> On Wed, May 14, 2008 at 9:26 AM, Jens Axboe <jens.axboe@oracle.com> 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
>
