1 .. SPDX-License-Identifier: GPL-2.0
2 .. include:: <isonum.txt>
4 .. |intel_pstate| replace:: :doc:`intel_pstate <intel_pstate>`
6 =======================
7 CPU Performance Scaling
8 =======================
10 :Copyright: |copy| 2017 Intel Corporation
12 :Author: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
15 The Concept of CPU Performance Scaling
16 ======================================
18 The majority of modern processors are capable of operating in a number of
19 different clock frequency and voltage configurations, often referred to as
20 Operating Performance Points or P-states (in ACPI terminology). As a rule,
21 the higher the clock frequency and the higher the voltage, the more instructions
22 can be retired by the CPU over a unit of time, but also the higher the clock
23 frequency and the higher the voltage, the more energy is consumed over a unit of
24 time (or the more power is drawn) by the CPU in the given P-state. Therefore
25 there is a natural tradeoff between the CPU capacity (the number of instructions
26 that can be executed over a unit of time) and the power drawn by the CPU.
28 In some situations it is desirable or even necessary to run the program as fast
29 as possible and then there is no reason to use any P-states different from the
30 highest one (i.e. the highest-performance frequency/voltage configuration
31 available). In some other cases, however, it may not be necessary to execute
32 instructions so quickly and maintaining the highest available CPU capacity for a
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
35 long for thermal or power supply capacity reasons or similar. To cover those
36 cases, there are hardware interfaces allowing CPUs to be switched between
37 different frequency/voltage configurations or (in the ACPI terminology) to be
38 put into different P-states.
40 Typically, they are used along with algorithms to estimate the required CPU
41 capacity, so as to decide which P-states to put the CPUs into. Of course, since
42 the utilization of the system generally changes over time, that has to be done
43 repeatedly on a regular basis. The activity by which this happens is referred
44 to as CPU performance scaling or CPU frequency scaling (because it involves
45 adjusting the CPU clock frequency).
48 CPU Performance Scaling in Linux
49 ================================
51 The Linux kernel supports CPU performance scaling by means of the ``CPUFreq``
52 (CPU Frequency scaling) subsystem that consists of three layers of code: the
53 core, scaling governors and scaling drivers.
55 The ``CPUFreq`` core provides the common code infrastructure and user space
56 interfaces for all platforms that support CPU performance scaling. It defines
57 the basic framework in which the other components operate.
59 Scaling governors implement algorithms to estimate the required CPU capacity.
60 As a rule, each governor implements one, possibly parametrized, scaling
63 Scaling drivers talk to the hardware. They provide scaling governors with
64 information on the available P-states (or P-state ranges in some cases) and
65 access platform-specific hardware interfaces to change CPU P-states as requested
68 In principle, all available scaling governors can be used with every scaling
69 driver. That design is based on the observation that the information used by
70 performance scaling algorithms for P-state selection can be represented in a
71 platform-independent form in the majority of cases, so it should be possible
72 to use the same performance scaling algorithm implemented in exactly the same
73 way regardless of which scaling driver is used. Consequently, the same set of
74 scaling governors should be suitable for every supported platform.
76 However, that observation may not hold for performance scaling algorithms
77 based on information provided by the hardware itself, for example through
78 feedback registers, as that information is typically specific to the hardware
79 interface it comes from and may not be easily represented in an abstract,
80 platform-independent way. For this reason, ``CPUFreq`` allows scaling drivers
81 to bypass the governor layer and implement their own performance scaling
82 algorithms. That is done by the |intel_pstate| scaling driver.
85 ``CPUFreq`` Policy Objects
86 ==========================
88 In some cases the hardware interface for P-state control is shared by multiple
89 CPUs. That is, for example, the same register (or set of registers) is used to
90 control the P-state of multiple CPUs at the same time and writing to it affects
91 all of those CPUs simultaneously.
93 Sets of CPUs sharing hardware P-state control interfaces are represented by
94 ``CPUFreq`` as struct cpufreq_policy objects. For consistency,
95 struct cpufreq_policy is also used when there is only one CPU in the given
98 The ``CPUFreq`` core maintains a pointer to a struct cpufreq_policy object for
99 every CPU in the system, including CPUs that are currently offline. If multiple
100 CPUs share the same hardware P-state control interface, all of the pointers
101 corresponding to them point to the same struct cpufreq_policy object.
103 ``CPUFreq`` uses struct cpufreq_policy as its basic data type and the design
104 of its user space interface is based on the policy concept.
110 First of all, a scaling driver has to be registered for ``CPUFreq`` to work.
111 It is only possible to register one scaling driver at a time, so the scaling
112 driver is expected to be able to handle all CPUs in the system.
114 The scaling driver may be registered before or after CPU registration. If
115 CPUs are registered earlier, the driver core invokes the ``CPUFreq`` core to
116 take a note of all of the already registered CPUs during the registration of the
117 scaling driver. In turn, if any CPUs are registered after the registration of
118 the scaling driver, the ``CPUFreq`` core will be invoked to take note of them
119 at their registration time.
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
123 logical CPU may be a physical single-core processor, or a single core in a
124 multicore processor, or a hardware thread in a physical processor or processor
125 core. In what follows "CPU" always means "logical CPU" unless explicitly stated
126 otherwise and the word "processor" is used to refer to the physical part
127 possibly including multiple logical CPUs.]
129 Once invoked, the ``CPUFreq`` core checks if the policy pointer is already set
130 for the given CPU and if so, it skips the policy object creation. Otherwise,
131 a new policy object is created and initialized, which involves the creation of
132 a new policy directory in ``sysfs``, and the policy pointer corresponding to
133 the given CPU is set to the new policy object's address in memory.
135 Next, the scaling driver's ``->init()`` callback is invoked with the policy
136 pointer of the new CPU passed to it as the argument. That callback is expected
137 to initialize the performance scaling hardware interface for the given CPU (or,
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
140 called for is new, to set parameters of the policy, like the minimum and maximum
141 frequencies supported by the hardware, the table of available frequencies (if
142 the set of supported P-states is not a continuous range), and the mask of CPUs
143 that belong to the same policy (including both online and offline CPUs). That
144 mask is then used by the core to populate the policy pointers for all of the
147 The next major initialization step for a new policy object is to attach a
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
150 later via ``sysfs``). First, a pointer to the new policy object is passed to
151 the governor's ``->init()`` callback which is expected to initialize all of the
152 data structures necessary to handle the given policy and, possibly, to add
153 a governor ``sysfs`` interface to it. Next, the governor is started by
154 invoking its ``->start()`` callback.
156 That callback is expected to register per-CPU utilization update callbacks for
157 all of the online CPUs belonging to the given policy with the CPU scheduler.
158 The utilization update callbacks will be invoked by the CPU scheduler on
159 important events, like task enqueue and dequeue, on every iteration of the
160 scheduler tick or generally whenever the CPU utilization may change (from the
161 scheduler's perspective). They are expected to carry out computations needed
162 to determine the P-state to use for the given policy going forward and to
163 invoke the scaling driver to make changes to the hardware in accordance with
164 the P-state selection. The scaling driver may be invoked directly from
165 scheduler context or asynchronously, via a kernel thread or workqueue, depending
166 on the configuration and capabilities of the scaling driver and the governor.
168 Similar steps are taken for policy objects that are not new, but were "inactive"
169 previously, meaning that all of the CPUs belonging to them were offline. The
170 only practical difference in that case is that the ``CPUFreq`` core will attempt
171 to use the scaling governor previously used with the policy that became
172 "inactive" (and is re-initialized now) instead of the default governor.
174 In turn, if a previously offline CPU is being brought back online, but some
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
178 into account. That is achieved by invoking the governor's ``->stop`` and
179 ``->start()`` callbacks, in this order, for the entire policy.
181 As mentioned before, the |intel_pstate| scaling driver bypasses the scaling
182 governor layer of ``CPUFreq`` and provides its own P-state selection algorithms.
183 Consequently, if |intel_pstate| is used, scaling governors are not attached to
184 new policy objects. Instead, the driver's ``->setpolicy()`` callback is invoked
185 to register per-CPU utilization update callbacks for each policy. These
186 callbacks are invoked by the CPU scheduler in the same way as for scaling
187 governors, but in the |intel_pstate| case they both determine the P-state to
188 use and change the hardware configuration accordingly in one go from scheduler
191 The policy objects created during CPU initialization and other data structures
192 associated with them are torn down when the scaling driver is unregistered
193 (which happens when the kernel module containing it is unloaded, for example) or
194 when the last CPU belonging to the given policy in unregistered.
197 Policy Interface in ``sysfs``
198 =============================
200 During the initialization of the kernel, the ``CPUFreq`` core creates a
201 ``sysfs`` directory (kobject) called ``cpufreq`` under
202 :file:`/sys/devices/system/cpu/`.
204 That directory contains a ``policyX`` subdirectory (where ``X`` represents an
205 integer number) for every policy object maintained by the ``CPUFreq`` core.
206 Each ``policyX`` directory is pointed to by ``cpufreq`` symbolic links
207 under :file:`/sys/devices/system/cpu/cpuY/` (where ``Y`` represents an integer
208 that may be different from the one represented by ``X``) for all of the CPUs
209 associated with (or belonging to) the given policy. The ``policyX`` directories
210 in :file:`/sys/devices/system/cpu/cpufreq` each contain policy-specific
211 attributes (files) to control ``CPUFreq`` behavior for the corresponding policy
212 objects (that is, for all of the CPUs associated with them).
214 Some of those attributes are generic. They are created by the ``CPUFreq`` core
215 and their behavior generally does not depend on what scaling driver is in use
216 and what scaling governor is attached to the given policy. Some scaling drivers
217 also add driver-specific attributes to the policy directories in ``sysfs`` to
218 control policy-specific aspects of driver behavior.
220 The generic attributes under :file:`/sys/devices/system/cpu/cpufreq/policyX/`
224 List of online CPUs belonging to this policy (i.e. sharing the hardware
225 performance scaling interface represented by the ``policyX`` policy
229 If the platform firmware (BIOS) tells the OS to apply an upper limit to
230 CPU frequencies, that limit will be reported through this attribute (if
233 The existence of the limit may be a result of some (often unintentional)
234 BIOS settings, restrictions coming from a service processor or another
235 BIOS/HW-based mechanisms.
237 This does not cover ACPI thermal limitations which can be discovered
238 through a generic thermal driver.
240 This attribute is not present if the scaling driver in use does not
244 Current frequency of the CPUs belonging to this policy as obtained from
245 the hardware (in KHz).
247 This is expected to be the frequency the hardware actually runs at.
248 If that frequency cannot be determined, this attribute should not
252 Maximum possible operating frequency the CPUs belonging to this policy
256 Minimum possible operating frequency the CPUs belonging to this policy
259 ``cpuinfo_transition_latency``
260 The time it takes to switch the CPUs belonging to this policy from one
261 P-state to another, in nanoseconds.
263 If unknown or if known to be so high that the scaling driver does not
264 work with the `ondemand`_ governor, -1 (:c:macro:`CPUFREQ_ETERNAL`)
265 will be returned by reads from this attribute.
268 List of all (online and offline) CPUs belonging to this policy.
270 ``scaling_available_governors``
271 List of ``CPUFreq`` scaling governors present in the kernel that can
272 be attached to this policy or (if the |intel_pstate| scaling driver is
273 in use) list of scaling algorithms provided by the driver that can be
274 applied to this policy.
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
278 listed by this attribute.]
281 Current frequency of all of the CPUs belonging to this policy (in kHz).
283 In the majority of cases, this is the frequency of the last P-state
284 requested by the scaling driver from the hardware using the scaling
285 interface provided by it, which may or may not reflect the frequency
286 the CPU is actually running at (due to hardware design and other
289 Some architectures (e.g. ``x86``) may attempt to provide information
290 more precisely reflecting the current CPU frequency through this
291 attribute, but that still may not be the exact current CPU frequency as
292 seen by the hardware at the moment.
295 The scaling driver currently in use.
298 The scaling governor currently attached to this policy or (if the
299 |intel_pstate| scaling driver is in use) the scaling algorithm
300 provided by the driver that is currently applied to this policy.
302 This attribute is read-write and writing to it will cause a new scaling
303 governor to be attached to this policy or a new scaling algorithm
304 provided by the scaling driver to be applied to it (in the
305 |intel_pstate| case), as indicated by the string written to this
306 attribute (which must be one of the names listed by the
307 ``scaling_available_governors`` attribute described above).
310 Maximum frequency the CPUs belonging to this policy are allowed to be
313 This attribute is read-write and writing a string representing an
314 integer to it will cause a new limit to be set (it must not be lower
315 than the value of the ``scaling_min_freq`` attribute).
318 Minimum frequency the CPUs belonging to this policy are allowed to be
321 This attribute is read-write and writing a string representing a
322 non-negative integer to it will cause a new limit to be set (it must not
323 be higher than the value of the ``scaling_max_freq`` attribute).
326 This attribute is functional only if the `userspace`_ scaling governor
327 is attached to the given policy.
329 It returns the last frequency requested by the governor (in kHz) or can
330 be written to in order to set a new frequency for the policy.
333 Generic Scaling Governors
334 =========================
336 ``CPUFreq`` provides generic scaling governors that can be used with all
337 scaling drivers. As stated before, each of them implements a single, possibly
338 parametrized, performance scaling algorithm.
340 Scaling governors are attached to policy objects and different policy objects
341 can be handled by different scaling governors at the same time (although that
342 may lead to suboptimal results in some cases).
344 The scaling governor for a given policy object can be changed at any time with
345 the help of the ``scaling_governor`` policy attribute in ``sysfs``.
347 Some governors expose ``sysfs`` attributes to control or fine-tune the scaling
348 algorithms implemented by them. Those attributes, referred to as governor
349 tunables, can be either global (system-wide) or per-policy, depending on the
350 scaling driver in use. If the driver requires governor tunables to be
351 per-policy, they are located in a subdirectory of each policy directory.
352 Otherwise, they are located in a subdirectory under
353 :file:`/sys/devices/system/cpu/cpufreq/`. In either case the name of the
354 subdirectory containing the governor tunables is the name of the governor
360 When attached to a policy object, this governor causes the highest frequency,
361 within the ``scaling_max_freq`` policy limit, to be requested for that policy.
363 The request is made once at that time the governor for the policy is set to
364 ``performance`` and whenever the ``scaling_max_freq`` or ``scaling_min_freq``
365 policy limits change after that.
370 When attached to a policy object, this governor causes the lowest frequency,
371 within the ``scaling_min_freq`` policy limit, to be requested for that policy.
373 The request is made once at that time the governor for the policy is set to
374 ``powersave`` and whenever the ``scaling_max_freq`` or ``scaling_min_freq``
375 policy limits change after that.
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
382 ``scaling_setspeed`` attribute of that policy.
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
389 scheduler's internal data structures directly.
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
393 should be changed for a given policy (that depends on whether or not the driver
394 is capable of changing the CPU frequency from scheduler context).
396 The actions of this governor for a particular CPU depend on the scheduling class
397 invoking its utilization update callback for that CPU. If it is invoked by the
398 RT or deadline scheduling classes, the governor will increase the frequency to
399 the allowed maximum (that is, the ``scaling_max_freq`` policy limit). In turn,
400 if it is invoked by the CFS scheduling class, the governor will use the
401 Per-Entity Load Tracking (PELT) metric for the root control group of the
402 given CPU as the CPU utilization estimate (see the *Per-entity load tracking*
403 LWN.net article [1]_ for a description of the PELT mechanism). Then, the new
404 CPU frequency to apply is computed in accordance with the formula
406 f = 1.25 * ``f_0`` * ``util`` / ``max``
408 where ``util`` is the PELT number, ``max`` is the theoretical maximum of
409 ``util``, and ``f_0`` is either the maximum possible CPU frequency for the given
410 policy (if the PELT number is frequency-invariant), or the current CPU frequency
413 This governor also employs a mechanism allowing it to temporarily bump up the
414 CPU frequency for tasks that have been waiting on I/O most recently, called
415 "IO-wait boosting". That happens when the :c:macro:`SCHED_CPUFREQ_IOWAIT` flag
416 is passed by the scheduler to the governor callback which causes the frequency
417 to go up to the allowed maximum immediately and then draw back to the value
418 returned by the above formula over time.
420 This governor exposes only one tunable:
423 Minimum time (in microseconds) that has to pass between two consecutive
424 runs of governor computations (default: 1000 times the scaling driver's
427 The purpose of this tunable is to reduce the scheduler context overhead
428 of the governor which might be excessive without it.
430 This governor generally is regarded as a replacement for the older `ondemand`_
431 and `conservative`_ governors (described below), as it is simpler and more
432 tightly integrated with the CPU scheduler, its overhead in terms of CPU context
433 switches and similar is less significant, and it uses the scheduler's own CPU
434 utilization metric, so in principle its decisions should not contradict the
435 decisions made by the other parts of the scheduler.
440 This governor uses CPU load as a CPU frequency selection metric.
442 In order to estimate the current CPU load, it measures the time elapsed between
443 consecutive invocations of its worker routine and computes the fraction of that
444 time in which the given CPU was not idle. The ratio of the non-idle (active)
445 time to the total CPU time is taken as an estimate of the load.
447 If this governor is attached to a policy shared by multiple CPUs, the load is
448 estimated for all of them and the greatest result is taken as the load estimate
449 for the entire policy.
451 The worker routine of this governor has to run in process context, so it is
452 invoked asynchronously (via a workqueue) and CPU P-states are updated from
453 there if necessary. As a result, the scheduler context overhead from this
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
457 reduces the CPU idle time (even though the CPU idle time is only reduced very
460 It generally selects CPU frequencies proportional to the estimated load, so that
461 the value of the ``cpuinfo_max_freq`` policy attribute corresponds to the load of
462 1 (or 100%), and the value of the ``cpuinfo_min_freq`` policy attribute
463 corresponds to the load of 0, unless when the load exceeds a (configurable)
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).
467 This governor exposes the following tunables:
470 This is how often the governor's worker routine should run, in
473 Typically, it is set to values of the order of 10000 (10 ms). Its
474 default value is equal to the value of ``cpuinfo_transition_latency``
475 for each policy this governor is attached to (but since the unit here
476 is greater by 1000, this means that the time represented by
477 ``sampling_rate`` is 1000 times greater than the transition latency by
480 If this tunable is per-policy, the following shell command sets the time
481 represented by it to be 750 times as high as the transition latency::
483 # echo `$(($(cat cpuinfo_transition_latency) * 750 / 1000)) > ondemand/sampling_rate
486 If the estimated CPU load is above this value (in percent), the governor
487 will set the frequency to the maximum value allowed for the policy.
488 Otherwise, the selected frequency will be proportional to the estimated
492 If set to 1 (default 0), it will cause the CPU load estimation code to
493 treat the CPU time spent on executing tasks with "nice" levels greater
494 than 0 as CPU idle time.
496 This may be useful if there are tasks in the system that should not be
497 taken into account when deciding what frequency to run the CPUs at.
498 Then, to make that happen it is sufficient to increase the "nice" level
499 of those tasks above 0 and set this attribute to 1.
501 ``sampling_down_factor``
502 Temporary multiplier, between 1 (default) and 100 inclusive, to apply to
503 the ``sampling_rate`` value if the CPU load goes above ``up_threshold``.
505 This causes the next execution of the governor's worker routine (after
506 setting the frequency to the allowed maximum) to be delayed, so the
507 frequency stays at the maximum level for a longer time.
509 Frequency fluctuations in some bursty workloads may be avoided this way
510 at the cost of additional energy spent on maintaining the maximum CPU
514 Reduction factor to apply to the original frequency target of the
515 governor (including the maximum value used when the ``up_threshold``
516 value is exceeded by the estimated CPU load) or sensitivity threshold
517 for the AMD frequency sensitivity powersave bias driver
518 (:file:`drivers/cpufreq/amd_freq_sensitivity.c`), between 0 and 1000
521 If the AMD frequency sensitivity powersave bias driver is not loaded,
522 the effective frequency to apply is given by
524 f * (1 - ``powersave_bias`` / 1000)
526 where f is the governor's original frequency target. The default value
527 of this attribute is 0 in that case.
529 If the AMD frequency sensitivity powersave bias driver is loaded, the
530 value of this attribute is 400 by default and it is used in a different
533 On Family 16h (and later) AMD processors there is a mechanism to get a
534 measured workload sensitivity, between 0 and 100% inclusive, from the
535 hardware. That value can be used to estimate how the performance of the
536 workload running on a CPU will change in response to frequency changes.
538 The performance of a workload with the sensitivity of 0 (memory-bound or
539 IO-bound) is not expected to increase at all as a result of increasing
540 the CPU frequency, whereas workloads with the sensitivity of 100%
541 (CPU-bound) are expected to perform much better if the CPU frequency is
544 If the workload sensitivity is less than the threshold represented by
545 the ``powersave_bias`` value, the sensitivity powersave bias driver
546 will cause the governor to select a frequency lower than its original
547 target, so as to avoid over-provisioning workloads that will not benefit
548 from running at higher CPU frequencies.
553 This governor uses CPU load as a CPU frequency selection metric.
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
559 which may not be suitable for systems with limited power supply capacity (e.g.
560 battery-powered). To achieve that, it changes the frequency in relatively
561 small steps, one step at a time, up or down - depending on whether or not a
562 (configurable) threshold has been exceeded by the estimated CPU load.
564 This governor exposes the following tunables:
567 Frequency step in percent of the maximum frequency the governor is
568 allowed to set (the ``scaling_max_freq`` policy limit), between 0 and
571 This is how much the frequency is allowed to change in one go. Setting
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
574 switch the frequency between the ``scaling_min_freq`` and
575 ``scaling_max_freq`` policy limits.
578 Threshold value (in percent, 20 by default) used to determine the
579 frequency change direction.
581 If the estimated CPU load is greater than this value, the frequency will
582 go up (by ``freq_step``). If the load is less than this value (and the
583 ``sampling_down_factor`` mechanism is not in effect), the frequency will
584 go down. Otherwise, the frequency will not be changed.
586 ``sampling_down_factor``
587 Frequency decrease deferral factor, between 1 (default) and 10
590 It effectively causes the frequency to go down ``sampling_down_factor``
591 times slower than it ramps up.
594 Frequency Boost Support
595 =======================
600 Some processors support a mechanism to raise the operating frequency of some
601 cores in a multicore package temporarily (and above the sustainable frequency
602 threshold for the whole package) under certain conditions, for example if the
603 whole chip is not fully utilized and below its intended thermal or power budget.
605 Different names are used by different vendors to refer to this functionality.
606 For Intel processors it is referred to as "Turbo Boost", AMD calls it
607 "Turbo-Core" or (in technical documentation) "Core Performance Boost" and so on.
608 As a rule, it also is implemented differently by different vendors. The simple
609 term "frequency boost" is used here for brevity to refer to all of those
612 The frequency boost mechanism may be either hardware-based or software-based.
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
617 whether or not to trigger boosting and when to do that.
619 The ``boost`` File in ``sysfs``
620 -------------------------------
622 This file is located under :file:`/sys/devices/system/cpu/cpufreq/` and controls
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
628 If the value in this file is 1, the frequency boost mechanism is enabled. This
629 means that either the hardware can be put into states in which it is able to
630 trigger boosting (in the hardware-based case), or the software is allowed 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
633 permission to use the frequency boost mechanism (which still may never be used
636 If the value in this file is 0, the frequency boost mechanism is disabled and
637 cannot be used at all.
639 The only values that can be written to this file are 0 and 1.
641 Rationale for Boost Control Knob
642 --------------------------------
644 The frequency boost mechanism is generally intended to help to achieve optimum
645 CPU performance on time scales below software resolution (e.g. below the
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
650 mechanism in the platform firmware (BIOS) setup, but that requires the system to
651 be restarted for the setting to be adjusted as desired, which may not be
652 practical at least in some cases. For example:
654 1. Boosting means overclocking the processor, although under controlled
655 conditions. Generally, the processor's energy consumption increases
656 as a result of increasing its frequency and voltage, even temporarily.
657 That may not be desirable on systems that switch to power sources of
658 limited capacity, such as batteries, so the ability to disable the boost
659 mechanism while the system is running may help there (but that depends on
662 2. In some situations deterministic behavior is more important than
663 performance or energy consumption (or both) and the ability to disable
664 boosting while the system is running may be useful then.
666 3. To examine the impact of the frequency boost mechanism itself, it is useful
667 to be able to run tests with and without boosting, preferably without
668 restarting the system in the meantime.
670 4. Reproducible results are important when running benchmarks. Since
671 the boosting functionality depends on the load of the whole package,
672 single-thread performance may vary because of it which may lead to
673 unreproducible results sometimes. That can be avoided by disabling the
674 frequency boost mechanism before running benchmarks sensitive to that
677 Legacy AMD ``cpb`` Knob
678 -----------------------
680 The AMD powernow-k8 scaling driver supports a ``sysfs`` knob very similar to
681 the global ``boost`` one. It is used for disabling/enabling the "Core
682 Performance Boost" feature of some AMD processors.
684 If present, that knob is located in every ``CPUFreq`` policy directory in
685 ``sysfs`` (:file:`/sys/devices/system/cpu/cpufreq/policyX/`) and is called
686 ``cpb``, which indicates a more fine grained control interface. The actual
687 implementation, however, works on the system-wide basis and setting that knob
688 for one policy causes the same value of it to be set for all of the other
689 policies at the same time.
691 That knob is still supported on AMD processors that support its underlying
692 hardware feature, but it may be configured out of the kernel (via the
693 :c:macro:`CONFIG_X86_ACPI_CPUFREQ_CPB` configuration option) and the global
694 ``boost`` knob is present regardless. Thus it is always possible use the
695 ``boost`` knob instead of the ``cpb`` one which is highly recommended, as that
696 is more consistent with what all of the other systems do (and the ``cpb`` knob
697 may not be supported any more in the future).
699 The ``cpb`` knob is never present for any processors without the underlying
700 hardware feature (e.g. all Intel ones), even if the
701 :c:macro:`CONFIG_X86_ACPI_CPUFREQ_CPB` configuration option is set.
707 .. [1] Jonathan Corbet, *Per-entity load tracking*,
708 https://lwn.net/Articles/531853/
projects / linux-2.6-microblaze.git / blob