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

  • To: <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • From: "Robert Stober" <rstober@xxxxxxxxxxxx>
  • Date: Thu, 14 Feb 2008 22:24:18 -0500
  • Cc: weiming <zephyr.zhao@xxxxxxxxx>
  • Delivery-date: Thu, 14 Feb 2008 19:25:02 -0800
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>
  • Thread-index: Achvfw4AaTZ2dpGtQoKvlboNg7262gAACSdQ
  • Thread-topic: [Xen-devel] A question no one can answer

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?
Thank you,

From: weiming [mailto:zephyr.zhao@xxxxxxxxx]
Sent: Thursday, February 14, 2008 7:02 PM
To: Robert Stober
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-devel] A question no one can answer

You mean QoS on memory bandwidth? It's an interesting question. (there are similar problems like QoS on shared L2)

I think it's hard to do.  Even for a native OS, it may only control this indirectly by adjusting the CPU time slice. And I don't think any commercial OS has implemented such feature yet.


On Thu, Feb 14, 2008 at 9:27 PM, Robert Stober <rstober@xxxxxxxxxxxx> wrote:
Good Afternoon,

Is it possible to limit a guest OS to a specific memory bandwidth
allocation? The purpose is to stop one guest OS from saturating the
memory bus.

Thank you,


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