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

[Diwaker Gupta <diwakergupta@xxxxxxxxx>]

> 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..

I'd be happy to. Just send them over, and let me know if you find
anything interesting.

> 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.

I don't trust the 10 second tests either, especially for such a high
RTT. Thats why I ran the tests for 80 seconds (thats 1000 RTTs, and
should give TCP enough time to stabilize). I'll get some numbers using
these options in any case.


> 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 ;)).

Yeah, but in my experience it usually picks up the "default" value as
set by the sysctl. I'll check the code.

> 
> 
> >>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.

I agree. But I think its a reasonable goal to expect dom0 performance
to match a VM performance across a variety of environments :)

-- 
Diwaker Gupta
http://resolute.ucsd.edu/diwaker

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