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Re: [Xen-users] Does I/O from Dom0 skip the PV backend drivers ?

To: Xen List <xen-users@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-users] Does I/O from Dom0 skip the PV backend drivers ?
From: "Fajar A. Nugraha" <fajar@xxxxxxxxx>
Date: Fri, 1 May 2009 09:53:47 +0700
Delivery-date: Thu, 30 Apr 2009 19:54:32 -0700
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On Thu, Apr 30, 2009 at 9:29 PM, Joe Armstrong <Joe.Armstrong@xxxxxxxxx> wrote:
> Thanks for the info - since I am running CentOs (with xen 3.0.3) it sounds
> like my best bet for real network numbers is to run on bare metal,

Actually it depends on what you're trying to do.
Network PV drivers are good enough so (for example) if you have a
recent enough CPU, you can easily saturate 1 GB link with only a minor
difference in CPU load.

If your benchmark is something like sysbench for MySQL, the limiting
factor is usually disk I/O. In this case testing on a native Linux and
testing on dom0 would yield very similar results.

> but disk
> I/O numbers from dom0 should be consistent with a bare metal machine.


Another thing to note is memory allocation. By default Xen
automatically baloons down dom0 memory as needed when domU is started,
but it does NOT automatically baloon-up later when domU is stopped. So
make sure your dom0 is using all memory (try "xm list" or "xm info")
before starting your benchmark.



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