admin-guide/pm/cpufreq.rst

28 In some situations it is desirable or even necessary to run the program as fast
31 available).  In some other cases, however, it may not be necessary to execute
33 relatively long time without utilizing it entirely may be regarded as wasteful.
34 It also may not be physically possible to maintain maximum CPU capacity for too
44 to as CPU performance scaling or CPU frequency scaling (because it involves
56 interfaces for all platforms that support CPU performance scaling.  It defines
71 platform-independent form in the majority of cases, so it should be possible
79 interface it comes from and may not be easily represented in an abstract,
90 control the P-state of multiple CPUs at the same time and writing to it affects
111 It is only possible to register one scaling driver at a time, so the scaling
121 In any case, the ``CPUFreq`` core is invoked to take note of any logical CPU it
122 has not seen so far as soon as it is ready to handle that CPU.  [Note that the
130 for the given CPU and if so, it skips the policy object creation.  Otherwise,
136 pointer of the new CPU passed to it as the argument.  That callback is expected
138 more precisely, for the set of CPUs sharing the hardware interface it belongs
139 to, represented by its policy object) and, if the policy object it has been
145 CPUs in it.
148 scaling governor to it (to begin with, that is the default scaling governor
149 determined by the kernel command line or configuration, but it may be changed
153 a governor ``sysfs`` interface to it.  Next, the governor is started by
175 other CPUs sharing the policy object with it are online already, there is no
176 need to re-initialize the policy object at all.  In that case, it only is
177 necessary to restart the scaling governor so that it can take the new online CPU
193 (which happens when the kernel module containing it is unloaded, for example) or
241 	support it.
260 	The time it takes to switch the CPUs belonging to this policy from one
276 	[Note that some governors are modular and it may be necessary to load a
277 	kernel module for the governor held by it to become available and be
285 	interface provided by it, which may or may not reflect the frequency
302 	This attribute is read-write and writing to it will cause a new scaling
304 	provided by the scaling driver to be applied to it (in the
314 	integer to it will cause a new limit to be set (it must not be lower
322 	non-negative integer to it will cause a new limit to be set (it must not
329 	It returns the last frequency requested by the governor (in kHz) or can
380 This governor does not do anything by itself.  Instead, it allows user space
381 to set the CPU frequency for the policy it is attached to by writing to the
387 This governor uses CPU utilization data available from the CPU scheduler.  It
388 generally is regarded as a part of the CPU scheduler, so it can access the
391 It runs entirely in scheduler context, although in some cases it may need to
392 invoke the scaling driver asynchronously when it decides that the CPU frequency
397 invoking its utilization update callback for that CPU.  If it is invoked by the
400 if it is invoked by the CFS scheduling class, the governor will use the
413 This governor also employs a mechanism allowing it to temporarily bump up the
428 	of the governor which might be excessive without it.
431 and `conservative`_ governors (described below), as it is simpler and more
433 switches and similar is less significant, and it uses the scheduler's own CPU
442 In order to estimate the current CPU load, it measures the time elapsed between
451 The worker routine of this governor has to run in process context, so it is
454 governor is minimum, but it causes additional CPU context switches to happen
455 relatively often and the CPU P-state updates triggered by it can be relatively
456 irregular.  Also, it affects its own CPU load metric by running code that
458 slightly by it).
460 It generally selects CPU frequencies proportional to the estimated load, so that
464 speedup threshold, in which case it will go straight for the highest frequency
465 it is allowed to use (the ``scaling_max_freq`` policy limit).
473 	Typically, it is set to values of the order of 10000 (10 ms).  Its
481 	represented by it to be 750 times as high as the transition latency::
492 	If set to 1 (default 0), it will cause the CPU load estimation code to
498 	Then, to make that happen it is sufficient to increase the "nice" level
530 	value of this attribute is 400 by default and it is used in a different
555 It estimates the CPU load in the same way as the `ondemand`_ governor described
556 above, but the CPU frequency selection algorithm implemented by it is different.
558 Namely, it avoids changing the frequency significantly over short time intervals
560 battery-powered).  To achieve that, it changes the frequency in relatively
572 	it to 0 will cause the default frequency step (5 percent) to be used
573 	and setting it to 100 effectively causes the governor to periodically
590 	It effectively causes the frequency to go down ``sampling_down_factor``
591 	times slower than it ramps up.
606 For Intel processors it is referred to as "Turbo Boost", AMD calls it
608 As a rule, it also is implemented differently by different vendors.  The simple
613 If it is hardware-based (e.g. on x86), the decision to trigger the boosting is
614 made by the hardware (although in general it requires the hardware to be put
615 into a special state in which it can control the CPU frequency within certain
616 limits).  If it is software-based (e.g. on ARM), the scaling driver decides
623 the "boost" setting for the whole system.  It is not present if the underlying
624 scaling driver does not support the frequency boost mechanism (or supports it,
625 but provides a driver-specific interface for controlling it, like
629 means that either the hardware can be put into states in which it is able to
631 trigger boosting (in the software-based case).  It does not mean that boosting
632 is actually in use at the moment on any CPUs in the system.  It only means a
646 scheduler tick interval) and it is demonstrably suitable for many workloads, but
647 it may lead to problems in certain situations.
649 For this reason, many systems make it possible to disable the frequency boost
666   3. To examine the impact of the frequency boost mechanism itself, it is useful
672      single-thread performance may vary because of it which may lead to
681 the global ``boost`` one.  It is used for disabling/enabling the "Core
688 for one policy causes the same value of it to be set for all of the other
692 hardware feature, but it may be configured out of the kernel (via the
694 ``boost`` knob is present regardless.  Thus it is always possible use the