RE: [Xen-users] strangeness with high BDP tests

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

> Any ideas why I'm getting such bad performance from the VMs 
> on high BDP links? I'm willing and interested to help in 
> debugging and fixing this issue, but I need some leads :)

The first thing to do is to look at the CPU usage in dom0 and domU. If
you can run them on different CPUs or even different hyperthreads it
might make the experiment simpler to understand. The first thing to find
out is whether you're maxed out on CPU, or whether this is an IO
blocking issue. Xm list should show you how much CPU each domain is
burning.

Secondly, enable performance counters in a Xen build, then use the user
space tools to read out the context switch rate. How does it compare
without the emulated BDP link?

Also, you might want to play around with the rate limiting function in
netback. If you set it to a few hundred Mb/s you might help promote
batching.

I'm also concerned that dummynet is pretty terible when operating at
such high speeds, and the whole thing might be just a bad interaction
between Xen's batching and dummynet's. Why not set up a real experiement
across Abilene just to check?

Ian

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