linux-kernel - Re: [PATCH 2/3] writeback: Record if the congestion was unnecessary

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Message-ID: <20100826182904.GC6805@cmpxchg.org>
Date:	Thu, 26 Aug 2010 20:29:04 +0200
From:	Johannes Weiner <hannes@...xchg.org>
To:	Mel Gorman <mel@....ul.ie>
Cc:	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com>,
	Wu Fengguang <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] writeback: Record if the congestion was unnecessary

On Thu, Aug 26, 2010 at 04:14:15PM +0100, Mel Gorman wrote:
> If congestion_wait() is called when there is no congestion, the caller
> will wait for the full timeout. This can cause unreasonable and
> unnecessary stalls. There are a number of potential modifications that
> could be made to wake sleepers but this patch measures how serious the
> problem is. It keeps count of how many congested BDIs there are. If
> congestion_wait() is called with no BDIs congested, the tracepoint will
> record that the wait was unnecessary.

I am not convinced that unnecessary is the right word.  On a workload
without any IO (i.e. no congestion_wait() necessary, ever), I noticed
the VM regressing both in time and in reclaiming the right pages when
simply removing congestion_wait() from the direct reclaim paths (the
one in __alloc_pages_slowpath and the other one in
do_try_to_free_pages).

So just being stupid and waiting for the timeout in direct reclaim
while kswapd can make progress seemed to do a better job for that
load.

I can not exactly pinpoint the reason for that behaviour, it would be
nice if somebody had an idea.

So personally I think it's a good idea to get an insight on the use of
congestion_wait() [patch 1] but I don't agree with changing its
behaviour just yet, or judging its usefulness solely on whether it
correctly waits for bdi congestion.
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