Re: [PATCH] mm: Implement swap prefetching tweaks

[Con Kolivas]

On Saturday 11 March 2006 16:33, Mike Galbraith wrote:
> On Sat, 2006-03-11 at 14:50 +1100, Con Kolivas wrote:
> > On Saturday 11 March 2006 09:35, Andrew Morton wrote:
> > > Con Kolivas <kernel@xxxxxxxxxxx> wrote:
> > > > +	/*
> > > > +	 * get_page_state is super expensive so we only perform it every
> > > > +	 * SWAP_CLUSTER_MAX prefetched_pages.
> > >
> > > nr_running() is similarly expensive btw.
> >
> > Yes which is why I do it just as infrequently as get_page_state.
> >
> > > > 	 * We also test if we're the only
> > > > +	 * task running anywhere. We want to have as little impact on all
> > > > +	 * resources (cpu, disk, bus etc). As this iterates over every cpu
> > > > +	 * we measure this infrequently.
> > > > +	 */
> > > > +	if (!(sp_stat.prefetched_pages % SWAP_CLUSTER_MAX)) {
> > > > +		unsigned long cpuload = nr_running();
> > > > +
> > > > +		if (cpuload > 1)
> > > > +			goto out;
> > >
> > > Sorry, this is just wrong.  If swap prefetch is useful then it's also
> > > useful if some task happens to be sitting over in the corner
> > > calculating pi.
> > >
> > > What's the actual problem here?  Someone's 3d game went blippy?  Why? 
> > > How much?  Are we missing a cond_resched()?
> >
> > No, it's pretty easy to reproduce, kprefetchd sits there in
> > uninterruptible sleep with one cpu on SMP pegged at 100% iowait due to
> > it. This tends to have noticeable effects everywhere on HT or SMP. On UP
> > the yielding helped it but even then it still causes blips. How much?
> > Well to be honest it's noticeable a shipload. Running a game, any game,
> > that uses 100% (and most fancy games do) causes stuttering on audio,
> > pauses and so on. This is evident on linux native games, games under
> > emulators or qemu and so on. That iowait really hurts, and tweaking just
> > priority doesn't help it in any way.
>
> That doesn't really make sense to me.  If a task can trigger audio
> dropout and stalls by sleeping, we have a serious problem.  In your
> SMP/HT case, I'd start crawling over the load balancing code.  I can't
> see how trivial CPU with non-saturated IO can cause dropout in the UP
> case either.  Am I missing something?

Clearly you, me and everyone else is missing something. I see it with each 
task bound to one cpu with cpu affinity so it's not a balancing issue. Try it 
yourself if you can instead of not believing me. Get a big dd reader 
(virtually no cpu and all io wait sleep) on one cpu and try and play a game 
on the other cpu. It dies rectally.

Cheers,
Con
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