Lines Matching refs:CPU
8 CPU Performance Scaling
16 The Concept of CPU Performance Scaling
23 can be retired by the CPU over a unit of time, but also the higher the clock
25 time (or the more power is drawn) by the CPU in the given P-state. Therefore
26 there is a natural tradeoff between the CPU capacity (the number of instructions
27 that can be executed over a unit of time) and the power drawn by the CPU.
33 instructions so quickly and maintaining the highest available CPU capacity for a
35 It also may not be physically possible to maintain maximum CPU capacity for too
41 Typically, they are used along with algorithms to estimate the required CPU
45 to as CPU performance scaling or CPU frequency scaling (because it involves
46 adjusting the CPU clock frequency).
49 CPU Performance Scaling in Linux
52 The Linux kernel supports CPU performance scaling by means of the ``CPUFreq``
53 (CPU Frequency scaling) subsystem that consists of three layers of code: the
57 interfaces for all platforms that support CPU performance scaling. It defines
60 Scaling governors implement algorithms to estimate the required CPU capacity.
66 access platform-specific hardware interfaces to change CPU P-states as requested
96 |struct cpufreq_policy| is also used when there is only one CPU in the given
100 every CPU in the system, including CPUs that are currently offline. If multiple
108 CPU Initialization
115 The scaling driver may be registered before or after CPU registration. If
122 In any case, the ``CPUFreq`` core is invoked to take note of any logical CPU it
123 has not seen so far as soon as it is ready to handle that CPU. [Note that the
124 logical CPU may be a physical single-core processor, or a single core in a
126 core. In what follows "CPU" always means "logical CPU" unless explicitly stated
131 for the given CPU and if so, it skips the policy object creation. Otherwise,
134 the given CPU is set to the new policy object's address in memory.
137 pointer of the new CPU passed to it as the argument. That callback is expected
138 to initialize the performance scaling hardware interface for the given CPU (or,
157 That callback is expected to register per-CPU utilization update callbacks for
158 all of the online CPUs belonging to the given policy with the CPU scheduler.
159 The utilization update callbacks will be invoked by the CPU scheduler on
161 scheduler tick or generally whenever the CPU utilization may change (from the
175 In turn, if a previously offline CPU is being brought back online, but some
178 necessary to restart the scaling governor so that it can take the new online CPU
186 to register per-CPU utilization update callbacks for each policy. These
187 callbacks are invoked by the CPU scheduler in the same way as for scaling
192 The policy objects created during CPU initialization and other data structures
195 when the last CPU belonging to the given policy in unregistered.
231 CPU frequencies, that limit will be reported through this attribute (if
287 the CPU is actually running at (due to hardware design and other
291 more precisely reflecting the current CPU frequency through this
292 attribute, but that still may not be the exact current CPU frequency as
382 to set the CPU frequency for the policy it is attached to by writing to the
388 This governor uses CPU utilization data available from the CPU scheduler. It
389 generally is regarded as a part of the CPU scheduler, so it can access the
393 invoke the scaling driver asynchronously when it decides that the CPU frequency
395 is capable of changing the CPU frequency from scheduler context).
397 The actions of this governor for a particular CPU depend on the scheduling class
398 invoking its utilization update callback for that CPU. If it is invoked by the
403 given CPU as the CPU utilization estimate (see the *Per-entity load tracking*
405 CPU frequency to apply is computed in accordance with the formula
410 ``util``, and ``f_0`` is either the maximum possible CPU frequency for the given
411 policy (if the PELT number is frequency-invariant), or the current CPU frequency
415 CPU frequency for tasks that have been waiting on I/O most recently, called
433 tightly integrated with the CPU scheduler, its overhead in terms of CPU context
434 switches and similar is less significant, and it uses the scheduler's own CPU
441 This governor uses CPU load as a CPU frequency selection metric.
443 In order to estimate the current CPU load, it measures the time elapsed between
445 time in which the given CPU was not idle. The ratio of the non-idle (active)
446 time to the total CPU time is taken as an estimate of the load.
453 invoked asynchronously (via a workqueue) and CPU P-states are updated from
455 governor is minimum, but it causes additional CPU context switches to happen
456 relatively often and the CPU P-state updates triggered by it can be relatively
457 irregular. Also, it affects its own CPU load metric by running code that
458 reduces the CPU idle time (even though the CPU idle time is only reduced very
461 It generally selects CPU frequencies proportional to the estimated load, so that
487 If the estimated CPU load is above this value (in percent), the governor
490 CPU load.
493 If set to 1 (default 0), it will cause the CPU load estimation code to
494 treat the CPU time spent on executing tasks with "nice" levels greater
495 than 0 as CPU idle time.
504 the ``sampling_rate`` value if the CPU load goes above ``up_threshold``.
511 at the cost of additional energy spent on maintaining the maximum CPU
517 value is exceeded by the estimated CPU load) or sensitivity threshold
537 workload running on a CPU will change in response to frequency changes.
541 the CPU frequency, whereas workloads with the sensitivity of 100%
542 (CPU-bound) are expected to perform much better if the CPU frequency is
549 from running at higher CPU frequencies.
554 This governor uses CPU load as a CPU frequency selection metric.
556 It estimates the CPU load in the same way as the `ondemand`_ governor described
557 above, but the CPU frequency selection algorithm implemented by it is different.
563 (configurable) threshold has been exceeded by the estimated CPU load.
582 If the estimated CPU load is greater than this value, the frequency will
616 into a special state in which it can control the CPU frequency within certain
646 CPU performance on time scales below software resolution (e.g. below the