Re: [PATCH]cfq-iosched: don't take requests with long distence as close

Zhang, Yanmin
2010-01-11T05:19:59Z

On Thu, 2010-01-07 at 09:30 -0500, Jeff Moyer wrote:
> Corrado Zoccolo <czoccolo@gmail.com> writes:
>
> > On Tue, Jan 5, 2010 at 10:16 PM, Jeff Moyer <jmoyer@redhat.com> wrote:
> >>
> >> For now, I'm leaning towards asking Jens to revert this. It may still
> >> be worth making sure that we don't merge a seeky queue with a non-seeky
> >> queue. I have a patch for that if folks are interested.
> > Jeff, can you send this patch to Yanmin, that is investigating a
> > regression apparently caused by excessive queue merge?
> >
> > http://lkml.org/lkml/2010/1/4/194
>
>
> You first have to back out Shaohua's patch, then apply this one.
Thanks for forwarding me the patches.
Actually, we found tiobench randread has about 20% regression since kernel
2.6.33-rc1, and fio randread has more than 40% regression.

>
> Cheers,
> Jeff
>
> cfq-iosched: don't allow merging with seeky queues
With your new patch applied on 2.6.33-rc1, I don't see improvement on
both tiobench and fio randread regression. I know unexpected merge/unmerge
is just one root cause of the regressions. A couple of other patches
are also related to them.

I also tried to apply both your patch and Corrado's patch at
http://lkml.org/lkml/2010/1/9/57. The result seems like the one when I just apply
Corrado's patch, with which regression almost disappears when low_latency=0. But
when low_latency=1, there is still about 25% regression.

>
> Shaohua Li noticed that cfq currently can merge with seeky queues, which
> causes unwanted merge/unmerge activity. We already know that the
> cur_cfqq is not seeky, so this patch just makes sure that the non-seeky
> queue is not merged with a seeky one.
>
> Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
>
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index 8df4fe5..3db9050 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -1677,6 +1677,10 @@ static inline int cfq_rq_close(struct cfq_data *cfqd, struct cfq_queue *cfqq,
> return cfq_dist_from_last(cfqd, rq) <= sdist;
> }
>
> +/*
> + * Search for a cfqq that is issuing non-seeky I/Os within the seek
> + * mean of the current cfqq.
> + */
> static struct cfq_queue *cfqq_close(struct cfq_data *cfqd,
> struct cfq_queue *cur_cfqq)
> {
> @@ -1701,7 +1705,14 @@ static struct cfq_queue *cfqq_close(struct cfq_data *cfqd,
> * will contain the closest sector.
> */
> __cfqq = rb_entry(parent, struct cfq_queue, p_node);
> - if (cfq_rq_close(cfqd, cur_cfqq, __cfqq->next_rq))
> + /*
> + * If the cfqq does not have enough seek samples, assume it is
> + * sequential until proven otherwise. If it is assumed that the
> + * queue is seeky first, then the close cooperator detection logic
> + * may never trigger as one queue strays further from the other(s).
> + */
> + if (cfq_rq_close(cfqd, cur_cfqq, __cfqq->next_rq) &&
> + (!sample_valid(__cfqq->seek_samples) || !CFQQ_SEEKY(__cfqq)))
> return __cfqq;
>
> if (blk_rq_pos(__cfqq->next_rq) < sector)
> @@ -1712,7 +1723,8 @@ static struct cfq_queue *cfqq_close(struct cfq_data *cfqd,
> return NULL;
>
> __cfqq = rb_entry(node, struct cfq_queue, p_node);
> - if (cfq_rq_close(cfqd, cur_cfqq, __cfqq->next_rq))
> + if (cfq_rq_close(cfqd, cur_cfqq, __cfqq->next_rq) &&
> + (!sample_valid(__cfqq->seek_samples) || !CFQQ_SEEKY(__cfqq)))
> return __cfqq;
>
> return NULL;
> --
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