Re: performance "regression" in cfq compared to anticipatory, deadline and noop

["Daniel J Blueman" <daniel.blueman@gmail.com>]

On Fri, May 16, 2008 at 8:57 AM, Jens Axboe <jens.axboe@oracle.com> wrote:
> On Fri, May 16 2008, Jens Axboe wrote:
>> On Fri, May 16 2008, Fabio Checconi wrote:
>> > > From: Jens Axboe <jens.axboe@oracle.com>
>> > > Date: Fri, May 16, 2008 08:40:03AM +0200
>> > >
>> > ...
>> > > I think we can improve this further without getting too involved. If a
>> > > 2nd request is seen in cfq_rq_enqueued(), then DO schedule a dispatch
>> > > since this likely means that we wont be doing more merges on the first
>> > > one.
>> > >
>> >
>> > But isn't there the risk that even the second request would be
>> > dispatched, while it still could have grown?
>>
>> Certainly, you'd only want to dispatch the first request. Ideally we'd
>> just get rid of this logic of 'did empty dispatch round' and only
>> dispatch requests once merging is done, it's basically the wrong thing
>> to do to make it visible to the io scheduler so soon. Well of course
>> even more ideally we'd always get big requests submitted, but
>> unfortunately many producers aren't that nice.
>>
>> The per-process plugging actually solves this nicely, since we do the
>> merging outside of the io scheduler. Perhaps just not dispatch on a
>> plugged queue would help a bit. I'm somewhat against this principle of
>> messing too much with dispatch logic in the schedulers, it'd be nicer to
>> solve this higher up.
>
> Something like this...
>
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index 5dfb7b9..5ab1a17 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -1775,6 +1775,9 @@ cfq_rq_enqueued(struct cfq_data *cfqd, struct cfq_queue *cfqq,
>
>        cic->last_request_pos = rq->sector + rq->nr_sectors;
>
> +       if (blk_queue_plugged(cfqd->queue))
> +               return;
> +
>        if (cfqq == cfqd->active_queue) {
>                /*
>                 * if we are waiting for a request for this queue, let it rip
> @@ -1784,7 +1787,7 @@ cfq_rq_enqueued(struct cfq_data *cfqd, struct cfq_queue *cfqq,
>                if (cfq_cfqq_wait_request(cfqq)) {
>                        cfq_mark_cfqq_must_dispatch(cfqq);
>                        del_timer(&cfqd->idle_slice_timer);
> -                       blk_start_queueing(cfqd->queue);
> +                       cfq_schedule_dispatch(cfqd);
>                }
>        } else if (cfq_should_preempt(cfqd, cfqq, rq)) {
>                /*
> @@ -1794,7 +1797,7 @@ cfq_rq_enqueued(struct cfq_data *cfqd, struct cfq_queue *cfqq,
>                 */
>                cfq_preempt_queue(cfqd, cfqq);
>                cfq_mark_cfqq_must_dispatch(cfqq);
> -               blk_start_queueing(cfqd->queue);
> +               cfq_schedule_dispatch(cfqd);
>        }
>  }
>
> @@ -1997,11 +2000,10 @@ static void cfq_kick_queue(struct work_struct *work)
>        struct cfq_data *cfqd =
>                container_of(work, struct cfq_data, unplug_work);
>        struct request_queue *q = cfqd->queue;
> -       unsigned long flags;
>
> -       spin_lock_irqsave(q->queue_lock, flags);
> +       spin_lock_irq(q->queue_lock);
>        blk_start_queueing(q);
> -       spin_unlock_irqrestore(q->queue_lock, flags);
> +       spin_unlock_irq(q->queue_lock);
>  }
>
>  /*

Platter speed at 64KB stride, but 16% (101MB/s) less performance at
4KB stride - perhaps merging isn't quite right?

Both traces at http://quora.org/blktrace-profiles-3.tar.bz2 ; let me
know if you'd like me to test Fabio's patch still.

Thanks,
  Daniel
-- 
Daniel J Blueman