Based on kernel version 4.10.8. Page generated on 2017-04-01 14:42 EST.
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. The decision about boost disable/enable is made either at hardware 21 (e.g. x86) or software (e.g ARM). 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 cpufreq core 52 driver exports a sysfs knob to enable or 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 The file is exported only when cpufreq driver supports boosting. 56 Explicitly changing the permissions and writing to that file anyway will 57 return EINVAL. 58 59 On supported CPUs one can write either a "0" or a "1" into this file. 60 This will either disable the boost functionality on all cores in the 61 whole system (0) or will allow the software or hardware to boost at will 62 (1). 63 64 Writing a "1" does not explicitly boost the system, but just allows the 65 CPU to boost at their discretion. Some implementations take external 66 factors like the chip's temperature into account, so boosting once does 67 not necessarily mean that it will occur every time even using the exact 68 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.