1Processor boosting control 2 3 - information for users - 4 5Quick guide for the impatient: 6-------------------- 7/sys/devices/system/cpu/cpufreq/boost 8controls the boost setting for the whole system. You can read and write 9that file with either "0" (boosting disabled) or "1" (boosting allowed). 10Reading or writing 1 does not mean that the system is boosting at this 11very moment, but only that the CPU _may_ raise the frequency at it's 12discretion. 13-------------------- 14 15Introduction 16------------- 17Some CPUs support a functionality to raise the operating frequency of 18some cores in a multi-core package if certain conditions apply, mostly 19if the whole chip is not fully utilized and below it's intended thermal 20budget. The decision about boost disable/enable is made either at hardware 21(e.g. x86) or software (e.g ARM). 22On Intel CPUs this is called "Turbo Boost", AMD calls it "Turbo-Core", 23in technical documentation "Core performance boost". In Linux we use 24the term "boost" for convenience. 25 26Rationale for disable switch 27---------------------------- 28 29Though the idea is to just give better performance without any user 30intervention, sometimes the need arises to disable this functionality. 31Most systems offer a switch in the (BIOS) firmware to disable the 32functionality at all, but a more fine-grained and dynamic control would 33be desirable: 341. 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. 392. To examine the impact of the boosting functionality it is helpful 40 to do tests with and without boosting. 413. 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 48User controlled switch 49---------------------- 50 51To allow the user to toggle the boosting functionality, the cpufreq core 52driver exports a sysfs knob to enable or disable it. There is a file: 53/sys/devices/system/cpu/cpufreq/boost 54which can either read "0" (boosting disabled) or "1" (boosting enabled). 55The file is exported only when cpufreq driver supports boosting. 56Explicitly changing the permissions and writing to that file anyway will 57return EINVAL. 58 59On supported CPUs one can write either a "0" or a "1" into this file. 60This will either disable the boost functionality on all cores in the 61whole system (0) or will allow the software or hardware to boost at will 62(1). 63 64Writing a "1" does not explicitly boost the system, but just allows the 65CPU to boost at their discretion. Some implementations take external 66factors like the chip's temperature into account, so boosting once does 67not necessarily mean that it will occur every time even using the exact 68same software setup. 69 70 71AMD legacy cpb switch 72--------------------- 73The AMD powernow-k8 driver used to support a very similar switch to 74disable or enable the "Core Performance Boost" feature of some AMD CPUs. 75This switch was instantiated in each CPU's cpufreq directory 76(/sys/devices/system/cpu[0-9]*/cpufreq) and was called "cpb". 77Though the per CPU existence hints at a more fine grained control, the 78actual implementation only supported a system-global switch semantics, 79which was simply reflected into each CPU's file. Writing a 0 or 1 into it 80would pull the other CPUs to the same state. 81For compatibility reasons this file and its behavior is still supported 82on 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, 84even with the config option set. 85This functionality is considered legacy and will be removed in some future 86kernel version. 87 88More fine grained boosting control 89---------------------------------- 90 91Technically it is possible to switch the boosting functionality at least 92on a per package basis, for some CPUs even per core. Currently the driver 93does not support it, but this may be implemented in the future. 94