Really hard subject. Perfect fairness requires 0 latency - which with a
CPU only being able to run one thing at a time is impossible. So what
latency ends up being is a measure for the convergence towards fairness.
Anyway - 200ms isn't too weird depending on the circumstances. We start
out with a 20ms latency for UP, we then multiply with 1+log2(nr_cpus)
which in say a quad core machine ends up with 60ms. That ought to mean
that under light load the max latency should not exceed twice that
(basically a consequence of the Nyquist-Shannon sampling theorem IIRC).
Now, if you get get under some load (by default: nr_running > 5) the
doing much (and after having looked up the original email I see its a
eeeeeeeee atom - which is dual cpu iirc, so that yields 40ms default) -
so 200 is definately on the high side.
What you can do to investigate this, is use the sched_wakeup tracer from
ftrace, that should give a function trace of the highest wakeup latency
showing what the kernel is doing.
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