Re: [PATCH] mm: Implement swap prefetching tweaks

[Peter Williams <pwil3058@bigpond.net.au>]

Con Kolivas wrote:
> On Saturday 11 March 2006 10:11, Peter Williams wrote:
>
> > 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.
> > >
> > >
> > > > 	 * 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.
> >
> > On SMP systems, something based on the run queues' raw_weighted_load
> > fields (comes with smpnice patch) might be more useful than nr_running()
> > as it contains information about the priority of the running tasks.
> > Perhaps (raw_weighted_load() > SCHED_LOAD_SCALE) or some variation,
> > where raw_weighted_load() is the sum of that field for all CPUs) would
> > suffice.  It would mean "there's more than the equivalent of one nice==0
> > task running" and shouldn't be any more expensive than nr_running().
> > Dividing SCHED_LOAD_SCALE by some number would be an obvious variation
> > to try as would taking into account this process's contribution to the
> > weighted load.
> >
> > Also if this was useful there's no real reason that raw_weighted_load
> > couldn't be made available on non SMP systems as well as SMP ones.
>
>
> That does seem reasonable, but I'm looking at total system load, not per 
> runqueue. So a global_weighted_load() function would be required to return 
> that.

Yes. That's why I said "something based on".

> Because despite what anyone seems to want to believe, reading from disk 
> hurts. Why it hurts so much I'm not really sure, but it's not a SCSI vs IDE 
> with or without DMA issue. It's not about tweaking parameters. It doesn't 
> seem to be only about cpu cycles. This is not a mistuned system that it 
> happens on. It just plain hurts if we do lots of disk i/o, perhaps it's 
> saturating the bus or something. Whatever it is, as much as I'd _like_ swap 
> prefetch to just keep working quietly at ultra ultra low priority, the disk 
> reads that swap prefetch does are not innocuous so I really do want them to 
> only be done when nothing else wants cpu.

Would you like to try a prototype version of the soft caps patch I'm 
working on to see if it will help?

Peter
--
Peter Williams                                   pwil3058@bigpond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
 -- Ambrose Bierce
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