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Re: [PATCH 6/7] mm: vmscan: Throttle reclaim if encountering too many di

To: Mel Gorman <mgorman@xxxxxxx>
Subject: Re: [PATCH 6/7] mm: vmscan: Throttle reclaim if encountering too many dirty pages under writeback
From: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Date: Thu, 18 Aug 2011 16:54:28 -0700
Cc: Linux-MM <linux-mm@xxxxxxxxx>, LKML <linux-kernel@xxxxxxxxxxxxxxx>, XFS <xfs@xxxxxxxxxxx>, Dave Chinner <david@xxxxxxxxxxxxx>, Christoph Hellwig <hch@xxxxxxxxxxxxx>, Johannes Weiner <jweiner@xxxxxxxxxx>, Wu Fengguang <fengguang.wu@xxxxxxxxx>, Jan Kara <jack@xxxxxxx>, Rik van Riel <riel@xxxxxxxxxx>, Minchan Kim <minchan.kim@xxxxxxxxx>
In-reply-to: <1312973240-32576-7-git-send-email-mgorman@xxxxxxx>
References: <1312973240-32576-1-git-send-email-mgorman@xxxxxxx> <1312973240-32576-7-git-send-email-mgorman@xxxxxxx>
On Wed, 10 Aug 2011 11:47:19 +0100
Mel Gorman <mgorman@xxxxxxx> 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?

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