Date: Thu, 18 Aug 2011 16:54:28 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Mel Gorman <mgorman@suse.de>
Cc: Linux-MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>,
        XFS
 <xfs@oss.sgi.com>, Dave Chinner <david@fromorbit.com>,
        Christoph Hellwig
 <hch@infradead.org>,
        Johannes Weiner <jweiner@redhat.com>,
        Wu Fengguang
 <fengguang.wu@intel.com>, Jan Kara <jack@suse.cz>,
        Rik van Riel
 <riel@redhat.com>, Minchan Kim <minchan.kim@gmail.com>
Subject: Re: [PATCH 6/7] mm: vmscan: Throttle reclaim if encountering too
 many dirty pages under writeback
Message-Id: <20110818165428.4f01a1b9.akpm@linux-foundation.org>
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	<1312973240-32576-7-git-send-email-mgorman@suse.de>
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On Wed, 10 Aug 2011 11:47:19 +0100
Mel Gorman <mgorman@suse.de> wrote:

> The percentage that must be in writeback depends on the priority. At
> default priority, all of them must be dirty. At DEF_PRIORITY-1, 50%
> of them must be, DEF_PRIORITY-2, 25% etc. i.e. as pressure increases
> the greater the likelihood the process will get throttled to allow
> the flusher threads to make some progress.

It'd be nice if the code comment were to capture this piece of implicit
arithmetic.  After all, it's a magic number and magic numbers should
stick out like sore thumbs.

And.. how do we know that the chosen magic numbers were optimal?