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Re: [Xen-users] Why does DomU CPU usage increase during live migration?

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Subject: Re: [Xen-users] Why does DomU CPU usage increase during live migration?
From: Javier Guerra <javier@xxxxxxxxxxx>
Date: Fri, 17 Apr 2009 09:26:18 -0500
Cc: "S. Ercan Yüzbasioglu" <ercan.yuzbasioglu@xxxxxxxxx>
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S. Ercan Yüzbasioglu wrote:
> I would expect that Dom0 should be the only domain whose CPU usage is
> supposed to increase. The migration process is supposed to be transparent to
> the DomU, right? So I cannot justify this behaviour. Does anybody have an
> idea about this?

two ideas:

1- Xen's Hypervisor is doing a heavy work, and charges it to the DomU's credit, 
therefore the scheduler gives less CPU time to the 'real' DomU, so it gets less 
idle time, so higher CPU usage.

2- the migration system uses MMU facilities to mark which pages have changed 
after copied (so they have to be recopied).  that marking makes the whole DomU 
RAM look a bit like virtual memory (backed by real RAM), temporarily increasing 
overhead, and making all processes in DomU slightly slower.


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