Chapter 16. Controlling power management transitions

• Write a value to the /dev/cpu_dma_latency file to change the maximum response time for processes in microseconds and hold the file descriptor open until low latency is required.
• Reference the /dev/cpu_dma_latency file in an application or a script.

Prerequisites

• You have administrator privileges.

Procedure

• Specify latency tolerance by writing a 32-bit number that represents a maximum response time in microseconds in /dev/cpu_dma_latency and keep the file descriptor open through the low-latency operation. A value of 0 disables C-state completely.

  For example:

  import os
  import os.path
  import signal
  import sys
  if not os.path.exists('/dev/cpu_dma_latency'):
   	print("no PM QOS interface on this system!")
   	sys.exit(1)
  fd = os.open('/dev/cpu_dma_latency', os.O_WRONLY)
   	 os.write(fd, b'\0\0\0\0')
   	 print("Press ^C to close /dev/cpu_dma_latency and exit")
      signal.pause()
  except KeyboardInterrupt:
      print("closing /dev/cpu_dma_latency")
      os.close(fd)
      sys.exit(0)

  Note

  The Power Management Quality of Service interface (pm_qos) interface is only active while it has an open file descriptor. Therefore, any script or program you use to access /dev/cpu_dma_latency must hold the file open until power-state transitions are allowed.