Re: Network throughput limits for local VM <-> VM communication

[Avi Kivity <avi@redhat.com>]

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.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.