linux-kernel - Re: [RFC 0/8] Variable Order Page Cache

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Message-ID: <Pine.LNX.4.64.0704200920350.20232@schroedinger.engr.sgi.com>
Date:	Fri, 20 Apr 2007 09:23:23 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Mel Gorman <mel@...net.ie>
cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Paul Jackson <pj@....com>, Dave Chinner <dgc@....com>,
	Andi Kleen <ak@...e.de>
Subject: Re: [RFC 0/8] Variable Order Page Cache

On Fri, 20 Apr 2007, Mel Gorman wrote:

> I believe there is an assumption in parts of reclaim that LRU pages are
> order-0. An interesting bug or two is likely to rear its head there.

Correct. We need to deal with reclaim etc.

> > Note that this is proof-of-concept. Lots of functionality is missing and
> > various issues have not been dealt with. Use of higher order pages may cause
> > memory fragmentation. Mel Gorman's anti-fragmentation work is probably
> > essential if we want to do this. We likely need actual defragmentation
> > support.
> > 
> 
> Ok, anti-fragmentation will help up to a point but it's awkward with ramfs
> because those pages are not reclaimable or migratable no matter what the
> order. Normal filesystems would fare much better fragmentation-wise.
> 
> The problem is that the mapping gfp_mask is normally GFP_HIGH_MOVABLE but it's
> GFP_HIGHUSER for ramfs. This patchset will increase the number of non-movable
> high-order allocations quite considerably and it will tend to fragment memory
> worse than we do currently. I can think of ways it can be dealt with 
> (even marking them RECLAIMABLE would help) so I'm not massively worried
> now but I'll keep it in mind as things develop.

Well I think we will have xfs support soon. Then we can deal with more 
issues and be more complete. What I wanted from this post was a consensus 
on how to proceed. There are many subsystems involved and I do not want to 
go off the deep end.
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