I was working on the Linux CFS scheduler and came across this tuneable parameter called sched_wakeup_granularity_ns. The use of this goes as follows as described in this paper.
This parameter controls the wake-up latency of a task, i.e., the amount of time it must lapse before it can preempt the current task.
Further looking up on this parameter from here :
If the difference between the virtual run time of current running process and the virtual run time of preempting process is bigger than the virtual run time of sched_wakeup_granularity_ns (here, transfer sched_wakeup_granularity_ns to a virtual run time with preempting process’s weight), the preemption happens.
The default value of this in my system is 2.5 milli seconds.
My question : If a process with a deadline <= 2.5 ms comes at time another low priority process is executing then, it won't get its chance to execute because of this minimum granularity and will always miss its deadline.
Am I right in my claim ? Why is the Linux kernel designed like that ?
Of course we can set this parameter to 0 ms and re-compile the kernel, but don't tasks with deadlines of 2.5 ms occur frequently in day to day life ? Video decoders, sound processing apps, and other latency critical workloads ?
