Documentation/cpu-freq/boost.txt

   1 Processor boosting control
   2
   3         - information for users -
   4
   5 Quick guide for the impatient:
   6 --------------------
   7 /sys/devices/system/cpu/cpufreq/boost
   8 controls the boost setting for the whole system. You can read and write
   9 that file with either "0" (boosting disabled) or "1" (boosting allowed).
  10 Reading or writing 1 does not mean that the system is boosting at this
  11 very moment, but only that the CPU _may_ raise the frequency at it's
  12 discretion.
  13 --------------------
  14
  15 Introduction
  16 -------------
  17 Some CPUs support a functionality to raise the operating frequency of
  18 some cores in a multi-core package if certain conditions apply, mostly
  19 if the whole chip is not fully utilized and below it's intended thermal
  20 budget. This is done without operating system control by a combination
  21 of hardware and firmware.
  22 On Intel CPUs this is called "Turbo Boost", AMD calls it "Turbo-Core",
  23 in technical documentation "Core performance boost". In Linux we use
  24 the term "boost" for convenience.
  25
  26 Rationale for disable switch
  27 ----------------------------
  28
  29 Though the idea is to just give better performance without any user
  30 intervention, sometimes the need arises to disable this functionality.
  31 Most systems offer a switch in the (BIOS) firmware to disable the
  32 functionality at all, but a more fine-grained and dynamic control would
  33 be desirable:
  34 1. While running benchmarks, reproducible results are important. Since
  35    the boosting functionality depends on the load of the whole package,
  36    single thread performance can vary. By explicitly disabling the boost
  37    functionality at least for the benchmark's run-time the system will run
  38    at a fixed frequency and results are reproducible again.
  39 2. To examine the impact of the boosting functionality it is helpful
  40    to do tests with and without boosting.
  41 3. Boosting means overclocking the processor, though under controlled
  42    conditions. By raising the frequency and the voltage the processor
  43    will consume more power than without the boosting, which may be
  44    undesirable for instance for mobile users. Disabling boosting may
  45    save power here, though this depends on the workload.
  46
  47
  48 User controlled switch
  49 ----------------------
  50
  51 To allow the user to toggle the boosting functionality, the acpi-cpufreq
  52 driver exports a sysfs knob to disable it. There is a file:
  53 /sys/devices/system/cpu/cpufreq/boost
  54 which can either read "0" (boosting disabled) or "1" (boosting enabled).
  55 Reading the file is always supported, even if the processor does not
  56 support boosting. In this case the file will be read-only and always
  57 reads as "0". Explicitly changing the permissions and writing to that
  58 file anyway will return EINVAL.
  59
  60 On supported CPUs one can write either a "0" or a "1" into this file.
  61 This will either disable the boost functionality on all cores in the
  62 whole system (0) or will allow the hardware to boost at will (1).
  63
  64 Writing a "1" does not explicitly boost the system, but just allows the
  65 CPU (and the firmware) to boost at their discretion. Some implementations
  66 take external factors like the chip's temperature into account, so
  67 boosting once does not necessarily mean that it will occur every time
  68 even using the exact same software setup.
  69
  70
  71 AMD legacy cpb switch
  72 ---------------------
  73 The AMD powernow-k8 driver used to support a very similar switch to
  74 disable or enable the "Core Performance Boost" feature of some AMD CPUs.
  75 This switch was instantiated in each CPU's cpufreq directory
  76 (/sys/devices/system/cpu[0-9]*/cpufreq) and was called "cpb".
  77 Though the per CPU existence hints at a more fine grained control, the
  78 actual implementation only supported a system-global switch semantics,
  79 which was simply reflected into each CPU's file. Writing a 0 or 1 into it
  80 would pull the other CPUs to the same state.
  81 For compatibility reasons this file and its behavior is still supported
  82 on AMD CPUs, though it is now protected by a config switch
  83 (X86_ACPI_CPUFREQ_CPB). On Intel CPUs this file will never be created,
  84 even with the config option set.
  85 This functionality is considered legacy and will be removed in some future
  86 kernel version.
  87
  88 More fine grained boosting control
  89 ----------------------------------
  90
  91 Technically it is possible to switch the boosting functionality at least
  92 on a per package basis, for some CPUs even per core. Currently the driver
  93 does not support it, but this may be implemented in the future.