Re: How how latent should non-preemptive scheduling be?

[Peter Zijlstra]

On Wed, 2008-09-17 at 14:54 -0700, Arjan van de Ven wrote:
> On Wed, 17 Sep 2008 22:48:55 +0100
> Sitsofe Wheeler <sitsofe@xxxxxxxxx> wrote:
> 
> > Arjan van de Ven wrote:
> > > this says you haven't done "make install" on the latencytop
> > > directory so it's not translating things for you.. can you do that
> > > please?
> > 
> > > Cause                                                Maximum
> > > 
> Percentage 
> 
> Scheduler: waiting for cpu                        208 msec         59.4 %
> 
> 
> you're rather CPU bound, and your process was woken up but didn't run for over 200 milliseconds..
> that sounds like a scheduler fairness issue!

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
expected latency starts to linearly grow with nr_running.

>From what I gather from the reply to this email the machine was not
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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