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

[Nivedita Singhvi <niv@xxxxxxxxxx>]

Diwaker Gupta wrote:
> > > 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.
>
> I had caught glimpses on the list of a top like utility for viewing
> CPU usage.. is that a reality yet? I haven't followed up on that
> thread. The problem is that xm list is fine for very coarse grained
> measurements, but its a pain to do real-time fine granularity
> measurements with that. Sure, I could always write my own little
> Python script using the xm interface, but it'll be great if we had
> something like top.
>
>
> > 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.
>
> Sorry if this is dumb, but whats the rate limiting function in
> netback? Is it a run-time parameter or something in the code? What
> does it do? If I set it too high, won't it lead to bad performance
> with low b/w flows? I guess I should just look at the code :)
Hi Diwaker! Sorry I'm coming to this thread late, I was out
sick the last couple of days. I just started looking into the
net flow control problem. Ian is speculating that the  rate
limiting function will actually help improve data get pushed
faster. We're looking into where exactly our latencies are.
If you could run some debug patches for me, I'd really appreciate
it..

Btw, have you tried using the -i and -I options to netperf?
-i 30, 10, will at least ensure a minimum of 10 runs for
each measurement, and -I can be used to specify a confidence
interval (99, 5). Even if it's consistent, I wouldn't trust the 10
second run time for the test.

Netperf uses setsockopt() to set its own buffer sizes, so
increasing the system sysctl values will not affect your test
in anyway (or shouldn't ;)).


> > 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?
>
> I think thats a separate debate. For now, I just want to get the same
> performance levels from a VM as from dom0, for all possible
> environments, dummynet just being one of them. Setting up a real
> experiment is a good idea though, I'm looking into it. BTW, where can
> I learn more on Xen's "batching"?
The question is how frequently should the frontend kick the
backend, and how frequently should the backend pass along packets
to the real device. Aggregating requests improves the efficiency
of the transfers but impacts latency.

thanks,
Nivedita




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