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

[Daniel J Blueman]

On Wed, May 14, 2008 at 9:26 AM, Jens Axboe <jens.axboe@xxxxxxxxxx> wrote:
> On Wed, May 14 2008, Daniel J Blueman wrote:
>> >  > > >  > >  > On Sun, 2008-05-11 at 14:14 +0100, Daniel J Blueman wrote:
>> >  > > >  > >  > > I've been experiencing this for a while also; an almost 50% regression
>> >  > > >  > >  > > is seen for single-process reads (ie sync) if slice_idle is 1ms or
>> >  > > >  > >  > > more (eg default of 8) [1], which seems phenomenal.
>> >  > > >  > >  > >
>> >  > > >  > >  > > Jens, is this the expected price to pay for optimal busy-spindle
>> >  > > >  > >  > > scheduling, a design issue, bug or am I missing something totally?
>> [snip]
>> >  > They seem to start out the same, but then CFQ gets interrupted by a
>> >  > timer unplug (which is also odd) and after that the request size drops.
>> >  > On most devices you don't notice, but some are fairly picky about
>> >  > request sizes. The end result is that CFQ has an average dispatch
>> >  > request size of 142kb, where AS is more than double that at 306kb. I'll
>> >  > need to analyze the data and look at the code a bit more to see WHY this
>> >  > happens.
>> >
>> >  Here's a test patch, I think we get into this situation due to CFQ being
>> >  a bit too eager to start queuing again. Not tested, I'll need to spend
>> >  some testing time on this. But I'd appreciate some feedback on whether
>> >  this changes the situation! The final patch will be a little more
>> >  involved.
>> >
>> >  diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
>> >  index b399c62..ebd8ce2 100644
>> >  --- a/block/cfq-iosched.c
>> >  +++ b/block/cfq-iosched.c
>> >  @@ -1775,18 +1775,8 @@ cfq_rq_enqueued(struct cfq_data *cfqd, struct cfq_queue *cfqq,
>> >
>> >         cic->last_request_pos = rq->sector + rq->nr_sectors;
>> >
>> >  -       if (cfqq == cfqd->active_queue) {
>> >  -               /*
>> >  -                * if we are waiting for a request for this queue, let it rip
>> >  -                * immediately and flag that we must not expire this queue
>> >  -                * just now
>> >  -                */
>> >  -               if (cfq_cfqq_wait_request(cfqq)) {
>> >  -                       cfq_mark_cfqq_must_dispatch(cfqq);
>> >  -                       del_timer(&cfqd->idle_slice_timer);
>> >  -                       blk_start_queueing(cfqd->queue);
>> >  -               }
>> >  -       } else if (cfq_should_preempt(cfqd, cfqq, rq)) {
>> >  +       if ((cfqq != cfqd->active_queue) &&
>> >  +                  cfq_should_preempt(cfqd, cfqq, rq)) {
>> >                 /*
>> >                  * not the active queue - expire current slice if it is
>> >                  * idle and has expired it's mean thinktime or this new queue
>>
>> I find this does address the issue (both with 64KB stride dd and
>> hdparm -t; presumably the requests getting merged). Tested on
>> 2.6.26-rc2 on Ubuntu HH 804 x86-64, with slice_idle defaulting to 8
>> and AHCI on ICH9; disk is ST3320613AS.
>>
>> Blktrace profiles from 'dd if=/dev/sda of=/dev/null bs=64k count=1000' are at:
>>
>> http://quora.org/blktrace-profiles.tar.bz2
>
> Goodie! I think the below patch is better - we do want to schedule the
> queue immediately, but we do not want to interrupt the queuer. So just
> kick the workqueue handling of the queue instead of entering the
> dispatcher directly. Can you test this one as well? Thanks!
>
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index f4e1006..e8c1941 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -1107,7 +1107,6 @@ static int cfq_dispatch_requests(struct request_queue *q, int force)
>
>                cfq_clear_cfqq_must_dispatch(cfqq);
>                cfq_clear_cfqq_wait_request(cfqq);
> -               del_timer(&cfqd->idle_slice_timer);
>
>                dispatched += __cfq_dispatch_requests(cfqd, cfqq, max_dispatch);
>        }
> @@ -1769,15 +1768,9 @@ cfq_rq_enqueued(struct cfq_data *cfqd, struct cfq_queue *cfqq,
>        cic->last_request_pos = rq->sector + rq->nr_sectors;
>
>        if (cfqq == cfqd->active_queue) {
> -               /*
> -                * if we are waiting for a request for this queue, let it rip
> -                * immediately and flag that we must not expire this queue
> -                * just now
> -                */
>                if (cfq_cfqq_wait_request(cfqq)) {
> -                       cfq_mark_cfqq_must_dispatch(cfqq);
>                        del_timer(&cfqd->idle_slice_timer);
> -                       blk_start_queueing(cfqd->queue);
> +                       kblockd_schedule_work(&cfqd->unplug_work);
>                }
>        } else if (cfq_should_preempt(cfqd, cfqq, rq)) {
>                /*
> @@ -1787,7 +1780,7 @@ 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);
> +               kblockd_schedule_work(&cfqd->unplug_work);
>        }
>  }

Applied on top of 2.6.26-rc2, I get platter-speed (118MB/s) with 'dd
if=/dev/sda of=/dev/null bs=64k' and 'hdparm -t', so looks good.
Identical testing without the patch (ie pure mainline) consistently
yields 65MB/s.

Blktrace profile at:

http://quora.org/blktrace-profiles-2.tar.bz2

I'll check for performance regressions with postmark on XFS; anything
else worth running while I've got this in hand?

Daniel
-- 
Daniel J Blueman
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