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

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Date:	Thu, 1 Mar 2007 20:06:25 -0800 (PST)
From:	Christoph Lameter <clameter@...r.sgi.com>
To:	Nick Piggin <npiggin@...e.de>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@...net.ie>, 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, Nick Piggin wrote:

> > I would say that anti-frag / defrag enables memory unplug.
> 
> Well that really depends. If you want to have any sort of guaranteed
> amount of unplugging or shrinking (or hugepage allocating), then antifrag
> doesn't work because it is a heuristic.

We would need additional measures such as real defrag and make more 
structure movable.

> One thing that worries me about anti-fragmentation is that people might
> actually start _using_ higher order pages in the kernel. Then fragmentation
> comes back, and it's worse because now it is not just the fringe hugepage or
> unplug users (who can anyway work around the fragmentation by allocating
> from reserve zones).

Yes, we (SGI) need exactly that: Use of higher order pages in the kernel 
in order to reduce overhead of managing page structs for large I/O and 
large memory applications. We need appropriate measures to deal with the 
fragmentation problem.

> > Thats a value judgement that I doubt. Zone based balancing is bad and has 
> > been repeatedly patched up so that it works with the usual loads.
> 
> Shouldn't we fix it instead of deciding it is broken and add another layer
> on top that supposedly does better balancing?

We need to reduce the real hardware zones as much as possible. Most high 
performance architectures have no need for additional DMA zones f.e. and
do not have to deal with the complexities that arise there.

> But just because zones are hardware _now_ doesn't mean they have to stay
> that way. The upshot is that a lot of work for zones is already there.

Well you cannot get there without the nodes. The control of memory 
allocations with user space support etc only comes with the nodes.

> > A. moveable/unmovable
> > B. DMA restrictions
> > C. container assignment.
> 
> There are alternatives to adding a new layer of virtual zones. We could try
> using zones, enven.

No merge them to one thing and handle them as one. No difference between 
zones and nodes anymore.
 
> zones aren't perfect right now, but they are quite similar to what you
> want (ie. blocks of memory). I think we should first try to generalise what
> we have rather than adding another layer.

Yes that would mean merging nodes and zones. So "nones".

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