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

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Date:	Sun, 1 Aug 2010 22:41:17 +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()

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. 

> 
> If the system 512MB memory, DEF_PRIORITY mean 128kB scan and It takes 4096
> shrink_page_list() calls to scan 128kB (i.e. 128kB/32=4096) memory.
> 4096 times 0.1sec stall makes crazy insane long stall. That shouldn't.

128K / (4K * SWAP_CLUSTER_MAX) = 1

> 
> In the other hand, this synchronous lumpy reclaim donesn't need this
> congestion_wait() at all. shrink_page_list(PAGEOUT_IO_SYNC) cause to
> call wait_on_page_writeback() and it provide sufficient waiting.

Absolutely I agree on you. 

> 
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> Reviewed-by: Wu Fengguang <fengguang.wu@...el.com>
Reviewed-by: Minchan Kim <minchan.kim@...il.com>

-- 
Kind regards,
Minchan Kim
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