Re: [PATCH] mm: Implement swap prefetching tweaks

[Con Kolivas]

On Saturday 11 March 2006 10:11, Peter Williams wrote:
> 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.
> >
> >>	 * 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. 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.

Cheers,
Con
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