Re: sluggish network performance in *one* domU

["Steven M. Bellovin" <smb%cs.columbia.edu@localhost>]

On Mon, 6 Jul 2009 22:05:31 +0200
Petar Bogdanovic <petar%smokva.net@localhost> wrote:

> Hello,
> 
> I'm experiencing a rather strange network problem within a Xen guest
> system.  The problem is bad network performance, it affects all tcp
> connections and only appears in one domU.
> 
> The hardware is running Xen 3.1.2, NetBSD 4.0 dom0 on i386.  There
> are 3 domUs running: two nearly identical NetBSD copies and one
> Debian system.
> 
> The `problematic' domU is one of those two NetBSD domUs.  I checked
> lots of stuff: the cpu is idle all the time, there is no disk or
> network I/O and I couldn't find anything in the logs.
> 
No solutions, but a few things to look at...

First -- try pkgsrc/benchmarks/ttcp rather than dd'ing /dev/urandom; it
avoids confounding network issues with host issues.

Second -- slow TCP generally indicates packet loss; the question is
where the packets are being dropped.  pkgsrc/net/mtr might help answer
that.

Third -- just try running a tcpdump on the domUs to see if there's some
background process sending a lot of data on that domU or perhaps on the
dom0.


                --Steve Bellovin, http://www.cs.columbia.edu/~smb