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Re: [Xen-devel] 4Gb memory limit circumventable?

To: Nuutti Kotivuori <naked@xxxxxx>
Subject: Re: [Xen-devel] 4Gb memory limit circumventable?
From: Ian Pratt <Ian.Pratt@xxxxxxxxxxxx>
Date: Mon, 29 Nov 2004 14:08:22 +0000
Cc: xen-devel@xxxxxxxxxxxxxxxxxxxxx, Ian.Pratt@xxxxxxxxxxxx
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> The 4Gb memory limit with Xen on x86 may prove to be a real issue to
> us. So I was wondering, are there any approaches that might not
> require too much work to support more than 4Gb of physical memory on a
> machine?
> For example, could all machines run in more or less HIGHMEM style mode
> (perhaps using PSE36 or PAE), and still share the paging tables as
> normally happens in Xen?
> In short - what would be the easiest approach to supporting more than
> 4Gb of physical memory in Xen and how much work would you estimate it
> to be?

You'd need to modify both Xen and the guest to understand PAE36
mode.  Not rocket science, but a fair sized chunk of work. BTW:
x86's loose about 5% of performance operating in PAE36 mode.

In short, we don't think it's worth the effort: it's much better
to switch to x86_64. I doubt there are that many x86 boxes out
there with more than 4GB of memory that this decision is going to
effect. It's certainly not possible to buy a new server that
isn't x86_64 these days...


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