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[Xen-devel] Re: PCI Passthrough & iperf Throughput Skew

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Subject: [Xen-devel] Re: PCI Passthrough & iperf Throughput Skew
From: Jambunathan K <jambunathan@xxxxxxxxxx>
Date: Tue, 27 Mar 2007 20:10:17 +0530
Cc: Sanjeev Jorapur <sanjeev@xxxxxxxxxx>
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Additional Data Point:

If I just exercise iperf in Dom0 or DomU *separately*, I get identical
throughput .  The 10:1 skew happens only when the iperf is running
concurrently on the Dom0 and DomU.

Jambunathan K.

Jambunathan K wrote:
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

Jambunathan K.

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