[Xen-users] e1000e NIC and MTU issues

[Graeme Smith <graemes78@xxxxxxxxx>]

Hello, 
I am observing a performance degradation (of about 100 Mbps dropl) between two domUs on different hosts when I set the MTU on dom0 to anything > 1500. The domUs are still configured with MTU of 1500, so effectively there isn't any traffic with bigger MTU ever generated. And the network is jumbo frame capable. 

Furthermore, when I check the interrupts/second for dom0 using 'mpstat' command while a throughput test is on, I observe this rate to have increased (from 6000 intrs/sec to 9000 intrs/sec) as soon as I change the dom0 MTU from <=1500 to >= 1501 (I have tried 1501 and 9000). I also observe this degradation only when changing the MTU on dom0 hosting the receiver domU (in a netperf test). 

The physical NIC on the host is Intel e100e, and the and platform is RedHat 5.2, Xen version
2.6.18-92.el5xen. It looks to me that this is an issue with the interrupt coalescing behavior of the NIC, and its interactions with Xen networking. Is the behavior of e1000e to different MTU settings well-known ?

However, when I do the throughput measurement between the hosts (dom0s and not domUs), I do not see any degradation with MTUs >= 1501. 

Any ideas?
GS

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