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Re: [Xen-devel] A question no one can answer

To: "Robert Stober" <rstober@xxxxxxxxxxxx>
Subject: Re: [Xen-devel] A question no one can answer
From: Alan Cox <alan@xxxxxxxxxxxxxxxxxxx>
Date: Fri, 15 Feb 2008 13:05:22 +0000
Cc: weiming <zephyr.zhao@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
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On Thu, 14 Feb 2008 22:24:18 -0500
"Robert Stober" <rstober@xxxxxxxxxxxx> wrote:

> Weiming,
> I agree that it is very hard, and that no one has done it. But
> nevertheless I suggest the following question to the Xen developers:
> Given the fact that memory bandwidth is shared amongst multiple cores on
> a single die, assume that one VM is running on each core. What is to
> stop one VM from saturating the memory bus, causing reduced performance
> of all the other VMs? This is the general multi-core problem, not
> specific to Xen. But it affects Xen greatly. What use is it to allocate
> memory to a VM if it can't use the memory because a process of another
> VM has saturated the memory bus?

Its perfectly doable on modern x86 - you use the profiling registers and
set them up so you get a threshold interrupt when too much main memory
traffic is counted (which you use to reschedule punishing the memory
user). There are research papers on it from quite a long time back but
afaik nobody ever implemented it in production although its not too hard
to do so might be an interesting project.


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