Re: [ck] Re: [PATCH] mm: yield during swap prefetching

Con Kolivas
2006-03-08T13:37:14Z

cc'ing Ingo...

On Wednesday 08 March 2006 10:32, Con Kolivas wrote:
> Andrew Morton writes:
> > Con Kolivas <kernel@kolivas.org> wrote:
> >> Swap prefetching doesn't use very much cpu but spends a lot of time
> >> waiting on disk in uninterruptible sleep. This means it won't get
> >> preempted often even at a low nice level since it is seen as sleeping
> >> most of the time. We want to minimise its cpu impact so yield where
> >> possible.
> >>
> >> Signed-off-by: Con Kolivas <kernel@kolivas.org>
> >> ---
> >> mm/swap_prefetch.c | 1 +
> >> 1 file changed, 1 insertion(+)
> >>
> >> Index: linux-2.6.15-ck5/mm/swap_prefetch.c
> >> ===================================================================
> >> --- linux-2.6.15-ck5.orig/mm/swap_prefetch.c 2006-03-02
> >> 14:00:46.000000000 +1100 +++
> >> linux-2.6.15-ck5/mm/swap_prefetch.c 2006-03-08 08:49:32.000000000 +1100
> >> @@ -421,6 +421,7 @@ static enum trickle_return trickle_swap(
> >>
> >> if (trickle_swap_cache_async(swp_entry, node) == TRICKLE_DELAY)
> >> break;
> >> + yield();
> >> }
> >>
> >> if (sp_stat.prefetched_pages) {
> >
> > yield() really sucks if there are a lot of runnable tasks. And the
> > amount of CPU which that thread uses isn't likely to matter anyway.
> >
> > I think it'd be better to just not do this. Perhaps alter the thread's
> > static priority instead? Does the scheduler have a knob which can be
> > used to disable a tasks's dynamic priority boost heuristic?
>
> We do have SCHED_BATCH but even that doesn't really have the desired
> effect. I know how much yield sucks and I actually want it to suck as much
> as yield does.

Thinking some more on this I wonder if SCHED_BATCH isn't a strong enough
scheduling hint if it's not suitable for such an application. Ingo do you
think we could make SCHED_BATCH tasks always wake up on the expired array?

Cheers,
Con