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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: weiming <zephyr.zhao@xxxxxxxxx>
Date: Thu, 14 Feb 2008 22:32:17 -0500
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
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Hi Robert,

The memory bus controller like Memory Controller Hub (MCH) on Intel's north bridge should arbitrate the traffic from different cores/processors. I guess the hardware can guarantee some kind of fairness so that no core would be starving and the other one is busy. (just my guess, I don't know the details)

However, control the bandwidth with priorities is difficult. Any one has ideas to this problem?

Weiming

On Thu, Feb 14, 2008 at 10:24 PM, 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?
 
Thank you,
 
Robert


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.

Weiming

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,

Robert

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