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Re: Ownership of machine pages: Was: [Xen-devel] Essay on an important X

To: "Magenheimer, Dan (HP Labs Fort Collins)" <dan.magenheimer@xxxxxx>
Subject: Re: Ownership of machine pages: Was: [Xen-devel] Essay on an important Xen decision (long)
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Wed, 11 Jan 2006 22:57:27 +0000
Cc: "Tian, Kevin" <kevin.tian@xxxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, xen-ia64-devel@xxxxxxxxxxxxxxxxxxx
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On 11 Jan 2006, at 22:49, Magenheimer, Dan (HP Labs Fort Collins) wrote:

context switches are added for any page related operation.

Hmmm... not really.  Page operations (such as alloc and free)
are relatively rare and can be batched (e.g. at domain startup).
And domU's are not going to be making policy decisions...
If dom0 is making policy decisions (e.g. this stick of RAM is
about to die, who do I steal memory from?), dom0 ownership
of the pages may even reduce context switches.

In a literal/extreme form of your model, any address space access/update would need to be translated and/or validated by domain0. That's certainly going to be a significant overhead. I believe that there is a balance to be struck between sufficient mechanism in Xen to ensure good performance while allowing flexible policy expression in the control plane. I think talking about this at the summit would be very interesting -- are your ideas solidified enough perhaps even for a short presentation?

 -- Keir

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