linux-kernel - Re: [PATCH 0/8] Create ZONE_MOVABLE to partition memory between movable and non-movable pages

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Date:	Fri, 26 Jan 2007 08:01:07 -0800 (PST)
From:	Christoph Lameter <clameter@....com>
To:	Mel Gorman <mel@....ul.ie>
cc:	Andrew Morton <akpm@...l.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Christoph Lameter <clameter@...r.sgi.com>
Subject: Re: [PATCH 0/8] Create ZONE_MOVABLE to partition memory between
 movable and non-movable pages

On Fri, 26 Jan 2007, Mel Gorman wrote:

> I haven't thought about it much so I probably am missing something. The major
> difference I see is when only one zone is present. In that case, a number of
> loops presumably get optimised away and the behavior is very different
> (presumably better although you point out no figures exist to prove it). Where
> there are two or more zones, the code paths should be similar whether there
> are 2, 3 or 4 zones present.

The balancing of allocations between zones is becoming unnecessary. Also 
in a NUMA system we then have zone == node which allows for a series of 
simplifications.

> As the common platforms will always have more than one zone, it'll be heavily
> tested and I'm guessing that distros are always going to have to ship kernels
> with ZONE_DMA for the devices that require it. The only platform I see that
> may have problems at the moment is IA64 which looks like the only platform
> that can have one and only one zone. I am guessing that Christoph will catch
> problems here fairly quickly although a non-optional ZONE_MOVABLE would throw
> a spanner into the works somewhat.

There are 6 platforms that have only one zone. These are not major 
platforms. In order for major platforms to go to a single zone in general 
we would have to implement a generic mechanism to do an allocation where 
one can specify the memory boundaries. Many DMA engines have different
limitations from what ZONE_DMA and ZONE_DMA32 can provide. If such a 
scheme would be implemented then those would be able to utilize memory 
better and the amount of bounce buffers would be reduced.
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