Re: [PATCH] mm: yield during swap prefetching

Andrew Morton
2006-03-07T23:24:47Z

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?