linux-kernel - Re: [RFC PATCH 0/3] Avoid the use of congestion

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Message-Id: <20100311154124.e1e23900.akpm@linux-foundation.org>
Date:	Thu, 11 Mar 2010 15:41:24 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mel Gorman <mel@....ul.ie>
Cc:	linux-mm@...ck.org, Nick Piggin <npiggin@...e.de>,
	Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com>,
	Chris Mason <chris.mason@...cle.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/3] Avoid the use of congestion_wait under zone
 pressure

On Mon,  8 Mar 2010 11:48:20 +0000
Mel Gorman <mel@....ul.ie> wrote:

> Under memory pressure, the page allocator and kswapd can go to sleep using
> congestion_wait(). In two of these cases, it may not be the appropriate
> action as congestion may not be the problem.

clear_bdi_congested() is called each time a write completes and the
queue is below the congestion threshold.

So if the page allocator or kswapd call congestion_wait() against a
non-congested queue, they'll wake up on the very next write completion.

Hence the above-quoted claim seems to me to be a significant mis-analysis and
perhaps explains why the patchset didn't seem to help anything?
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