linux-kernel - Re: [PATCH] Add __GFP_MOVABLE for callers to flag allocations that may be migrated

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Date:	Mon, 4 Dec 2006 12:17:26 -0800 (PST)
From:	Christoph Lameter <clameter@....com>
To:	Andrew Morton <akpm@...l.org>
cc:	Mel Gorman <mel@...net.ie>,
	Linux Memory Management List <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Add __GFP_MOVABLE for callers to flag allocations that
 may be migrated

On Mon, 4 Dec 2006, Andrew Morton wrote:

> > The multi zone approach does not work with NUMA. NUMA only supports a 
> > single zone for memory policy control etc.
> 
> Wot?  memory policies are a per-vma thing?

They only apply to "policy_zone" of a node. policy_zone can only take a 
single type of zone (has been like it forever). Multiple zones could 
become a nightmare with an exploding number of zones on zonelists. I.e. 
instead of 1k zones on a nodelist we now have 2k for two or even 4k if you 
want to have support for memory policies for 4 zones per node. We will 
then increase the search time through zonelists and have to manage all the 
memory in the different zones. Balancing is going to be difficult.

> I suspect you'll have to live with that.  I've yet to see a vaguely sane
> proposal to otherwise prevent unreclaimable, unmoveable kernel allocations
> from landing in a hot-unpluggable physical memory region.

Mel's approach already mananges memory in a chunks of MAX_ORDER. It is 
easy to just restrict the unmovable types of allocation to a section of 
the zone.

Then we should be doing some work to cut down the number of unmovable 
allocations.
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