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clinuxlinux-kernelembedded-linuxprocfs

How to know linux scheduler time slice?

I'm looking for the value of the time slice (or quantum) of my Linux kernel.

Specific Questions:

  • Is there a /proc file which expose such an information ?
  • (Or) Is it well-defined in the Linux header of my distributions ?
  • (Or) Is there a C function of the Linux API (maybe sysinfo) that expose this value ?
Solution

  • The quantum allocated for a particular process may vary:

    You can tune "slice" by adjusting sched_latency_ns and sched_min_granularity_ns, but note that "slice" is not a fixed quantum. Also note that CFS preemption decisions are based upon instantaneous state. A task may have received a full (variable) "slice" of CPU time, but preemption will be triggered only if a more deserving task is available, so a "slice" is not the "max uninterrupted CPU time" that you may expect it to be.. but it is somewhat similar.

    This is because the Completely Fair Scheduler, the default Linux scheduler, assigns a proportion of the processor to a process rather than a fixed timeslice. That means the timeslice for each process is proportional to the current load and weighted by the process' priority value.

    For special-purpose realtime processes which use SCHED_RR, the default timeslice is defined in the Linux kernel as RR_TIMESLICE in include/linux/sched/rt.h.

    /*
 * default timeslice is 100 msecs (used only for SCHED_RR tasks).
 * Timeslices get refilled after they expire.
 */
#define RR_TIMESLICE            (100 * HZ / 1000)

    You can use sched_rr_get_interval() to get the SCHED_RR interval for a specific SCHED_RR process.