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16 different clock frequency and voltage configurations, often referred to as
25 In some situations it is desirable or even necessary to run the program as fast
26 as possible and then there is no reason to use any P-states different from the
28 available). In some other cases, however, it may not be necessary to execute
31 It also may not be physically possible to maintain maximum CPU capacity for too
32 long for thermal or power supply capacity reasons or similar. To cover those
33 cases, there are hardware interfaces allowing CPUs to be switched between
34 different frequency/voltage configurations or (in the ACPI terminology) to be
37 Typically, they are used along with algorithms to estimate the required CPU
38 capacity, so as to decide which P-states to put the CPUs into. Of course, since
39 the utilization of the system generally changes over time, that has to be done
41 to as CPU performance scaling or CPU frequency scaling (because it involves
56 Scaling governors implement algorithms to estimate the required CPU capacity.
60 Scaling drivers talk to the hardware. They provide scaling governors with
62 access platform-specific hardware interfaces to change CPU P-states as requested
69 to use the same performance scaling algorithm implemented in exactly the same
75 feedback registers, as that information is typically specific to the hardware
78 to bypass the governor layer and implement their own performance scaling
86 CPUs. That is, for example, the same register (or set of registers) is used to
87 control the P-state of multiple CPUs at the same time and writing to it affects
95 The ``CPUFreq`` core maintains a pointer to a |struct cpufreq_policy| object for
98 corresponding to them point to the same |struct cpufreq_policy| object.
107 First of all, a scaling driver has to be registered for ``CPUFreq`` to work.
108 It is only possible to register one scaling driver at a time, so the scaling
109 driver is expected to be able to handle all CPUs in the system.
112 CPUs are registered earlier, the driver core invokes the ``CPUFreq`` core to
115 the scaling driver, the ``CPUFreq`` core will be invoked to take note of them
118 In any case, the ``CPUFreq`` core is invoked to take note of any logical CPU it
119 has not seen so far as soon as it is ready to handle that CPU. [Note that the
123 otherwise and the word "processor" is used to refer to the physical part
129 a new policy directory in ``sysfs``, and the policy pointer corresponding to
130 the given CPU is set to the new policy object's address in memory.
133 pointer of the new CPU passed to it as the argument. That callback is expected
134 to initialize the performance scaling hardware interface for the given CPU (or,
136 to, represented by its policy object) and, if the policy object it has been
137 called for is new, to set parameters of the policy, like the minimum and maximum
140 that belong to the same policy (including both online and offline CPUs). That
141 mask is then used by the core to populate the policy pointers for all of the
144 The next major initialization step for a new policy object is to attach a
145 scaling governor to it (to begin with, that is the default scaling governor
147 via ``sysfs``). First, a pointer to the new policy object is passed to the
148 governor's ``->init()`` callback which is expected to initialize all of the
149 data structures necessary to handle the given policy and, possibly, to add
150 a governor ``sysfs`` interface to it. Next, the governor is started by
153 That callback it expected to register per-CPU utilization update callbacks for
154 all of the online CPUs belonging to the given policy with the CPU scheduler.
158 scheduler's perspective). They are expected to carry out computations needed
159 to determine the P-state to use for the given policy going forward and to
160 invoke the scaling driver to make changes to the hardware in accordance with
166 previously, meaning that all of the CPUs belonging to them were offline. The
168 to use the scaling governor previously used with the policy that became
173 need to re-initialize the policy object at all. In that case, it only is
174 necessary to restart the scaling governor so that it can take the new online CPU
180 Consequently, if |intel_pstate| is used, scaling governors are not attached to
182 to register per-CPU utilization update callbacks for each policy. These
184 governors, but in the |intel_pstate| case they both determine the P-state to
191 when the last CPU belonging to the given policy in unregistered.
203 Each ``policyX`` directory is pointed to by ``cpufreq`` symbolic links
206 associated with (or belonging to) the given policy. The ``policyX`` directories
208 attributes (files) to control ``CPUFreq`` behavior for the corresponding policy
213 and what scaling governor is attached to the given policy. Some scaling drivers
214 also add driver-specific attributes to the policy directories in ``sysfs`` to
221 List of online CPUs belonging to this policy (i.e. sharing the hardware
226 If the platform firmware (BIOS) tells the OS to apply an upper limit to
241 Current frequency of the CPUs belonging to this policy as obtained from
244 This is expected to be the frequency the hardware actually runs at.
249 Maximum possible operating frequency the CPUs belonging to this policy
253 Minimum possible operating frequency the CPUs belonging to this policy
257 The time it takes to switch the CPUs belonging to this policy from one
258 P-state to another, in nanoseconds.
260 If unknown or if known to be so high that the scaling driver does not
265 List of all (online and offline) CPUs belonging to this policy.
269 be attached to this policy or (if the |intel_pstate| scaling driver is
271 applied to this policy.
273 [Note that some governors are modular and it may be necessary to load a
274 kernel module for the governor held by it to become available and be
278 Current frequency of all of the CPUs belonging to this policy (in kHz).
283 the CPU is actually running at (due to hardware design and other
286 Some architectures (e.g. ``x86``) may attempt to provide information
295 The scaling governor currently attached to this policy or (if the
297 provided by the driver that is currently applied to this policy.
299 This attribute is read-write and writing to it will cause a new scaling
300 governor to be attached to this policy or a new scaling algorithm
301 provided by the scaling driver to be applied to it (in the
302 |intel_pstate| case), as indicated by the string written to this
307 Maximum frequency the CPUs belonging to this policy are allowed to be
311 integer to it will cause a new limit to be set (it must not be lower
315 Minimum frequency the CPUs belonging to this policy are allowed to be
319 non-negative integer to it will cause a new limit to be set (it must not
324 is attached to the given policy.
327 be written to in order to set a new frequency for the policy.
337 Scaling governors are attached to policy objects and different policy objects
339 may lead to suboptimal results in some cases).
344 Some governors expose ``sysfs`` attributes to control or fine-tune the scaling
345 algorithms implemented by them. Those attributes, referred to as governor
347 scaling driver in use. If the driver requires governor tunables to be
357 When attached to a policy object, this governor causes the highest frequency,
358 within the ``scaling_max_freq`` policy limit, to be requested for that policy.
360 The request is made once at that time the governor for the policy is set to
367 When attached to a policy object, this governor causes the lowest frequency,
368 within the ``scaling_min_freq`` policy limit, to be requested for that policy.
370 The request is made once at that time the governor for the policy is set to
378 to set the CPU frequency for the policy it is attached to by writing to the
388 It runs entirely in scheduler context, although in some cases it may need to
395 RT or deadline scheduling classes, the governor will increase the frequency to
401 CPU frequency to apply is computed in accordance with the formula
410 This governor also employs a mechanism allowing it to temporarily bump up the
413 is passed by the scheduler to the governor callback which causes the frequency
414 to go up to the allowed maximum immediately and then draw back to the value
420 Minimum time (in microseconds) that has to pass between two consecutive
424 The purpose of this tunable is to reduce the scheduler context overhead
439 In order to estimate the current CPU load, it measures the time elapsed between
442 time to the total CPU time is taken as an estimate of the load.
444 If this governor is attached to a policy shared by multiple CPUs, the load is
448 The worker routine of this governor has to run in process context, so it is
451 governor is minimum, but it causes additional CPU context switches to happen
457 It generally selects CPU frequencies proportional to the estimated load, so that
458 the value of the ``cpuinfo_max_freq`` policy attribute corresponds to the load of
460 corresponds to the load of 0, unless when the load exceeds a (configurable)
462 it is allowed to use (the ``scaling_max_freq`` policy limit).
470 Typically, it is set to values of the order of 10000 (10 ms). Its
471 default value is equal to the value of ``cpuinfo_transition_latency``
472 for each policy this governor is attached to (but since the unit here
478 represented by it to be 750 times as high as the transition latency::
484 will set the frequency to the maximum value allowed for the policy.
485 Otherwise, the selected frequency will be proportional to the estimated
489 If set to 1 (default 0), it will cause the CPU load estimation code to
494 taken into account when deciding what frequency to run the CPUs at.
495 Then, to make that happen it is sufficient to increase the "nice" level
496 of those tasks above 0 and set this attribute to 1.
499 Temporary multiplier, between 1 (default) and 100 inclusive, to apply to
503 setting the frequency to the allowed maximum) to be delayed, so the
511 Reduction factor to apply to the original frequency target of the
519 the effective frequency to apply is given by
530 On Family 16h (and later) AMD processors there is a mechanism to get a
532 hardware. That value can be used to estimate how the performance of the
533 workload running on a CPU will change in response to frequency changes.
536 IO-bound) is not expected to increase at all as a result of increasing
538 (CPU-bound) are expected to perform much better if the CPU frequency is
543 will cause the governor to select a frequency lower than its original
544 target, so as to avoid over-provisioning workloads that will not benefit
557 battery-powered). To achieve that, it changes the frequency in relatively
565 allowed to set (the ``scaling_max_freq`` policy limit), between 0 and
568 This is how much the frequency is allowed to change in one go. Setting
569 it to 0 will cause the default frequency step (5 percent) to be used
570 and setting it to 100 effectively causes the governor to periodically
575 Threshold value (in percent, 20 by default) used to determine the
587 It effectively causes the frequency to go down ``sampling_down_factor``
597 Some processors support a mechanism to raise the operating frequency of some
602 Different names are used by different vendors to refer to this functionality.
603 For Intel processors it is referred to as "Turbo Boost", AMD calls it
606 term "frequency boost" is used here for brevity to refer to all of those
610 If it is hardware-based (e.g. on x86), the decision to trigger the boosting is
611 made by the hardware (although in general it requires the hardware to be put
614 whether or not to trigger boosting and when to do that.
626 means that either the hardware can be put into states in which it is able to
627 trigger boosting (in the hardware-based case), or the software is allowed to
630 permission to use the frequency boost mechanism (which still may never be used
636 The only values that can be written to this file are 0 and 1.
641 The frequency boost mechanism is generally intended to help to achieve optimum
644 it may lead to problems in certain situations.
646 For this reason, many systems make it possible to disable the frequency boost
647 mechanism in the platform firmware (BIOS) setup, but that requires the system to
648 be restarted for the setting to be adjusted as desired, which may not be
654 That may not be desirable on systems that switch to power sources of
655 limited capacity, such as batteries, so the ability to disable the boost
660 performance or energy consumption (or both) and the ability to disable
663 3. To examine the impact of the frequency boost mechanism itself, it is useful
664 to be able to run tests with and without boosting, preferably without
669 single-thread performance may vary because of it which may lead to
671 frequency boost mechanism before running benchmarks sensitive to that
677 The AMD powernow-k8 scaling driver supports a ``sysfs`` knob very similar to
685 for one policy causes the same value of it to be set for all of the other