linux-kernel - Re: [PATCH 8/8] vmscan: Kick flusher threads to clean pages when reclaim is encountering dirty pages

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Date:	Tue, 27 Jul 2010 22:55:25 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Mel Gorman <mel@....ul.ie>
Cc:	"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>,
	Christoph Hellwig <hch@...radead.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH 8/8] vmscan: Kick flusher threads to clean pages when
 reclaim is encountering dirty pages

On Tue, Jul 27, 2010 at 10:40:26PM +0800, Mel Gorman wrote:
> On Tue, Jul 27, 2010 at 10:34:23PM +0800, Wu Fengguang wrote:
> > > If you plan to keep wakeup_flusher_threads(), a simpler form may be
> > > sufficient, eg.
> > > 
> > >         laptop_mode ? 0 : (nr_dirty * 16)
> > 
> > This number is not sensitive because the writeback code may well round
> > it up to some more IO efficient value (currently 4MB). AFAIK the
> > nr_pages parameters passed by all existing flusher callers are some
> > rule-of-thumb value, and far from being an exact number.
> > 
> 
> I get that it's a rule of thumb but decided I would still pass in some value
> related to nr_dirty that was bounded in some manner.
> Currently, that bound is 4MB but maybe it should have been bound to
> MAX_WRITEBACK_PAGES (which is 4MB for x86, but could be anything
> depending on the base page size).

I see your worry about much bigger page size making

        vmscan batch size > writeback batch size

and it's a legitimate worry.

Thanks,
Fengguang
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