Date:	Tue, 10 Jun 2008 23:16:42 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Paul Mundt <lethal@...ux-sh.org>
Cc:	Rik van Riel <riel@...hat.com>, clameter@....com,
	linux-kernel@...r.kernel.org, lee.schermerhorn@...com,
	kosaki.motohiro@...fujitsu.com, linux-mm@...ck.org,
	eric.whitney@...com, Andi Kleen <andi@...stfloor.org>,
	Ingo Molnar <mingo@...e.hu>, Andy Whitcroft <apw@...dowen.org>
Subject: Re: [PATCH -mm 13/25] Noreclaim LRU Infrastructure

On Wed, 11 Jun 2008 14:09:15 +0900 Paul Mundt <lethal@...ux-sh.org> wrote:

> On Tue, Jun 10, 2008 at 02:33:34PM -0700, Andrew Morton wrote:
> > On Tue, 10 Jun 2008 15:37:02 -0400
> > Rik van Riel <riel@...hat.com> wrote:
> > 
> > > On Tue, 10 Jun 2008 12:17:23 -0700 (PDT)
> > > Christoph Lameter <clameter@....com> wrote:
> > > 
> > > > On Sun, 8 Jun 2008, Andrew Morton wrote:
> > > > 
> > > > > And it will take longer to get those problems sorted out if 32-bt
> > > > > machines aren't even compiing the new code in.
> > > > 
> > > > The problem is going to be less if we dependedn on 
> > > > CONFIG_PAGEFLAGS_EXTENDED instead of 64 bit. This means that only certain 
> > > > 32bit NUMA/sparsemem configs cannot do this due to lack of page flags.
> > > > 
> > > > I did the pageflags rework in part because of Rik's project.
> > > 
> > > I think your pageflags work freed up a number of bits on 32
> > > bit systems, unless someone compiles a 32 bit system with
> > > support for 4 memory zones (2 bits ZONE_SHIFT) and 64 NUMA
> > > nodes (6 bits NODE_SHIFT), in which case we should still
> > > have 24 bits for flags.
> > > 
> > > Of course, having 64 NUMA nodes and a ZONE_SHIFT of 2 on
> > > a 32 bit system is probably total insanity already.  I
> > > suspect very few people compile 32 bit with NUMA at all,
> > > except if it is an architecture that uses DISCONTIGMEM
> > > instead of zones, in which case ZONE_SHIFT is 0, which
> > > will free up space too :)
> > 
> > Maybe it's time to bite the bullet and kill i386 NUMA support.  afaik
> > it's just NUMAQ and a 2-node NUMAish machine which IBM made (as400?)
> > 
> > arch/sh uses NUMA for 32-bit, I believe. But I don't know what its
> > maximum node count is.  The default for sh NODES_SHIFT is 3.  
> 
> In terms of memory nodes, systems vary from 2 up to 16 or so. It gets
> gradually more complex in the SMP cases where we are 3-4 levels deep in
> various types of memories that we expose as nodes (ie, 4-8 CPUs with a
> dozen different memories or so at various interconnect levels).

Thanks.

Andi has suggested that we can remove the node-ID encoding from
page.flags on x86 because that info is available elsewhere, although a
bit more slowly.

<looks at page_zone(), wonders whether we care about performance anyway>

There wouldn't be much point in doing that unless we did it for all
32-bit architectures.  How much trouble would it cause sh?

> As far as testing goes, it's part of the regular build and regression
> testing for a number of boards, which we verify on a daily basis
> (although admittedly -mm gets far less testing, even though that's where
> most of the churn in this area tends to be).

Oh well, that's what -rc is for :(

It would be good if someone over there could start testing linux-next. 
Once I get my act together that will include most-of-mm anyway.

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