RE: [Xen-devel] vif interface dropping 80% packets

Ian Pratt
Tue, 06 Feb 2007 13:43:09 -0000

> I''m trying to characterize the performance of 10G ethernet to a
user
> domain.
> The machine has 2 configured ethernet cards, eth0 is a 1G card, and
eth2 is
> a 10G card.  (eth1 is another 1G card and eth3 is another 10G, but
these
> are not configured)
> I have network bridge set up on eth2 with defaults, nothing special,
just:
>    /etc/xen/scripts/network-bridge start vifnum=2
> I have a user domain that attaches eth1 to this vif, and when I try
netperf
> tests, I get tons of packet loss in the vif interface.
>
> As can be seen above, vif3.1 drops about 80% of the packets it should
> transmit.  Is this expected due to something in the dom0 -> domU
interface
> just not being able to keep up, or is there something I can do about
it?
> It almost looks like the dom0->domU link is throttled at about 1Gb/s.
I
> say this because the 10G link is running about 50-60% capacity when
the
> drop rate is 80%.

First off, try switching to 3.0.4 and use the included 2.6.16-xen
kernel.

How are you measuring the bandwidth? It''s much more realistic to use
TCP
as open-loop UDP blasts aren''t used by any real protocols.

> Everything has MTU 1500, btw - I noticed that Xen is pretty unhappy
with an
> MTU of 9000.

In what way?  I''d expect larger MTUs to work OK, though I
haven''t tested
them myself.

Does the larger MTU work in dom0, but not through to other domU''s?

You may need to set the MTU on the bridge and all vif''s.


Ian