linux-kernel - Re: The performance and behaviour of the anti-fragmentation related patches

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Message-Id: <20070302102315.e0728a42.akpm@linux-foundation.org>
Date:	Fri, 2 Mar 2007 10:23:15 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Christoph Lameter <clameter@...r.sgi.com>
Cc:	Rik van Riel <riel@...hat.com>, Mel Gorman <mel@...net.ie>,
	npiggin@...e.de, mingo@...e.hu, jschopp@...tin.ibm.com,
	arjan@...radead.org, torvalds@...ux-foundation.org,
	mbligh@...igh.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: The performance and behaviour of the anti-fragmentation related
 patches

On Fri, 2 Mar 2007 10:15:36 -0800 (PST)
Christoph Lameter <clameter@...r.sgi.com> wrote:

> On Fri, 2 Mar 2007, Andrew Morton wrote:
> 
> > > One particular case is a 32GB system with a database that takes most
> > > of memory.  The amount of actually freeable page cache memory is in
> > > the hundreds of MB.
> > 
> > Where's the rest of the memory? tmpfs?  mlocked?  hugetlb?
> 
> The memory is likely in use but there is enough memory free in unmapped 
> clean pagecache pages so that we occasionally are able to free pages. Then 
> the app is reading more from disk replenishing that ...
> Thus we are forever cycling through the LRU lists moving pages between 
> the lists aging etc etc. Can lead to a livelock.

Guys, with this level of detail thses problems will never be fixed.

> > > A third scenario is where a system has way more RAM than swap, and not
> > > a whole lot of freeable page cache.  In this case, the VM ends up
> > > spending WAY too much CPU time scanning and shuffling around essentially
> > > unswappable anonymous memory and tmpfs files.
> > 
> > Well we've allegedly fixed that, but it isn't going anywhere without
> > testing.
> 
> We have fixed the case in which we compile the kernel without swap. Then 
> anonymous pages behave like mlocked pages. Did we do more than that?

oh yeah, we took the ran-out-of-swapcache code out.  But if we're going to
do this thing, we should find some way to bring it back.

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