Re: [PATCH] mm: Implement swap prefetching tweaks

[Mike Galbraith <efault@gmx.de>]

On Sat, 2006-03-11 at 16:50 +1100, Con Kolivas wrote:
> 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@kolivas.org> 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.

I said it didn't make sense to me, not that I didn't believe you.  If I
had a real SMP box, I would look into it, but all I have is HT.

If you're creating a lot of traffic, I can see it causing problems.  I
was under the impression that you were doing minimal IO and absolutely
trivial CPU.  That's what didn't make sense to me to be clear.

	-Mike

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