Processor boosting control - information for users - Quick guide for the impatient: -------------------- /sys/devices/system/cpu/cpufreq/boost controls the boost setting for the whole system. You can read and write that file with either "0" (boosting disabled) or "1" (boosting allowed). Reading or writing 1 does not mean that the system is boosting at this very moment, but only that the CPU _may_ raise the frequency at it's discretion. -------------------- Introduction ------------- Some CPUs support a functionality to raise the operating frequency of some cores in a multi-core package if certain conditions apply, mostly if the whole chip is not fully utilized and below it's intended thermal budget. The decision about boost disable/enable is made either at hardware (e.g. x86) or software (e.g ARM). On Intel CPUs this is called "Turbo Boost", AMD calls it "Turbo-Core", in technical documentation "Core performance boost". In Linux we use the term "boost" for convenience. Rationale for disable switch ---------------------------- Though the idea is to just give better performance without any user intervention, sometimes the need arises to disable this functionality. Most systems offer a switch in the (BIOS) firmware to disable the functionality at all, but a more fine-grained and dynamic control would be desirable: 1. While running benchmarks, reproducible results are important. Since the boosting functionality depends on the load of the whole package, single thread performance can vary. By explicitly disabling the boost functionality at least for the benchmark's run-time the system will run at a fixed frequency and results are reproducible again. 2. To examine the impact of the boosting functionality it is helpful to do tests with and without boosting. 3. Boosting means overclocking the processor, though under controlled conditions. By raising the frequency and the voltage the processor will consume more power than without the boosting, which may be undesirable for instance for mobile users. Disabling boosting may save power here, though this depends on the workload. User controlled switch ---------------------- To allow the user to toggle the boosting functionality, the cpufreq core driver exports a sysfs knob to enable or disable it. There is a file: /sys/devices/system/cpu/cpufreq/boost which can either read "0" (boosting disabled) or "1" (boosting enabled). The file is exported only when cpufreq driver supports boosting. Explicitly changing the permissions and writing to that file anyway will return EINVAL. On supported CPUs one can write either a "0" or a "1" into this file. This will either disable the boost functionality on all cores in the whole system (0) or will allow the software or hardware to boost at will (1). Writing a "1" does not explicitly boost the system, but just allows the CPU to boost at their discretion. Some implementations take external factors like the chip's temperature into account, so boosting once does not necessarily mean that it will occur every time even using the exact same software setup. AMD legacy cpb switch --------------------- The AMD powernow-k8 driver used to support a very similar switch to disable or enable the "Core Performance Boost" feature of some AMD CPUs. This switch was instantiated in each CPU's cpufreq directory (/sys/devices/system/cpu[0-9]*/cpufreq) and was called "cpb". Though the per CPU existence hints at a more fine grained control, the actual implementation only supported a system-global switch semantics, which was simply reflected into each CPU's file. Writing a 0 or 1 into it would pull the other CPUs to the same state. For compatibility reasons this file and its behavior is still supported on AMD CPUs, though it is now protected by a config switch (X86_ACPI_CPUFREQ_CPB). On Intel CPUs this file will never be created, even with the config option set. This functionality is considered legacy and will be removed in some future kernel version. More fine grained boosting control ---------------------------------- Technically it is possible to switch the boosting functionality at least on a per package basis, for some CPUs even per core. Currently the driver does not support it, but this may be implemented in the future.