Re: tiobench read 50% regression with 2.6.30-rc1

["Zhang, Yanmin" <yanmin_zhang@linux.intel.com>]

On Tue, 2009-04-14 at 14:14 +0200, Jens Axboe wrote:
> On Fri, Apr 10 2009, Zhang, Yanmin wrote:
> > On Thu, 2009-04-09 at 11:57 +0200, Jens Axboe wrote:
> > > On Thu, Apr 09 2009, Zhang, Yanmin wrote:
> > > > Comparing with 2.6.29's result, tiobench (read) has about 50% regression
> > > > with 2.6.30-rc1 on all my machines. Bisect down to below patch.
> > > > 
> > > > b029195dda0129b427c6e579a3bb3ae752da3a93 is first bad commit
> > > > commit b029195dda0129b427c6e579a3bb3ae752da3a93
> > > > Author: Jens Axboe <jens.axboe@oracle.com>
> > > > Date:   Tue Apr 7 11:38:31 2009 +0200

> > > 
> > > It's not a huge surprise that we regressed there. I'll get this fixed up
> > > next week. Can you I talk you into trying to change the 'quantum' sysfs
> > > variable for the drive? It's in /sys/block/xxx/queue/iosched where xxx
> > > is your drive(s). It's set to 4, if you could try progressively larger
> > > settings and retest, that would help get things started.
> > I tried 4,8,16,64,128 and didn't find result difference.
> 
> Can you try with this patch?
The new patch does fix it.


> 
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index a4809de..66f00e5 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -1905,10 +1905,17 @@ cfq_rq_enqueued(struct cfq_data *cfqd, struct cfq_queue *cfqq,
>  		 * Remember that we saw a request from this process, but
>  		 * don't start queuing just yet. Otherwise we risk seeing lots
>  		 * of tiny requests, because we disrupt the normal plugging
> -		 * and merging.
> +		 * and merging. If the request is already larger than a single
> +		 * page, let it rip immediately. For that case we assume that
> +		 * merging is already done.
>  		 */
> -		if (cfq_cfqq_wait_request(cfqq))
> +		if (cfq_cfqq_wait_request(cfqq)) {
> +			if (blk_rq_bytes(rq) > PAGE_CACHE_SIZE) {
> +				del_timer(&cfqd->idle_slice_timer);
> +				blk_start_queueing(cfqd->queue);
> +			}
>  			cfq_mark_cfqq_must_dispatch(cfqq);
> +		}
>  	} else if (cfq_should_preempt(cfqd, cfqq, rq)) {
>  		/*
>  		 * not the active queue - expire current slice if it is
>