vif interface dropping 80% packets

["Reese Faucette" <reese@myri.com>]

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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.

vif3.1    Link encap:Ethernet  HWaddr FE:FF:FF:FF:FF:FF
          inet6 addr: fe80::fcff:ffff:feff:ffff/64 Scope:Link
          UP BROADCAST RUNNING NOARP  MTU:1500  Metric:1
          RX packets:165602 errors:0 dropped:0 overruns:0 frame:0
          TX packets:12982847 errors:0 dropped:48987465 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:8679444 (8.2 MiB)  TX bytes:20888548648 (19.4 GiB)

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%.

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

thanks,
-reese

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