Re: [PATCH] mm: yield during swap prefetching

[Con Kolivas]

Andrew Morton writes:

> Con Kolivas <kernel@xxxxxxxxxxx> 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@xxxxxxxxxxx>
> > ---
> >  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.

Cheers,
Con

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