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 09:23:12 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Minchan Kim <minchan.kim@...il.com>
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 Thu, Sep 16, 2010 at 04:59:49PM +0900, Minchan Kim wrote:
> 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. 
> 

My bad, when the description was first written, both were affected and I
neglected to correct the description. I'm still debating with myself as
to whether the kswapd congestion_wait() should be wait_iff_congested()
or not.

Thanks

> "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
> 

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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