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Re: [Qemu-discuss] E1000 & virtio will not respect set MTU

From: Markus Kovero
Subject: Re: [Qemu-discuss] E1000 & virtio will not respect set MTU
Date: Wed, 01 Feb 2012 21:52:02 +0200
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As we can see here, packet length varies over what it should be, when
using realtek, packet length is strictly 1448, which fits uplink fine
and upload degration will not occur.

This is not a pmtu problem either, same thing happens when pmtu
discovery is disabled.
This also happens on Windows, so I'm thinking its not distribution related.

Thanks for any assistance

Hi, It seems I have classical "oh, okay nevermind"-scenario here, had more time to look into it and it seems I was too eager to jump on conclusions with strictly mtu-related issue, ignoring the fact I was merely witnessing gso/tso in operation without fully realizing that offloading can actually sink so deeply under virtualization layer and actually function (properly). Performance degradation seems to come from other layers than kvm/qemu itself, I was able to benchmark speeds up to 3Gbps from virtual guest to host server using virtio. I can't explain why realtek just works at this point, maybe due it's lacking some feature that is partly to blame here, have to dig deeper. (:

Thanks and sorry for mailing list noise(:

Markus Kovero

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