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SubjectRe: Scheduler latency


On Thu, 22 Jan 1998, Richard Gooch wrote:
> > > > slower than the one in 2.0.25. Results of timing sched_yield(2):
> > > >
> > > > PPro/180 running 2.1.79:
> > > > SCHED_OTHER class: 56 microseconds
> > > > SCHED_FIFO class: 3 microseconds
> > > >
> > > > P133 running 2.0.25:
> > > > SCHED_OTHER class: 4 microseconds
> > > > SCHED_FIFO class: 4 microseconds
> >
> > 2.0.25 doesn't actually do anything that all when you run "sched_yield()".
> > There was a missing "need_resched = 1" in the 2.0.x series, so
> > sched_yield() actually didn't do anything at all. Which is what you want
> > if you want to benchmark how fast sched_yield() is, but it is NOT what you
> > want if you actually want to _do_ sched_yield()..
>
> Hm. I didn't realise that. Yesterday when I checked the sources I was
> looking at 2.0.33 :-/
> Note that both the machines I tested this on have two CPUs and have
> SMP=1. Running the test in a single CPU (SMP=0) PPro 200 running
> 2.0.31:
> SCHED_OTHER: 13 microseconds

Ok, 2.0.31 should have the sched_yield() fix already, so now the numbers
are ok. Can you also test a SMP-2.0.31 in order to be able to compare
validly against the 2.1.79 case? It is certainly possible that this has
gotten slower, but the 2.0.25 numbers aren't something we can compare
against (essentially, all kernels before 2.0.31 are fundamentally broken
wrt sched_yield()).

Linus


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