linux-kernel - Re: performance regression from block merge

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Date:	Thu, 21 Dec 2006 21:18:18 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Andrew Morton <akpm@...l.org>
Cc:	linux-kernel@...r.kernel.org, Linus Torvalds <torvalds@...l.org>
Subject: Re: performance regression from block merge

On Thu, Dec 21 2006, Andrew Morton wrote:
> On Thu, 21 Dec 2006 20:47:41 +0100
> Jens Axboe <jens.axboe@...cle.com> wrote:
> 
> > On Thu, Dec 21 2006, Jens Axboe wrote:
> > > On Thu, Dec 21 2006, Andrew Morton wrote:
> > > > 
> > > > Jens, elapsed time for `mke2fs /dev/hdc5' with the anticipatory scheduler
> > > > (at least) has gone from nine seconds to sixty as a result of the recent
> > > > block merge.
> > > 
> > > About when? I'll double check and test here, I'm assuming you mean since
> > > 2.6.19?
> 
> In yesterday's merge.  I double-checked by testing yesterday's-Linus versus
> yesterday's-Linus-plus-yesterday's-git-block.  The latter was slow.
> 
> > Auch, brown paper bag time, I spotted an obvious typo in the recent
> > merge. Does this fix it? It should be safe to kill the ->special check,
> > but lets leave that for another time.
> > 
> > diff --git a/block/elevator.c b/block/elevator.c
> > index 62c7a30..536be74 100644
> > --- a/block/elevator.c
> > +++ b/block/elevator.c
> > @@ -82,7 +82,7 @@ inline int elv_rq_merge_ok(struct reques
> >  	/*
> >  	 * must be same device and not a special request
> >  	 */
> > -	if (rq->rq_disk != bio->bi_bdev->bd_disk || !rq->special)
> > +	if (rq->rq_disk != bio->bi_bdev->bd_disk || rq->special)
> >  		return 0;
> >  
> >  	if (!elv_iosched_allow_merge(rq, bio))
> > 
> 
> I shan't be near that machine for eight hours or so; shall test then.

I already verified it, that was the bug! I'll send it upstream.

-- 
Jens Axboe

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