo Boost", AMD calls it "Turbo-Core",
23	in technical documentation "Core performance boost". In Linux we use
24	the term "boost" for convenience.
26	Rationale for disable switch
27	----------------------------
29	Though the idea is to just give better performance without any user
30	intervention, sometimes the need arises to disable this functionality.
31	Most systems offer a switch in the (BIOS) firmware to disable the
32	functionality at all, but a more fine-grained and dynamic control would
33	be desirable:
34	1. While running benchmarks, reproducible results are important. Since
35	   the boosting functionality depends on the load of the whole package,
36	   single thread performance can vary. By explicitly disabling the boost
37	   functionality at least for the benchmark's run-time the system will run
38	   at a fixed frequency and results are reproducible again.
39	2. To examine the impact of the boosting functionality it is helpful
40	   to do tests with and without boosting.
41	3. Boosting means overclocking the processor, though under controlled
42	   conditions. By raising the frequency and the voltage the processor
43	   will consume more power than without the boosting, which may be
44	   undesirable for instance for mobile users. Disabling boosting may
45	   save power here, though this depends on the workload.
48	User controlled switch
49	----------------------
51	To allow the user to toggle the boosting functionality, the cpufreq core
52	driver exports a sysfs knob to enable or disable it. There is a file:
53	/sys/devices/system/cpu/cpufreq/boost
54	which can either read "0" (boosting disabled) or "1" (boosting enabled).
55	The file is exported only when cpufreq driver supports boosting.
56	Explicitly changing the permissions and writing to that file anyway will
57	return EINVAL.
59	On supported CPUs one can write either a "0" or a "1" into this file.
60	This will either disable the boost functionality on all cores in the
61	whole system (0) or will allow the software or hardware to boost at will
62	(1).
64	Writing a "1" does not explicitly boost the system, but just allows the
65	CPU to boost at their discretion. Some implementations take external
66	factors like the chip's temperature into account, so boosting once does
67	not necessarily mean that it will occur every time even using the exact
68	same software setup.
71	AMD legacy cpb switch
72	---------------------
73	The AMD powernow-k8 driver used to support a very similar switch to
74	disable or enable the "Core Performance Boost" feature of some AMD CPUs.
75	This switch was instantiated in each CPU's cpufreq directory
76	(/sys/devices/system/cpu[0-9]*/cpufreq) and was called "cpb".
77	Though the per CPU existence hints at a more fine grained control, the
78	actual implementation only supported a system-global switch semantics,
79	which was simply reflected into each CPU's file. Writing a 0 or 1 into it
80	would pull the other CPUs to the same state.
81	For compatibility reasons this file and its behavior is still supported
82	on AMD CPUs, though it is now protected by a config switch
83	(X86_ACPI_CPUFREQ_CPB). On Intel CPUs this file will never be created,
84	even with the config option set.
85	This functionality is considered legacy and will be removed in some future
86	kernel version.
88	More fine grained boosting control
89	----------------------------------
91	Technically it is possible to switch the boosting functionality at least
92	on a per package basis, for some CPUs even per core. Currently the driver
93	does not support it, but this may be implemented in the future.
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