admin-guide/pm/cpufreq.rst

1 .. SPDX-License-Identifier: GPL-2.0
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
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
38 put into different P-states.
41 capacity, so as to decide which P-states to put the CPUs into.  Of course, since
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
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
80 platform-independent way.  For this reason, ``CPUFreq`` allows scaling drivers
88 In some cases the hardware interface for P-state control is shared by multiple
90 control the P-state of multiple CPUs at the same time and writing to it affects
93 Sets of CPUs sharing hardware P-state control interfaces are represented by
100 CPUs share the same hardware P-state control interface, all of the pointers
123 logical CPU may be a physical single-core processor, or a single core in a
135 Next, the scaling driver's ``->init()`` callback is invoked with the policy
142 the set of supported P-states is not a continuous range), and the mask of CPUs
151 the governor's ``->init()`` callback which is expected to initialize all of the
154 invoking its ``->start()`` callback.
156 That callback is expected to register per-CPU utilization update callbacks for
162 to determine the P-state to use for the given policy going forward and to
164 the P-state selection.  The scaling driver may be invoked directly from
172 "inactive" (and is re-initialized now) instead of the default governor.
176 need to re-initialize the policy object at all.  In that case, it only is
178 into account.  That is achieved by invoking the governor's ``->stop`` and
179 ``->start()`` callbacks, in this order, for the entire policy.
182 governor layer of ``CPUFreq`` and provides its own P-state selection algorithms.
184 new policy objects.  Instead, the driver's ``->setpolicy()`` callback is invoked
185 to register per-CPU utilization update callbacks for each policy.  These
187 governors, but in the |intel_pstate| case they both determine the P-state to
210 in :file:`/sys/devices/system/cpu/cpufreq` each contain policy-specific
217 also add driver-specific attributes to the policy directories in ``sysfs`` to
218 control policy-specific aspects of driver behavior.
235 	BIOS/HW-based mechanisms.
261 	P-state to another, in nanoseconds.
264 	work with the `ondemand`_ governor, -1 (:c:macro:`CPUFREQ_ETERNAL`)
287 	In the majority of cases, this is the frequency of the last P-state
306 	This attribute is read-write and writing to it will cause a new scaling
317 	This attribute is read-write and writing a string representing an
319 	than the value of the ``scaling_min_freq`` attribute).
325 	This attribute is read-write and writing a string representing a
326 	non-negative integer to it will cause a new limit to be set (it must not
327 	be higher than the value of the ``scaling_max_freq`` attribute).
351 Some governors expose ``sysfs`` attributes to control or fine-tune the scaling
353 tunables, can be either global (system-wide) or per-policy, depending on the
355 per-policy, they are located in a subdirectory of each policy directory.
362 ---------------
372 -------------
382 -------------
389 -------------
405 Per-Entity Load Tracking (PELT) metric for the root control group of the
406 given CPU as the CPU utilization estimate (see the *Per-entity load tracking*
414 policy (if the PELT number is frequency-invariant), or the current CPU frequency
419 "IO-wait boosting".  That happens when the :c:macro:`SCHED_CPUFREQ_IOWAIT` flag
442 ------------
448 time in which the given CPU was not idle.  The ratio of the non-idle (active)
456 invoked asynchronously (via a workqueue) and CPU P-states are updated from
459 relatively often and the CPU P-state updates triggered by it can be relatively
468 speedup threshold, in which case it will go straight for the highest frequency
483 	If this tunable is per-policy, the following shell command sets the time
498 	than 0 as CPU idle time.
520 	value is exceeded by the estimated CPU load) or sensitivity threshold
528 		f * (1 - ``powersave_bias`` / 1000)
542 	The performance of a workload with the sensitivity of 0 (memory-bound or
543 	IO-bound) is not expected to increase at all as a result of increasing
545 	(CPU-bound) are expected to perform much better if the CPU frequency is
548 	If the workload sensitivity is less than the threshold represented by
550 	will cause the governor to select a frequency lower than its original
551 	target, so as to avoid over-provisioning workloads that will not benefit
552 	from running at higher CPU frequencies.
555 ----------------
564 battery-powered).  To achieve that, it changes the frequency in relatively
565 small steps, one step at a time, up or down - depending on whether or not a
566 (configurable) threshold has been exceeded by the estimated CPU load.
582 	Threshold value (in percent, 20 by default) used to determine the
585 	If the estimated CPU load is greater than this value, the frequency will
586 	go up (by ``freq_step``).  If the load is less than this value (and the
595 	times slower than it ramps up.
602 ----------
606 threshold for the whole package) under certain conditions, for example if the
611 "Turbo-Core" or (in technical documentation) "Core Performance Boost" and so on.
616 The frequency boost mechanism may be either hardware-based or software-based.
617 If it is hardware-based (e.g. on x86), the decision to trigger the boosting is
620 limits).  If it is software-based (e.g. on ARM), the scaling driver decides
624 -------------------------------
629 but provides a driver-specific interface for controlling it, like
634 trigger boosting (in the hardware-based case), or the software is allowed to
635 trigger boosting (in the software-based case).  It does not mean that boosting
646 --------------------------------
666   2. In some situations deterministic behavior is more important than
676      single-thread performance may vary because of it which may lead to
682 -----------------------
684 The AMD powernow-k8 scaling driver supports a ``sysfs`` knob very similar to
691 implementation, however, works on the system-wide basis and setting that knob
711 .. [1] Jonathan Corbet, *Per-entity load tracking*,