1.. SPDX-License-Identifier: GPL-2.0 2.. include:: <isonum.txt> 3 4.. |intel_pstate| replace:: :doc:`intel_pstate <intel_pstate>` 5 6======================= 7CPU Performance Scaling 8======================= 9 10:Copyright: |copy| 2017 Intel Corporation 11 12:Author: Rafael J. Wysocki <rafael.j.wysocki@intel.com> 13 14 15The Concept of CPU Performance Scaling 16====================================== 17 18The majority of modern processors are capable of operating in a number of 19different clock frequency and voltage configurations, often referred to as 20Operating Performance Points or P-states (in ACPI terminology). As a rule, 21the higher the clock frequency and the higher the voltage, the more instructions 22can be retired by the CPU over a unit of time, but also the higher the clock 23frequency and the higher the voltage, the more energy is consumed over a unit of 24time (or the more power is drawn) by the CPU in the given P-state. Therefore 25there is a natural tradeoff between the CPU capacity (the number of instructions 26that can be executed over a unit of time) and the power drawn by the CPU. 27 28In some situations it is desirable or even necessary to run the program as fast 29as possible and then there is no reason to use any P-states different from the 30highest one (i.e. the highest-performance frequency/voltage configuration 31available). In some other cases, however, it may not be necessary to execute 32instructions so quickly and maintaining the highest available CPU capacity for a 33relatively long time without utilizing it entirely may be regarded as wasteful. 34It also may not be physically possible to maintain maximum CPU capacity for too 35long for thermal or power supply capacity reasons or similar. To cover those 36cases, there are hardware interfaces allowing CPUs to be switched between 37different frequency/voltage configurations or (in the ACPI terminology) to be 38put into different P-states. 39 40Typically, they are used along with algorithms to estimate the required CPU 41capacity, so as to decide which P-states to put the CPUs into. Of course, since 42the utilization of the system generally changes over time, that has to be done 43repeatedly on a regular basis. The activity by which this happens is referred 44to as CPU performance scaling or CPU frequency scaling (because it involves 45adjusting the CPU clock frequency). 46 47 48CPU Performance Scaling in Linux 49================================ 50 51The Linux kernel supports CPU performance scaling by means of the ``CPUFreq`` 52(CPU Frequency scaling) subsystem that consists of three layers of code: the 53core, scaling governors and scaling drivers. 54 55The ``CPUFreq`` core provides the common code infrastructure and user space 56interfaces for all platforms that support CPU performance scaling. It defines 57the basic framework in which the other components operate. 58 59Scaling governors implement algorithms to estimate the required CPU capacity. 60As a rule, each governor implements one, possibly parametrized, scaling 61algorithm. 62 63Scaling drivers talk to the hardware. They provide scaling governors with 64information on the available P-states (or P-state ranges in some cases) and 65access platform-specific hardware interfaces to change CPU P-states as requested 66by scaling governors. 67 68In principle, all available scaling governors can be used with every scaling 69driver. That design is based on the observation that the information used by 70performance scaling algorithms for P-state selection can be represented in a 71platform-independent form in the majority of cases, so it should be possible 72to use the same performance scaling algorithm implemented in exactly the same 73way regardless of which scaling driver is used. Consequently, the same set of 74scaling governors should be suitable for every supported platform. 75 76However, that observation may not hold for performance scaling algorithms 77based on information provided by the hardware itself, for example through 78feedback registers, as that information is typically specific to the hardware 79interface it comes from and may not be easily represented in an abstract, 80platform-independent way. For this reason, ``CPUFreq`` allows scaling drivers 81to bypass the governor layer and implement their own performance scaling 82algorithms. That is done by the |intel_pstate| scaling driver. 83 84 85``CPUFreq`` Policy Objects 86========================== 87 88In some cases the hardware interface for P-state control is shared by multiple 89CPUs. That is, for example, the same register (or set of registers) is used to 90control the P-state of multiple CPUs at the same time and writing to it affects 91all of those CPUs simultaneously. 92 93Sets of CPUs sharing hardware P-state control interfaces are represented by 94``CPUFreq`` as struct cpufreq_policy objects. For consistency, 95struct cpufreq_policy is also used when there is only one CPU in the given 96set. 97 98The ``CPUFreq`` core maintains a pointer to a struct cpufreq_policy object for 99every CPU in the system, including CPUs that are currently offline. If multiple 100CPUs share the same hardware P-state control interface, all of the pointers 101corresponding to them point to the same struct cpufreq_policy object. 102 103``CPUFreq`` uses struct cpufreq_policy as its basic data type and the design 104of its user space interface is based on the policy concept. 105 106 107CPU Initialization 108================== 109 110First of all, a scaling driver has to be registered for ``CPUFreq`` to work. 111It is only possible to register one scaling driver at a time, so the scaling 112driver is expected to be able to handle all CPUs in the system. 113 114The scaling driver may be registered before or after CPU registration. If 115CPUs are registered earlier, the driver core invokes the ``CPUFreq`` core to 116take a note of all of the already registered CPUs during the registration of the 117scaling driver. In turn, if any CPUs are registered after the registration of 118the scaling driver, the ``CPUFreq`` core will be invoked to take note of them 119at their registration time. 120 121In any case, the ``CPUFreq`` core is invoked to take note of any logical CPU it 122has not seen so far as soon as it is ready to handle that CPU. [Note that the 123logical CPU may be a physical single-core processor, or a single core in a 124multicore processor, or a hardware thread in a physical processor or processor 125core. In what follows "CPU" always means "logical CPU" unless explicitly stated 126otherwise and the word "processor" is used to refer to the physical part 127possibly including multiple logical CPUs.] 128 129Once invoked, the ``CPUFreq`` core checks if the policy pointer is already set 130for the given CPU and if so, it skips the policy object creation. Otherwise, 131a new policy object is created and initialized, which involves the creation of 132a new policy directory in ``sysfs``, and the policy pointer corresponding to 133the given CPU is set to the new policy object's address in memory. 134 135Next, the scaling driver's ``->init()`` callback is invoked with the policy 136pointer of the new CPU passed to it as the argument. That callback is expected 137to initialize the performance scaling hardware interface for the given CPU (or, 138more precisely, for the set of CPUs sharing the hardware interface it belongs 139to, represented by its policy object) and, if the policy object it has been 140called for is new, to set parameters of the policy, like the minimum and maximum 141frequencies supported by the hardware, the table of available frequencies (if 142the set of supported P-states is not a continuous range), and the mask of CPUs 143that belong to the same policy (including both online and offline CPUs). That 144mask is then used by the core to populate the policy pointers for all of the 145CPUs in it. 146 147The next major initialization step for a new policy object is to attach a 148scaling governor to it (to begin with, that is the default scaling governor 149determined by the kernel command line or configuration, but it may be changed 150later via ``sysfs``). First, a pointer to the new policy object is passed to 151the governor's ``->init()`` callback which is expected to initialize all of the 152data structures necessary to handle the given policy and, possibly, to add 153a governor ``sysfs`` interface to it. Next, the governor is started by 154invoking its ``->start()`` callback. 155 156That callback is expected to register per-CPU utilization update callbacks for 157all of the online CPUs belonging to the given policy with the CPU scheduler. 158The utilization update callbacks will be invoked by the CPU scheduler on 159important events, like task enqueue and dequeue, on every iteration of the 160scheduler tick or generally whenever the CPU utilization may change (from the 161scheduler's perspective). They are expected to carry out computations needed 162to determine the P-state to use for the given policy going forward and to 163invoke the scaling driver to make changes to the hardware in accordance with 164the P-state selection. The scaling driver may be invoked directly from 165scheduler context or asynchronously, via a kernel thread or workqueue, depending 166on the configuration and capabilities of the scaling driver and the governor. 167 168Similar steps are taken for policy objects that are not new, but were "inactive" 169previously, meaning that all of the CPUs belonging to them were offline. The 170only practical difference in that case is that the ``CPUFreq`` core will attempt 171to use the scaling governor previously used with the policy that became 172"inactive" (and is re-initialized now) instead of the default governor. 173 174In turn, if a previously offline CPU is being brought back online, but some 175other CPUs sharing the policy object with it are online already, there is no 176need to re-initialize the policy object at all. In that case, it only is 177necessary to restart the scaling governor so that it can take the new online CPU 178into account. That is achieved by invoking the governor's ``->stop`` and 179``->start()`` callbacks, in this order, for the entire policy. 180 181As mentioned before, the |intel_pstate| scaling driver bypasses the scaling 182governor layer of ``CPUFreq`` and provides its own P-state selection algorithms. 183Consequently, if |intel_pstate| is used, scaling governors are not attached to 184new policy objects. Instead, the driver's ``->setpolicy()`` callback is invoked 185to register per-CPU utilization update callbacks for each policy. These 186callbacks are invoked by the CPU scheduler in the same way as for scaling 187governors, but in the |intel_pstate| case they both determine the P-state to 188use and change the hardware configuration accordingly in one go from scheduler 189context. 190 191The policy objects created during CPU initialization and other data structures 192associated 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 194when the last CPU belonging to the given policy in unregistered. 195 196 197Policy Interface in ``sysfs`` 198============================= 199 200During the initialization of the kernel, the ``CPUFreq`` core creates a 201``sysfs`` directory (kobject) called ``cpufreq`` under 202:file:`/sys/devices/system/cpu/`. 203 204That directory contains a ``policyX`` subdirectory (where ``X`` represents an 205integer number) for every policy object maintained by the ``CPUFreq`` core. 206Each ``policyX`` directory is pointed to by ``cpufreq`` symbolic links 207under :file:`/sys/devices/system/cpu/cpuY/` (where ``Y`` represents an integer 208that may be different from the one represented by ``X``) for all of the CPUs 209associated with (or belonging to) the given policy. The ``policyX`` directories 210in :file:`/sys/devices/system/cpu/cpufreq` each contain policy-specific 211attributes (files) to control ``CPUFreq`` behavior for the corresponding policy 212objects (that is, for all of the CPUs associated with them). 213 214Some of those attributes are generic. They are created by the ``CPUFreq`` core 215and their behavior generally does not depend on what scaling driver is in use 216and what scaling governor is attached to the given policy. Some scaling drivers 217also add driver-specific attributes to the policy directories in ``sysfs`` to 218control policy-specific aspects of driver behavior. 219 220The generic attributes under :file:`/sys/devices/system/cpu/cpufreq/policyX/` 221are the following: 222 223``affected_cpus`` 224 List of online CPUs belonging to this policy (i.e. sharing the hardware 225 performance scaling interface represented by the ``policyX`` policy 226 object). 227 228``bios_limit`` 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 231 present). 232 233 The existence of the limit may be a result of some (often unintentional) 234 BIOS settings, restrictions coming from a service processor or other 235 BIOS/HW-based mechanisms. 236 237 This does not cover ACPI thermal limitations which can be discovered 238 through a generic thermal driver. 239 240 This attribute is not present if the scaling driver in use does not 241 support it. 242 243``cpuinfo_cur_freq`` 244 Current frequency of the CPUs belonging to this policy as obtained from 245 the hardware (in KHz). 246 247 This is expected to be the frequency the hardware actually runs at. 248 If that frequency cannot be determined, this attribute should not 249 be present. 250 251``cpuinfo_avg_freq`` 252 An average frequency (in KHz) of all CPUs belonging to a given policy, 253 derived from a hardware provided feedback and reported on a time frame 254 spanning at most few milliseconds. 255 256 This is expected to be based on the frequency the hardware actually runs 257 at and, as such, might require specialised hardware support (such as AMU 258 extension on ARM). If one cannot be determined, this attribute should 259 not be present. 260 261 Note that failed attempt to retrieve current frequency for a given 262 CPU(s) will result in an appropriate error, i.e.: EAGAIN for CPU that 263 remains idle (raised on ARM). 264 265``cpuinfo_max_freq`` 266 Maximum possible operating frequency the CPUs belonging to this policy 267 can run at (in kHz). 268 269``cpuinfo_min_freq`` 270 Minimum possible operating frequency the CPUs belonging to this policy 271 can run at (in kHz). 272 273``cpuinfo_transition_latency`` 274 The time it takes to switch the CPUs belonging to this policy from one 275 P-state to another, in nanoseconds. 276 277``related_cpus`` 278 List of all (online and offline) CPUs belonging to this policy. 279 280``scaling_available_frequencies`` 281 List of available frequencies of the CPUs belonging to this policy 282 (in kHz). 283 284``scaling_available_governors`` 285 List of ``CPUFreq`` scaling governors present in the kernel that can 286 be attached to this policy or (if the |intel_pstate| scaling driver is 287 in use) list of scaling algorithms provided by the driver that can be 288 applied to this policy. 289 290 [Note that some governors are modular and it may be necessary to load a 291 kernel module for the governor held by it to become available and be 292 listed by this attribute.] 293 294``scaling_cur_freq`` 295 Current frequency of all of the CPUs belonging to this policy (in kHz). 296 297 In the majority of cases, this is the frequency of the last P-state 298 requested by the scaling driver from the hardware using the scaling 299 interface provided by it, which may or may not reflect the frequency 300 the CPU is actually running at (due to hardware design and other 301 limitations). 302 303 Some architectures (e.g. ``x86``) may attempt to provide information 304 more precisely reflecting the current CPU frequency through this 305 attribute, but that still may not be the exact current CPU frequency as 306 seen by the hardware at the moment. This behavior though, is only 307 available via c:macro:``CPUFREQ_ARCH_CUR_FREQ`` option. 308 309``scaling_driver`` 310 The scaling driver currently in use. 311 312``scaling_governor`` 313 The scaling governor currently attached to this policy or (if the 314 |intel_pstate| scaling driver is in use) the scaling algorithm 315 provided by the driver that is currently applied to this policy. 316 317 This attribute is read-write and writing to it will cause a new scaling 318 governor to be attached to this policy or a new scaling algorithm 319 provided by the scaling driver to be applied to it (in the 320 |intel_pstate| case), as indicated by the string written to this 321 attribute (which must be one of the names listed by the 322 ``scaling_available_governors`` attribute described above). 323 324``scaling_max_freq`` 325 Maximum frequency the CPUs belonging to this policy are allowed to be 326 running at (in kHz). 327 328 This attribute is read-write and writing a string representing an 329 integer to it will cause a new limit to be set (it must not be lower 330 than the value of the ``scaling_min_freq`` attribute). 331 332``scaling_min_freq`` 333 Minimum frequency the CPUs belonging to this policy are allowed to be 334 running at (in kHz). 335 336 This attribute is read-write and writing a string representing a 337 non-negative integer to it will cause a new limit to be set (it must not 338 be higher than the value of the ``scaling_max_freq`` attribute). 339 340``scaling_setspeed`` 341 This attribute is functional only if the `userspace`_ scaling governor 342 is attached to the given policy. 343 344 It returns the last frequency requested by the governor (in kHz) or can 345 be written to in order to set a new frequency for the policy. 346 347 348Generic Scaling Governors 349========================= 350 351``CPUFreq`` provides generic scaling governors that can be used with all 352scaling drivers. As stated before, each of them implements a single, possibly 353parametrized, performance scaling algorithm. 354 355Scaling governors are attached to policy objects and different policy objects 356can be handled by different scaling governors at the same time (although that 357may lead to suboptimal results in some cases). 358 359The scaling governor for a given policy object can be changed at any time with 360the help of the ``scaling_governor`` policy attribute in ``sysfs``. 361 362Some governors expose ``sysfs`` attributes to control or fine-tune the scaling 363algorithms implemented by them. Those attributes, referred to as governor 364tunables, can be either global (system-wide) or per-policy, depending on the 365scaling driver in use. If the driver requires governor tunables to be 366per-policy, they are located in a subdirectory of each policy directory. 367Otherwise, they are located in a subdirectory under 368:file:`/sys/devices/system/cpu/cpufreq/`. In either case the name of the 369subdirectory containing the governor tunables is the name of the governor 370providing them. 371 372``performance`` 373--------------- 374 375When attached to a policy object, this governor causes the highest frequency, 376within the ``scaling_max_freq`` policy limit, to be requested for that policy. 377 378The request is made once at that time the governor for the policy is set to 379``performance`` and whenever the ``scaling_max_freq`` or ``scaling_min_freq`` 380policy limits change after that. 381 382``powersave`` 383------------- 384 385When attached to a policy object, this governor causes the lowest frequency, 386within the ``scaling_min_freq`` policy limit, to be requested for that policy. 387 388The request is made once at that time the governor for the policy is set to 389``powersave`` and whenever the ``scaling_max_freq`` or ``scaling_min_freq`` 390policy limits change after that. 391 392``userspace`` 393------------- 394 395This governor does not do anything by itself. Instead, it allows user space 396to set the CPU frequency for the policy it is attached to by writing to the 397``scaling_setspeed`` attribute of that policy. Though the intention may be to 398set an exact frequency for the policy, the actual frequency may vary depending 399on hardware coordination, thermal and power limits, and other factors. 400 401``schedutil`` 402------------- 403 404This governor uses CPU utilization data available from the CPU scheduler. It 405generally is regarded as a part of the CPU scheduler, so it can access the 406scheduler's internal data structures directly. 407 408It runs entirely in scheduler context, although in some cases it may need to 409invoke the scaling driver asynchronously when it decides that the CPU frequency 410should be changed for a given policy (that depends on whether or not the driver 411is capable of changing the CPU frequency from scheduler context). 412 413The actions of this governor for a particular CPU depend on the scheduling class 414invoking its utilization update callback for that CPU. If it is invoked by the 415RT or deadline scheduling classes, the governor will increase the frequency to 416the allowed maximum (that is, the ``scaling_max_freq`` policy limit). In turn, 417if it is invoked by the CFS scheduling class, the governor will use the 418Per-Entity Load Tracking (PELT) metric for the root control group of the 419given CPU as the CPU utilization estimate (see the *Per-entity load tracking* 420LWN.net article [1]_ for a description of the PELT mechanism). Then, the new 421CPU frequency to apply is computed in accordance with the formula 422 423 f = 1.25 * ``f_0`` * ``util`` / ``max`` 424 425where ``util`` is the PELT number, ``max`` is the theoretical maximum of 426``util``, and ``f_0`` is either the maximum possible CPU frequency for the given 427policy (if the PELT number is frequency-invariant), or the current CPU frequency 428(otherwise). 429 430This governor also employs a mechanism allowing it to temporarily bump up the 431CPU frequency for tasks that have been waiting on I/O most recently, called 432"IO-wait boosting". That happens when the :c:macro:`SCHED_CPUFREQ_IOWAIT` flag 433is passed by the scheduler to the governor callback which causes the frequency 434to go up to the allowed maximum immediately and then draw back to the value 435returned by the above formula over time. 436 437This governor exposes only one tunable: 438 439``rate_limit_us`` 440 Minimum time (in microseconds) that has to pass between two consecutive 441 runs of governor computations (default: 1.5 times the scaling driver's 442 transition latency or the maximum 2ms). 443 444 The purpose of this tunable is to reduce the scheduler context overhead 445 of the governor which might be excessive without it. 446 447This governor generally is regarded as a replacement for the older `ondemand`_ 448and `conservative`_ governors (described below), as it is simpler and more 449tightly integrated with the CPU scheduler, its overhead in terms of CPU context 450switches and similar is less significant, and it uses the scheduler's own CPU 451utilization metric, so in principle its decisions should not contradict the 452decisions made by the other parts of the scheduler. 453 454``ondemand`` 455------------ 456 457This governor uses CPU load as a CPU frequency selection metric. 458 459In order to estimate the current CPU load, it measures the time elapsed between 460consecutive invocations of its worker routine and computes the fraction of that 461time in which the given CPU was not idle. The ratio of the non-idle (active) 462time to the total CPU time is taken as an estimate of the load. 463 464If this governor is attached to a policy shared by multiple CPUs, the load is 465estimated for all of them and the greatest result is taken as the load estimate 466for the entire policy. 467 468The worker routine of this governor has to run in process context, so it is 469invoked asynchronously (via a workqueue) and CPU P-states are updated from 470there if necessary. As a result, the scheduler context overhead from this 471governor is minimum, but it causes additional CPU context switches to happen 472relatively often and the CPU P-state updates triggered by it can be relatively 473irregular. Also, it affects its own CPU load metric by running code that 474reduces the CPU idle time (even though the CPU idle time is only reduced very 475slightly by it). 476 477It generally selects CPU frequencies proportional to the estimated load, so that 478the value of the ``cpuinfo_max_freq`` policy attribute corresponds to the load of 4791 (or 100%), and the value of the ``cpuinfo_min_freq`` policy attribute 480corresponds to the load of 0, unless when the load exceeds a (configurable) 481speedup threshold, in which case it will go straight for the highest frequency 482it is allowed to use (the ``scaling_max_freq`` policy limit). 483 484This governor exposes the following tunables: 485 486``sampling_rate`` 487 This is how often the governor's worker routine should run, in 488 microseconds. 489 490 Typically, it is set to values of the order of 2000 (2 ms). Its 491 default value is to add a 50% breathing room 492 to ``cpuinfo_transition_latency`` on each policy this governor is 493 attached to. The minimum is typically the length of two scheduler 494 ticks. 495 496 If this tunable is per-policy, the following shell command sets the time 497 represented by it to be 1.5 times as high as the transition latency 498 (the default):: 499 500 # echo `$(($(cat cpuinfo_transition_latency) * 3 / 2))` > ondemand/sampling_rate 501 502``up_threshold`` 503 If the estimated CPU load is above this value (in percent), the governor 504 will set the frequency to the maximum value allowed for the policy. 505 Otherwise, the selected frequency will be proportional to the estimated 506 CPU load. 507 508``ignore_nice_load`` 509 If set to 1 (default 0), it will cause the CPU load estimation code to 510 treat the CPU time spent on executing tasks with "nice" levels greater 511 than 0 as CPU idle time. 512 513 This may be useful if there are tasks in the system that should not be 514 taken into account when deciding what frequency to run the CPUs at. 515 Then, to make that happen it is sufficient to increase the "nice" level 516 of those tasks above 0 and set this attribute to 1. 517 518``sampling_down_factor`` 519 Temporary multiplier, between 1 (default) and 100 inclusive, to apply to 520 the ``sampling_rate`` value if the CPU load goes above ``up_threshold``. 521 522 This causes the next execution of the governor's worker routine (after 523 setting the frequency to the allowed maximum) to be delayed, so the 524 frequency stays at the maximum level for a longer time. 525 526 Frequency fluctuations in some bursty workloads may be avoided this way 527 at the cost of additional energy spent on maintaining the maximum CPU 528 capacity. 529 530``powersave_bias`` 531 Reduction factor to apply to the original frequency target of the 532 governor (including the maximum value used when the ``up_threshold`` 533 value is exceeded by the estimated CPU load) or sensitivity threshold 534 for the AMD frequency sensitivity powersave bias driver 535 (:file:`drivers/cpufreq/amd_freq_sensitivity.c`), between 0 and 1000 536 inclusive. 537 538 If the AMD frequency sensitivity powersave bias driver is not loaded, 539 the effective frequency to apply is given by 540 541 f * (1 - ``powersave_bias`` / 1000) 542 543 where f is the governor's original frequency target. The default value 544 of this attribute is 0 in that case. 545 546 If the AMD frequency sensitivity powersave bias driver is loaded, the 547 value of this attribute is 400 by default and it is used in a different 548 way. 549 550 On Family 16h (and later) AMD processors there is a mechanism to get a 551 measured workload sensitivity, between 0 and 100% inclusive, from the 552 hardware. That value can be used to estimate how the performance of the 553 workload running on a CPU will change in response to frequency changes. 554 555 The performance of a workload with the sensitivity of 0 (memory-bound or 556 IO-bound) is not expected to increase at all as a result of increasing 557 the CPU frequency, whereas workloads with the sensitivity of 100% 558 (CPU-bound) are expected to perform much better if the CPU frequency is 559 increased. 560 561 If the workload sensitivity is less than the threshold represented by 562 the ``powersave_bias`` value, the sensitivity powersave bias driver 563 will cause the governor to select a frequency lower than its original 564 target, so as to avoid over-provisioning workloads that will not benefit 565 from running at higher CPU frequencies. 566 567``conservative`` 568---------------- 569 570This governor uses CPU load as a CPU frequency selection metric. 571 572It estimates the CPU load in the same way as the `ondemand`_ governor described 573above, but the CPU frequency selection algorithm implemented by it is different. 574 575Namely, it avoids changing the frequency significantly over short time intervals 576which may not be suitable for systems with limited power supply capacity (e.g. 577battery-powered). To achieve that, it changes the frequency in relatively 578small steps, one step at a time, up or down - depending on whether or not a 579(configurable) threshold has been exceeded by the estimated CPU load. 580 581This governor exposes the following tunables: 582 583``freq_step`` 584 Frequency step in percent of the maximum frequency the governor is 585 allowed to set (the ``scaling_max_freq`` policy limit), between 0 and 586 100 (5 by default). 587 588 This is how much the frequency is allowed to change in one go. Setting 589 it to 0 will cause the default frequency step (5 percent) to be used 590 and setting it to 100 effectively causes the governor to periodically 591 switch the frequency between the ``scaling_min_freq`` and 592 ``scaling_max_freq`` policy limits. 593 594``down_threshold`` 595 Threshold value (in percent, 20 by default) used to determine the 596 frequency change direction. 597 598 If the estimated CPU load is greater than this value, the frequency will 599 go up (by ``freq_step``). If the load is less than this value (and the 600 ``sampling_down_factor`` mechanism is not in effect), the frequency will 601 go down. Otherwise, the frequency will not be changed. 602 603``sampling_down_factor`` 604 Frequency decrease deferral factor, between 1 (default) and 10 605 inclusive. 606 607 It effectively causes the frequency to go down ``sampling_down_factor`` 608 times slower than it ramps up. 609 610 611Frequency Boost Support 612======================= 613 614Background 615---------- 616 617Some processors support a mechanism to raise the operating frequency of some 618cores in a multicore package temporarily (and above the sustainable frequency 619threshold for the whole package) under certain conditions, for example if the 620whole chip is not fully utilized and below its intended thermal or power budget. 621 622Different names are used by different vendors to refer to this functionality. 623For Intel processors it is referred to as "Turbo Boost", AMD calls it 624"Turbo-Core" or (in technical documentation) "Core Performance Boost" and so on. 625As a rule, it also is implemented differently by different vendors. The simple 626term "frequency boost" is used here for brevity to refer to all of those 627implementations. 628 629The frequency boost mechanism may be either hardware-based or software-based. 630If it is hardware-based (e.g. on x86), the decision to trigger the boosting is 631made by the hardware (although in general it requires the hardware to be put 632into a special state in which it can control the CPU frequency within certain 633limits). If it is software-based (e.g. on ARM), the scaling driver decides 634whether or not to trigger boosting and when to do that. 635 636The ``boost`` File in ``sysfs`` 637------------------------------- 638 639This file is located under :file:`/sys/devices/system/cpu/cpufreq/` and controls 640the "boost" setting for the whole system. It is not present if the underlying 641scaling driver does not support the frequency boost mechanism (or supports it, 642but provides a driver-specific interface for controlling it, like 643|intel_pstate|). 644 645If the value in this file is 1, the frequency boost mechanism is enabled. This 646means that either the hardware can be put into states in which it is able to 647trigger boosting (in the hardware-based case), or the software is allowed to 648trigger boosting (in the software-based case). It does not mean that boosting 649is actually in use at the moment on any CPUs in the system. It only means a 650permission to use the frequency boost mechanism (which still may never be used 651for other reasons). 652 653If the value in this file is 0, the frequency boost mechanism is disabled and 654cannot be used at all. 655 656The only values that can be written to this file are 0 and 1. 657 658Rationale for Boost Control Knob 659-------------------------------- 660 661The frequency boost mechanism is generally intended to help to achieve optimum 662CPU performance on time scales below software resolution (e.g. below the 663scheduler tick interval) and it is demonstrably suitable for many workloads, but 664it may lead to problems in certain situations. 665 666For this reason, many systems make it possible to disable the frequency boost 667mechanism in the platform firmware (BIOS) setup, but that requires the system to 668be restarted for the setting to be adjusted as desired, which may not be 669practical at least in some cases. For example: 670 671 1. Boosting means overclocking the processor, although under controlled 672 conditions. Generally, the processor's energy consumption increases 673 as a result of increasing its frequency and voltage, even temporarily. 674 That may not be desirable on systems that switch to power sources of 675 limited capacity, such as batteries, so the ability to disable the boost 676 mechanism while the system is running may help there (but that depends on 677 the workload too). 678 679 2. In some situations deterministic behavior is more important than 680 performance or energy consumption (or both) and the ability to disable 681 boosting while the system is running may be useful then. 682 683 3. To examine the impact of the frequency boost mechanism itself, it is useful 684 to be able to run tests with and without boosting, preferably without 685 restarting the system in the meantime. 686 687 4. Reproducible results are important when running benchmarks. Since 688 the boosting functionality depends on the load of the whole package, 689 single-thread performance may vary because of it which may lead to 690 unreproducible results sometimes. That can be avoided by disabling the 691 frequency boost mechanism before running benchmarks sensitive to that 692 issue. 693 694Legacy AMD ``cpb`` Knob 695----------------------- 696 697The AMD powernow-k8 scaling driver supports a ``sysfs`` knob very similar to 698the global ``boost`` one. It is used for disabling/enabling the "Core 699Performance Boost" feature of some AMD processors. 700 701If present, that knob is located in every ``CPUFreq`` policy directory in 702``sysfs`` (:file:`/sys/devices/system/cpu/cpufreq/policyX/`) and is called 703``cpb``, which indicates a more fine grained control interface. The actual 704implementation, however, works on the system-wide basis and setting that knob 705for one policy causes the same value of it to be set for all of the other 706policies at the same time. 707 708That knob is still supported on AMD processors that support its underlying 709hardware feature, but it may be configured out of the kernel (via the 710:c:macro:`CONFIG_X86_ACPI_CPUFREQ_CPB` configuration option) and the global 711``boost`` knob is present regardless. Thus it is always possible use the 712``boost`` knob instead of the ``cpb`` one which is highly recommended, as that 713is more consistent with what all of the other systems do (and the ``cpb`` knob 714may not be supported any more in the future). 715 716The ``cpb`` knob is never present for any processors without the underlying 717hardware feature (e.g. all Intel ones), even if the 718:c:macro:`CONFIG_X86_ACPI_CPUFREQ_CPB` configuration option is set. 719 720 721References 722========== 723 724.. [1] Jonathan Corbet, *Per-entity load tracking*, 725 https://lwn.net/Articles/531853/ 726