linux-kernel - Re: [RFC]cfq-iosched: fix a kbuild regression

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Message-ID: <20100317133019.GV5768@kernel.dk>
Date:	Wed, 17 Mar 2010 14:30:19 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	linux-kernel@...r.kernel.org, czoccolo@...il.com,
	vgoyal@...hat.com, jmoyer@...hat.com, guijianfeng@...fujitsu.com,
	alex.shi@...el.com
Subject: Re: [RFC]cfq-iosched: fix a kbuild regression

On Tue, Mar 16 2010, Shaohua Li wrote:
> Alex Shi reported a kbuild regression which is about 10% performance lost.
> He bisected to this commit: 3dde36ddea3e07dd025c4c1ba47edec91606fec0.
> The reason is cfqq_close() can't find close cooperator. If we store the seek
> distance to the value before the commit like below, the regression fully goes
> away. If this is too invasive, just changing the cfq_rq_close() for the
> !for_preempt is ok too.

Corrado, any objections to widening the seek threshold?

> 
> Reported-by: Alex Shi <alex.shi@...el.com>
> Signed-off-by: Shaohua Li <shaohua.li@...el.com>
> 
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index dee9d93..fcae456 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -46,7 +46,7 @@ static const int cfq_hist_divisor = 4;
>  #define CFQ_HW_QUEUE_MIN	(5)
>  #define CFQ_SERVICE_SHIFT       12
>  
> -#define CFQQ_SEEK_THR		(sector_t)(8 * 100)
> +#define CFQQ_SEEK_THR		(sector_t)(8 * 1024)
>  #define CFQQ_SECT_THR_NONROT	(sector_t)(2 * 32)
>  #define CFQQ_SEEKY(cfqq)	(hweight32(cfqq->seek_history) > 32/8)
>  

-- 
Jens Axboe

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