[people/ms/linux.git] / Documentation / cpu-freq / boost.txt

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.
Commit	Line	Data
615b7300 AP	1	Processor boosting control
	2
	3	- information for users -
	4
	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	--------------------
	14
	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
0636f0c3 LM	20	budget. The decision about boost disable/enable is made either at hardware
0636f0c3 LM	21	(e.g. x86) or software (e.g ARM).
615b7300 AP	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.
	25
	26	Rationale for disable switch
	27	----------------------------
	28
	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.
	46
	47
	48	User controlled switch
	49	----------------------
	50
0636f0c3 LM	51	To allow the user to toggle the boosting functionality, the cpufreq core
0636f0c3 LM	52	driver exports a sysfs knob to enable or disable it. There is a file:
615b7300 AP	53	/sys/devices/system/cpu/cpufreq/boost
615b7300 AP	54	which can either read "0" (boosting disabled) or "1" (boosting enabled).
0636f0c3 LM	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.
615b7300 AP	58
	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
0636f0c3 LM	61	whole system (0) or will allow the software or hardware to boost at will
0636f0c3 LM	62	(1).
615b7300 AP	63
615b7300 AP	64	Writing a "1" does not explicitly boost the system, but just allows the
0636f0c3 LM	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.
615b7300 AP	69
	70
	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.
	87
	88	More fine grained boosting control
	89	----------------------------------
	90
	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.