>  > > >  > >  > On Sun, 2008-05-11 at 14:14 +0100, Daniel J Blueman wrote:
quoted text
>  > > >  > >  > > 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]
quoted text
>  > 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

Let me know when you have an updated patch to test,
  Daniel
-- 
Daniel J Blueman
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