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

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Date:	Mon, 11 Jan 2010 13:20:09 +0800
From:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	Corrado Zoccolo <czoccolo@...il.com>,
	Shaohua Li <shaohua.li@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"jens.axboe@...cle.com" <jens.axboe@...cle.com>,
	"Zhang, Yanmin" <yanmin.zhang@...el.com>
Subject: Re: [PATCH]cfq-iosched: don't take requests with long distence as 
 close

On Thu, 2010-01-07 at 09:30 -0500, Jeff Moyer wrote:
> Corrado Zoccolo <czoccolo@...il.com> writes:
> 
> > On Tue, Jan 5, 2010 at 10:16 PM, Jeff Moyer <jmoyer@...hat.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@...hat.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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