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I am trying to test throughput between two computers directly connected through 1 GbE 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. This is my post ever post(question) on any forum :) usually I find answers....

Additional info! Two centos systems One system is Xen 4.2 host (but it shouldn't be the problem) Checked with varying buffer sizes in /pro/sys/net/ipv4

forrestGump
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Just a thought... With MTUs at that size you start heading towards the memory page limit, though admittedly at 5000-5050 you should have eclipsed it already for a 4K page size (default for Xen 3.0). Still, I'd just wonder if your memory is getting fragmented. Try upping your memory page size so that you are sure that what you want to fill in a frame will fit in one page of memory and see if that helps.

... Actually, the answer to your question might be here: http://comments.gmane.org/gmane.network.iperf.user/485

longofest
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