set_dma_latency

set_dma_latency

Overview

You can use the set_dma_latency tool to set the value of the /dev/cpu_dma_latency file. This allows us to control or disable the use of P and C states on the processor.

The ability to control C and P states is useful during metrics evaluation as entering and leaving C and P states incurs some overheads, which can inject noise into the metrics results.

Detail

The program takes a single 32-bit microsecond argument, opens the /dev/cpu_dma_latency file, and writes the value to that file as a 32 bit value. The program then sleeps, as the file must be kept open for the written value to continue to have effect.

Upon success the program prints its PID on stdout.

Upon failure the program returns with a non-zero return code.

To terminate the program, and thus close /dev/cpu_dma_latency and restore the original settings, send the program a SIGTERM. This can be achieved using kill(2) and the PID returned by the program.

Normally you must execute the program with root privileges to allow the program to write to the /dev/cpu_dma_latency file.

Notes

If you use this program to reduce CPU power and frequency variations, you might also want to investigate disabling 'turbo mode'.

If your system is using the intel_pstate driver, this can be achieved by use of the /sys/devices/system/cpu/intel_pstate/no_turbo file.

References

See the Linux kernel pm qos documentation for additional information about /dev/cpu_dma_latency and other related features.