Testing xen with Varying MTU

posted in Hypervisor
Friday, January 31 2014, 12:30 AM
Arjun
Arjun
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I am trying to test throughput between two computers directly connected through 1 Gb and testing with iperf. I am getting a throughput around 980Mbps when MTU is between 5000 and 5050, however it drastically falls to around 680Mbps anything above MTU=5050. I have checked varying window sizes, but with same result. Increasing the MTU should decrease the overheads and thereby should increase the bandwidth or at least should not fall. I can't figure out this strange behavior. By the way testing TCP throughput. Any help ! and thanks guys.

Additional info! Two centos systmes, One of them is Xen 4.2 host(Dom0). Checked with varying buffer sizes in /pro/sys/net/ipv4, but same result. Latency is 0.2 ms. I have set up bridge for eth0. Is linux bridge and Xen bridge (backend) is the same, I am little confused a bit here.

After this I want to test throughput of VMs sitting on Xen 4.2 with varying amount of MTU.
Actually I am doing my thesis on virtualization and I chose Xen :)
Thanks guys ! Any help much appreciated....
Responses (2)
  • Accepted Answer

    Arjun
    Arjun
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    Friday, January 31 2014, 02:46 AM - #permalink
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    Is it something to do with page size on Xen ?
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  • Accepted Answer

    Friday, January 31 2014, 03:18 PM - #permalink
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    Well, I have little experience mucking with MTU settings, but I did find a little info on the subject.

    There is an interesting (but, sadly, undated) research paper out of Japan which might be helpful:

    http://web-ext.u-aizu.ac.jp/~hitoshi/RESEARCH/XenLRO_iccet.pdf

    An HP support article of unknown vintage says setting MTU around 1500 is optimal:

    http://h10025.www1.hp.com/ewfrf/wc/document?cc=us&lc=en&docname=c03103258

    If you want to use Jumbo Frames, it appears that MTU=9000 is best. See:

    http://wiki.xen.org/wiki/Network_Throughput_and_Performance_Guide

    Someone else was trying to adjust network performance using the MTU only to find that enabling GSO changes things:

    http://choon.net/forum/read.php?22,462515

    There's also an old (2006) paper which discusses Xen MTU settings, but I don't know if it is still accurate:

    https://www.usenix.org/legacy/event/usenix06/tech/menon/menon_html/paper.html

    If nothing here satisfies, then I'd suggest querying the xen-users mailing list. You will find a number of Xen developers who hang out there. They might be able to give you additional insight.

    Russ
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