[Xen-users] Lots of udp (multicast) packet loss in domU

[Mike Kazmier <DaKaZ@xxxxxxxxx>]

Hello,

After a few of us have spent a week google'ing around for answers, I feel 
compelled to ask this question: how do I stop packet loss between my dom0 and 
domU?  We are currently running a xen-3.3.0 (and have tried with xen-3.2.1) on 
a Gentoo system with 2.6.18 kernel for both domU and dom0.  We have also tried 
a 2.6.25 kernel for dom0 with exactly the same results.

The goal is to run our multicast processing application in a domU with the 
BRIDGED configuration.  Note: we do not have any problem if we put the network 
interfaces into PCI Passthrough and use them exclusively in the domU, but that 
is less than ideal as occasionally other domU's need to communicate with those 
feeds.  

So, when we have little (sub 100 Mbps) of multicast traffic coming, everything 
is fine.  Over that, we start to see packet loss approaching 5%.  But the loss 
is only seen in the domU.  I can run our realtime analysis tool in the dom0 and 
domU at the same time, on the same multicast feed and found that in the dom0, 
all packets are accounted for.  Initially, I found a large number of dropped 
frames on the VIF interface, but after running " echo -n 256 > 
/sys/class/net/eth2/rxbuf_min " all reported errors have gone away (ie, dom0 
does not report dropping any packets) but we still see loss.

Any suggestions at all would be greatly appreciated.  I know we have something 
borked on our end since this white paper shows high domU udp performace: 
http://www.cesnet.cz/doc/techzpravy/2008/effects-of-virtualisation-on-network-processing
 - but at the same time a qoogle search shows tons of issues around UDP packet 
loss with Xen.  Any help would be greatly appreciaited.  

The hardware is an 8 core, 2.33 GHz penryn (Xeon) based system.  We have Intel 
quad GigE nics with the latest intel IGB drivers.  Here is the current domU 
config:

----------------------------------
# general
name    = "xenF";
memory  = 1024;
vcpus = 6;
builder = "linux";
on_poweroff = "destroy";
on_reboot = "restart";
on_crash = "restart";
cpu_weight = 1024  # xenF processes get 4 times default (256) priority threads
cpu_cap = 600  #Don't use more than 6 real CPUs

# This lets us use the xm console
extra = " console=xvc0 xencons=xvc0";

# booting
kernel  = "/boot/vmlinuz-2.6.18-xen-domU";

# virtual harddisk
disk = [ "file:/var/xen/domU-xenF,xvda1,w" ];
root = "/dev/xvda1 ro";

# virtual network
vif    = [ "bridge=xenbr0", "bridge=xenbr1", "bridge=xenbr2"]


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