linux-kernel - Re: [PATCH 1/4] x86_64: Switch to SPARSE

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Message-ID: <Pine.LNX.4.64.0704021534100.25602@schroedinger.engr.sgi.com>
Date:	Mon, 2 Apr 2007 15:37:56 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Andi Kleen <ak@...e.de>
cc:	Dave Hansen <hansendc@...ibm.com>, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, Martin Bligh <mbligh@...gle.com>,
	linux-mm@...ck.org,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH 1/4] x86_64: Switch to SPARSE_VIRTUAL

On Tue, 3 Apr 2007, Andi Kleen wrote:

> If it works I would be inclined to replaced old sparsemem with Christoph's
> new one on x86-64. Perhaps that could cut down the bewildering sparsemem
> ifdef jungle that is there currently.
> 
> But I presume it won't work on 32bit because of the limited address space?

Not in general but it will work in non PAE mode. 4GB need 2^(32-21+4) = 
16MB pages. This would require the mapping on 4 4MB pages.

For 64GB you'd need 256M which would be a quarter of low mem. Probably 
takes up too much of low mem.

> > But, without some cold, hard, data, we mere mortals without the 1024-way
> > machines can only guess. ;)
> 
> Yep.
> 
> -Andi (who would be scared of a 1024 way x86 machine)

- Christoph (who does not believe that Andi has to wait long for 
something bigger)

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