Re: virtio net regression

[Antoine Martin <antoine@nagafix.co.uk>]

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Wireshark was showing a huge amount of invalid packets (wrong checksum)
- - that was the cause of the slowdown.
Simply rebooting the host into 2.6.28.9 fixed *everything*, regardless
of whether the guests use virtio or ne2k_pci/etc.
The guests are still running 2.6.29.1, but I am not likely to try that
release again on the host anytime soon! Ouch!

Antoine

Antoine Martin wrote:
> Hi,
> 
> I've got some hosts that were happily running the 2.6.25.x host kernel,
> kvm-84, kernel.org kvm modules.
> The guests were running 2.6.25 to 2.6.29.x quite happily.
> Network was using virtio.
> Since I upgraded one of the hosts (Intel dual core) to 2.6.29.x
> yesterday, the virtio network performance of the guests on it dropped
> dramatically. (for some reason another AMD host did not seem to be
> affected...)
> Here are the tests I performed using wget and scp:
> * guest to guest: fast
> * guest to host: fast
> * host to internet: fast
> * guest to internet: slow!!!
> I was normally getting ~5MB/s to the host (speed to the internet was
> limited by the capacity of the DSL line), but since the upgrade the
> performance had dropped to around 20KB/s!
> Strangely enough, I could open many new connections to the guest and get
> more chunks all at 20KB/s!
> I switched the guests to using ne2k_pci and the performance has been
> restored...
> 
> And this is where it gets even weirder...
> UDP packets get corrupted using ne2k_pci and rtl8139cp but not with
> virtio...
> So I can get performance or UDP, but not both...
> 
> Let me know if there is anything more I can provide to help fix this
> regression.
> I can reproduce the problem quite easily without causing problems on the
> host.
> 
> Cheers
> Antoine
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