Re: [PATCH] mm: yield during swap prefetching

Con Kolivas
2006-03-07T23:32:17Z

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.

Cheers,
Con