On 2011-07-13 19:10, Matthew Wilcox wrote:
> On Tue, Jul 12, 2011 at 10:30:35PM +0200, Jens Axboe wrote:
>> It's probably the grouping, we need to do something about that. Does the
>> below patch make it behave as you expect?
>
> "something", absolutely. But there is benefit from doing some aggregation
> (we tried disabling it entirely with the "well-known OLTP benchmark" and
> performance went down).
Yep, that's why the current solution is somewhat middle of the road...
> Ideally we'd do something like "if the softirq is taking up more than 10%
> of a core, split the grouping". Do we have enough stats to do that kind
> of monitoring?
I don't think we have those stats, though it could/should be pulled from
the ksoftirqX threads. We could have some metric, ala
dest_cpu = get_group_completion_cpu(rq->cpu);
if (ksoftirqd_of(dest_cpu) >= 90% busy)
dest_cpu = rq->cpu;
to send things completely local to the submitter of the IO, IFF the
current CPU is close to running at full tilt.
--
Jens Axboe
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