PCI Passthrough & iperf Throughput Skew

[Jambunathan K <jambunathan@netxen.com>]

I am seeing a strange behaviour with Xen-3.0.4 which seems to suggest
that Dom0 has either higher Scheduling affinity or Interrupt affinity.

I am in the initial stages of investigation and it is possible that my
hunch is totally wrong. I would like to hear comments or questions
from the experts.

I have a multifunction NIC card, with one function being used by Dom0
and another one exported to DomU using PCI passthrough

Dom0 and DomU have almost the same memory allocated to each
(~1.3G). The ports themselves are operating at the same speed and
share the same IRQ.

iperf (as a client) is started on Dom0 in the same way as it is
started on DomU.

iperf results are skewed in favour of Dom0 in the ratio 10:1.

Given all other things are similar wouldn't it be normal to expect
throughputs in the ratio of 1:1.

(ps:- The 2 ports on the Xen machine is connected
"point-to-point-wise" to 2 ports on another machine running Linux
natively)


Regards,
Jambunathan K.