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

  • To: <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • From: "Reese Faucette" <reese@xxxxxxxx>
  • Date: Tue, 6 Feb 2007 14:52:48 -0800
  • Delivery-date: Tue, 06 Feb 2007 14:52:49 -0800
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>

You are probably using jumbo packets for native, right? What is the MTU when you get 9.88 Gb/s? I was thinking MTU=1500 when I said unrealistic. Not sure why you cannot get higher MTU in Xen to work. I have not tried MTU > 1500 bytes in Xen myself but
I believe Herbert Xu has done that successfully.

ok, true - got me there. TCP test with native linux sending to dom0 and 1500 MTU is 9.1GB/s and 23% of 1 CPU used on receiver.

If you get larger MTU to work, keep in mind that in Xen there is an extra data copy to transfer the packet from dom0 to guest which will have a higher impact on performance
for larger MTUs when compared to linux.

of course - i don't expect there to be no impact, but the numbers i was getting with mtu=1500 were horrid, like 70Mb/s using netperf with TCP_SENDFILE. a slower sender actually resulted in better throughput due to fewer dropped packets. (e.g. using TCP_STREAM got 220Mb/s, better but still stinky)

MTU=9000 does work, I had failed to set one of the vif interfaces - plenty of interfaces to change ;-)
The performance is better with 9k MTU, as expected:
TCP_STREAM and TCP_SENDFILE both get ~1.8Gb/s, xentop reports 87% utilization for dom0, so I guess that's about all I can expect without code changes.

If I understand the Xen bridge network flow, though, it's a real bummer that

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

I can try, but the issue i had before was that 2.6.16 did not have the
ethernet driver I need for the main ethernet connection:
0000:06:00.1 Ethernet controller: Intel Corporation Enterprise Southbridge DPT LAN Copper (rev 01)
It comes with 2.6.18, but not 2.6.16, and the driver does not compile under
2.6.16, so i was stuck with 2.6.18.

before I spend a lot of time converting to 3.0.4, a couple of questions:
1) any reason to expect better perf on 3.0.4 ?
2) if yes, Will the patch "fedora-36252.patch" work on the 2.6.16 kernel included with 3.0.4, or have conflicts crept in?


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