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

  • To: Ian Pratt <m+Ian.Pratt@xxxxxxxxxxxx>
  • From: Diwaker Gupta <diwakergupta@xxxxxxxxx>
  • Date: Sun, 3 Apr 2005 23:13:15 -0700
  • Cc: ian.pratt@xxxxxxxxxxxx, xen-users@xxxxxxxxxxxxxxxxxxx
  • Delivery-date: Mon, 04 Apr 2005 06:15:23 +0000
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  • List-id: Xen user discussion <xen-users.lists.xensource.com>

> > 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 :)

> 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"?

Diwaker Gupta

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