linux-kernel - Re: [PATCH 7/8] writeback: Do not sleep on the congestion queue if there are no congested BDIs

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Date:	Thu, 16 Sep 2010 16:59:49 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Mel Gorman <mel@....ul.ie>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org,
	Linux Kernel List <linux-kernel@...r.kernel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Subject: Re: [PATCH 7/8] writeback: Do not sleep on the congestion queue if
 there are no congested BDIs

On Wed, Sep 15, 2010 at 01:27:50PM +0100, Mel Gorman wrote:
> If congestion_wait() is called with no BDI congested, the caller will sleep
> for the full timeout and this may be an unnecessary sleep. This patch adds
> a wait_iff_congested() that checks congestion and only sleeps if a BDI is
> congested else, it calls cond_resched() to ensure the caller is not hogging
> the CPU longer than its quota but otherwise will not sleep.
> 
> This is aimed at reducing some of the major desktop stalls reported during
> IO. For example, while kswapd is operating, it calls congestion_wait()
> but it could just have been reclaiming clean page cache pages with no
> congestion. Without this patch, it would sleep for a full timeout but after
> this patch, it'll just call schedule() if it has been on the CPU too long.
> Similar logic applies to direct reclaimers that are not making enough
> progress.

I confused due to kswapd you mentioned.
This patch affects only direct reclaim.
Please, complete the description. 

"This patch affects direct reclaimer to reduce stall"
Otherwise, looks good to me. 

> 
> Signed-off-by: Mel Gorman <mel@....ul.ie>
Reviewed-by: Minchan Kim <minchan.kim@...il.com>

-- 
Kind regards,
Minchan Kim
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