linux-kernel - Re: [PATCH 7/8] writeback: sync old inodes first in background writeback

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Message-ID: <20100723122015.GA8210@localhost>
Date:	Fri, 23 Jul 2010 20:20:15 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Mel Gorman <mel@....ul.ie>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andreas Mohr <andi@...as.de>,
	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>, Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Minchan Kim <minchan.kim@...il.com>
Subject: Re: [PATCH 7/8] writeback: sync old inodes first in background
 writeback

> For the case of of a heavy dirtier (dd) and concurrent light dirtiers
> (some random processes), the light dirtiers won't be easily throttled.
> task_dirty_limit() handles that case well. It will give light dirtiers
> higher threshold than heavy dirtiers so that only the latter will be
> dirty throttled.

The caveat is, the real dirty throttling threshold is not exactly the
value specified by vm.dirty_ratio or vm.dirty_bytes. Instead it's some
value slightly lower than it. That real value differs for each process,
which is a nice trick to throttle heavy dirtiers first. If I remember
it right, that's invented by Peter and Andrew.

Thanks,
Fengguang
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