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

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Message-ID: <4B9A3049.7010602@linux.vnet.ibm.com>
Date:	Fri, 12 Mar 2010 13:15:05 +0100
From:	Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com>
To:	Mel Gorman <mel@....ul.ie>
CC:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	Nick Piggin <npiggin@...e.de>,
	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

Mel Gorman wrote:
> On Fri, Mar 12, 2010 at 02:05:26AM -0500, Andrew Morton wrote:
>> On Fri, 12 Mar 2010 07:39:26 +0100 Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com> wrote:
>>
>>>
>>> Andrew Morton wrote:
>>>> On Mon,  8 Mar 2010 11:48:20 +0000
>>>> Mel Gorman <mel@....ul.ie> wrote:
[...]

>> If not, we broke it again.
>>
> 
> We were broken with respect to this in the first place. That
> cond_reched() is badly placed and waiting on congestion when congestion
> might not be involved is also a bit odd.
> 
> It's possible that Christian's specific problem would also be addressed
> by the following patch. Christian, willing to test?

Will is here, but no chance before monday/tuesday to get a free machine 
slot - I'll post results as soon as I get them.

> It still feels a bit unnatural though that the page allocator waits on
> congestion when what it really cares about is watermarks. Even if this
> patch works for Christian, I think it still has merit so will kick it a
> few more times.

In whatever way I can look at it watermark_wait should be supperior to 
congestion_wait. Because as Mel points out waiting for watermarks is 
what is semantically correct there.

If there eventually some day comes a solution without any of those waits 
I'm fine too - e.g. by closing whatever races we have and fixing that 
one context can never run into this in direct_reclaim:
1. free pages with try_to_free
2. not getting one in the subsequent get_page call

But as long as we have a wait - watermark waiting > congestion waiting 
(IMHO).

> ==== CUT HERE ====
> page-allocator: Attempt page allocation immediately after direct reclaim
[...]
-- 

Grüsse / regards, Christian Ehrhardt
IBM Linux Technology Center, System z Linux Performance
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