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SubjectRe: [patch 0/9] mutex subsystem, -V4
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On Mon, 2005-12-26 at 18:15 +0100, Mike Galbraith wrote:
> At 03:11 AM 12/26/2005 -0800, Andrew Morton wrote:
> >Arjan van de Ven <arjan@infradead.org> wrote:
> > >
> > >
> > > > hm. 16 CPUs hitting the same semaphore at great arrival rates. The cost
> > > > of a short spin is much less than the cost of a sleep/wakeup. The
> > machine
> > > > was doing 100,000 - 200,000 context switches per second.
> > >
> > > interesting.. this might be a good indication that a "spin a bit first"
> > > mutex slowpath for some locks might be worth implementing...
> >
> >If we see a workload which is triggering such high context switch rates,
> >maybe. But I don't think we've seen any such for a long time.
>
> Hmm. Is there a real workload where such a high context switch rate is
> necessary? Every time I've seen a high (100,000 - 200,000 is beyond absurd
> on my little box, but...) context switch rate, it's been because something
> sucked.

I can trivially produce 20K per second on my little sub Ghz box so 100K
on a busy server is certainly plausible. Especially if for the purposes
of this discussion we are also worried about -rt + IRQ threading where
each IRQ costs two context switches (more if it raises a softirq).

Lee

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