Subject: Re: [PATCH] mm: Implement swap prefetching tweaks
From: Mike Galbraith <efault@gmx.de>
To: Con Kolivas <kernel@kolivas.org>
Cc: Andrew Morton <akpm@osdl.org>, linux-kernel@vger.kernel.org,
       ck@vds.kolivas.org
In-Reply-To: <200603111450.39305.kernel@kolivas.org>
References: <200603102054.20077.kernel@kolivas.org>
	 <20060310143545.74a9a92a.akpm@osdl.org>
	 <200603111450.39305.kernel@kolivas.org>
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Date: Sat, 11 Mar 2006 06:33:59 +0100
Message-Id: <1142055239.7819.47.camel@homer>
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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?

	-Mike

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