linux-kernel - Re: [PATCH] vmscan: synchronous lumpy reclaim don't call congestion

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Message-Id: <20100802131016.4F7D.A69D9226@jp.fujitsu.com>
Date:	Mon,  2 Aug 2010 13:13:21 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	kosaki.motohiro@...fujitsu.com,
	Wu Fengguang <fengguang.wu@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>, Andy Whitcroft <apw@...dowen.org>,
	Rik van Riel <riel@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Dave Chinner <david@...morbit.com>,
	Chris Mason <chris.mason@...cle.com>,
	Nick Piggin <npiggin@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andreas Mohr <andi@...as.de>, Bill Davidsen <davidsen@....com>,
	Ben Gamari <bgamari.foss@...il.com>
Subject: Re: [PATCH] vmscan: synchronous lumpy reclaim don't call congestion_wait()

> Hi KOSAKI, 
> 
> On Sun, Aug 01, 2010 at 06:12:47PM +0900, KOSAKI Motohiro wrote:
> > rebased onto Wu's patch
> > 
> > ----------------------------------------------
> > From 35772ad03e202c1c9a2252de3a9d3715e30d180f Mon Sep 17 00:00:00 2001
> > From: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> > Date: Sun, 1 Aug 2010 17:23:41 +0900
> > Subject: [PATCH] vmscan: synchronous lumpy reclaim don't call congestion_wait()
> > 
> > congestion_wait() mean "waiting for number of requests in IO queue is
> > under congestion threshold".
> > That said, if the system have plenty dirty pages, flusher thread push
> > new request to IO queue conteniously. So, IO queue are not cleared
> > congestion status for a long time. thus, congestion_wait(HZ/10) is
> > almostly equivalent schedule_timeout(HZ/10).
> Just a nitpick. 
> Why is it a problem?
> HZ/10 is upper bound we intended.  If is is rahter high, we can low it. 
> But totally I agree on this patch. It would be better to remove it 
> than lowing. 

because all of _unnecessary_ sleep is evil. the problem is, congestion_wait()
mean "wait until queue congestion will be cleared, iow, wait all of IO". 
but we want to wait until _my_ IO finished.

So, if flusher thread conteniously push new IO into the queue, that makes
big difference.

Thanks.


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