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

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Date:	Mon, 2 Aug 2010 13:38:29 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	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()

On Mon, Aug 2, 2010 at 1:13 PM, KOSAKI Motohiro
<kosaki.motohiro@...fujitsu.com> wrote:
>> 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.
>

Agree. Please include this explanation in description to make it kind
if you resent this patch.
Thanks


-- 
Kind regards,
Minchan Kim
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