title: sched_wakeup_granularity_ns in CFS correctly designed or not?
body: <p>I was working on the Linux CFS scheduler and came across this tuneable parameter called <code>sched_wakeup_granularity_ns</code>. The use of this goes as follows as described in <a href="http://rouskas.csc.ncsu.edu/Publications/Conferences/ICC-SPS-2015.pdf" rel="nofollow noreferrer">this paper</a>.</p>
  
  <blockquote>
    <p>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.</p>
  </blockquote>
  
  <p>Further looking up on this parameter from <a href="https://www.systutorials.com/239998/sched_min_granularity_ns-sched_latency_ns-cfs-affect-timeslice-processes/" rel="nofollow noreferrer">here</a> :</p>
  
  <blockquote>
    <p>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.</p>
  </blockquote>
  
  <p>The default value of this in my system is 2.5 milli seconds.</p>
  
  <p>My question : If a process with a deadline &lt;= 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. </p>
  
  <p>Am I right in my claim ? Why is the Linux kernel designed like that ? </p>
  
  <p>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 ?</p>