> Subject: Re: Network throughput limits for local VM <-> VM
> communication
>
> On 06/17/2009 11:12 AM, Fischer, Anna wrote:
> >
> > For the tests I run now (with vlan= enabled) I am actually using both
> TCP and UDP, and I see the problem with virtio_net for both protocols.
> What I am wondering about though is that I do not seem to have any
> problems if I communicate directly between the two guests (if I plug
> then into the same bridge and put them onto the same networks), so why
> do I only see the problem of stalling network communication when there
> is a routing VM in the network path? Is this just because the system is
> even more overloaded in that case? Or could this be an issue related to
> a dual NIC configuration or the fact that I run multiple bridges on the
> same physical machine?
> >
>
> My guess is that somewhere there's a queue that's shorter that the
> virtio queue, or its usable size fluctuates (because it is shared with
> something else). So TCP flow control doesn't work, and UDP doesn't
> have
> a chance.
>
> > When you say "We are working on fixing this." - which code parts are
> you working on? Is this in the QEMU network I/O processing code or is
> this virtio_net related?
> >
>
> tap. virtio, qemu, maybe more. It's a difficult problem.
>
> > Retry with "the fixed configuration"? You mean setting the vlan=
> parameter? I have already used the vlan= parameter for the latest
> tests, and so the CPU utilization issues I am talking about are
> happening with that configuration.
> >
>
> Yeah.
>
> Can you compare total data sent and received as seen by the guests?
> That would confirm that packets being dropped causes the slowdown.
Yes, I will check on that and report back.
It still does not answer my question on why I only see low CPU utilization numbers with the e1000 virtual device model. There is no network stalling or packet drops or any other obvious issues when running with that model, but I am still seeing low CPU utilization numbers. What is preventing KVM here to use more of the host CPU capacity when the host is not doing anything else but run virtual machines? Is there any way that I can get higher CPU utilization out of KVM?
Thanks,
Anna
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