[Xen-devel] RE: [Xen-users] Network performance - sending from VM to VM using TCP

["Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>]

 > I have been simulating a network using dummynet and 
> evaluating it using netperf. Xen3.0-unstable is used and the 
> VMs are vmlinuz-2.6.11-xenU. The simulated link is 300Mbps 
> with 80ms RTT.
> Using netperf, I sent data using TCP from domain-0 of machine 
> 1 to domain-0 of machine 2. Then I repeat the experiment, but 
> this time from VM-1 of machine 1 to VM-1 of machine 2.
> 
> However, the performance across the two VMs is substantially 
> worse than that across domain-0. Here's the result:

Someone else was having problems with low performance via dummynet a
couple of months back. It's presumably dummynet's packet scheduling
causing some bad interaction with the batch processing of packets in
netfront/back.

The first step to understanding this is probably to capture a tcpdump
and look at it with tcptrace to see what's happening with window sizes
and scheduling of packets.

Ian

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