[fedora-virt] slightly OT: Comparing apples with apples?

Gianluca Sforna giallu at gmail.com
Fri Apr 24 09:55:54 UTC 2009

On Fri, Apr 24, 2009 at 11:33 AM, Daniel P. Berrange
<berrange at redhat.com> wrote:
> On Fri, Apr 24, 2009 at 09:10:47AM +0100, Mark McLoughlin wrote:
>> > The most common complain was that competing for the host's resources
>> > was not fair, something along the line: Fedora starts first becasue
>> > has no grub, grabs some critical resource and Ubuntu has to wait for
>> > it before continuing.
>> >
>> > So basically the question is: do you think there could be any reason
>> > why such a test can be unfair to one of the VMs?
>> Okay, seriously - I think it's a reasonable experiment.
>> However, the fact that you have two guests competing for resources is
>> always going to make people suspicious. It may well be a deterministic
>> experiment, but you're always going to have a hard time convincing
>> people of that.
> This would be particularly true if the VMs were over-committing on
> CPU resources. eg if you had a 2 cpu host, and gave both VMs 2 vcpus
> each, then you'd have 4 vcpus total comparing for 2 pcpus.

Setup was on a 2 CPU host, each guest was assigned 1vcpu

> will also likely compete on disk I/O during boot and impact each
> other that way.

I expected this of course; I just assumed reasonable there was no
reason for the host kernel to serve data in an uneven way.

> Which reminds me, that you need to be careful with
> the QEMU disk caching modes. By default QEMU will use the host OS
> disk cache, so if one of the VM's data was already in the host cache
> and the other's wasn't, one would have an unfair I/O advtange.

Ah, ok. Suspecting something like this, I repeated the experiment
multiple times to see if there was any fluctuation. After the first
three runs, there was not

> I'd recommend running with cache=off for the -driver parameters to
> ensure they a guarenteed to be using Direct IO,avoiding any cache
> on the host OS.
Is this exposed in the virt-manager interface (F10) ?

>> Personally, I'd do it by timing each VM on its own and comparing the
>> boot times.
> You've also got to be sure both VMs are being run with comparable
> configs. eg, it'd be totally unfair to compare Fedora VM with IDE
> disk and RTL8139 nic against a Ubuntu VM using virtio disk & net,
> or vica-verca.

The setup was done with the same (default) options offered by
virt-manager. I also choose the "Generic" OS type for both.

Thank you very much

Gianluca Sforna


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