linux-kernel - Re: [PATCH]cfq-iosched: don't stop async queue with async requests pending

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Message-ID: <4B4E8590.8060403@cn.fujitsu.com>
Date:	Thu, 14 Jan 2010 10:46:40 +0800
From:	Gui Jianfeng <guijianfeng@...fujitsu.com>
To:	Shaohua Li <shaohua.li@...el.com>
CC:	jens.axboe@...cle.com, linux-kernel@...r.kernel.org,
	jmoyer@...hat.com, vgoyal@...hat.com, yanmin_zhang@...ux.intel.com
Subject: Re: [PATCH]cfq-iosched: don't stop async queue with async requests
 pending

Shaohua Li wrote:
> My SSD speed of direct write is about 80m/s, while I test page writeback,
> the speed can only go to 68m/s. Below patch fixes this.
> It appears we missused cfq_should_idle in cfq_may_dispatch. cfq_should_idle
> means a queue should idle because it's seekless sync queue or it's the last queue,
> which is to maintain service tree time slice. So it doesn't mean the
> last queue is always a sync queue. If the last queue is asyn queue,
> we definitely shouldn't stop dispatch requests because of pending async
> requests.
> 
> Signed-off-by: Shaohua Li <shaohua.li@...el.com>
> 
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index 918c7fd..8198079 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -2222,7 +2222,8 @@ static bool cfq_may_dispatch(struct cfq_data *cfqd, struct cfq_queue *cfqq)
>  	/*
>  	 * Drain async requests before we start sync IO
>  	 */
> -	if (cfq_should_idle(cfqd, cfqq) && cfqd->rq_in_driver[BLK_RW_ASYNC])
> +	if (cfq_cfqq_sync(cfqq) && cfq_should_idle(cfqd, cfqq)
> +		&& cfqd->rq_in_driver[BLK_RW_ASYNC])
>  		return false;

It seems only sync queue could open the idle window, so i don't think we really need this.

Thanks,
Gui

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