Re: [Qemu-devel] TCP Segementation Offloading

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Sun, May 01, 2016 at 02:31:57PM +0200, Ingo Krabbe wrote:
> Good Mayday Qemu Developers,
> 
> today I tried to find a reference to a networking problem, that seems to be of quite general nature: TCP Segmentation Offloading (TSO) in virtual environments.
> 
> When I setup TAP network adapter for a virtual machine and put it into a host bridge, the known best practice is to manually set "tso off gso off" with ethtool, for the guest driver if I use a hardware emulation, such as e1000 and/or "tso off gso off" for the host driver and/or for the bridge adapter, if I use the virtio driver, as otherwise you experience (sometimes?) performance problems or even lost packages.

I can't parse this sentence.  In what cases do you think it's a "known
best practice" to disable tso and gso?  Maybe a table would be a clearer
way to communicate this.

Can you provide a link to the source claiming tso and gso should be
disabled?

> I haven't found a complete analysis of the background of these problems, but there seem to be some effects on MTU based fragmentation and UDP checksums.
> 
> There is a tso related bug on launchpad, but the context of this bug is too narrow, for the generality of the problem.
> 
> Also it seems that there is a problem in LXC contexts too (I found such a reference, without detailed description in a Post about Xen setup).
> 
> My question now is: Is there a bug in the driver code and shouldn't this be documented somewhere in wiki.qemu.org? Where there developments about this topic in the past or is there any planned/ongoing work todo on the qemu drivers?
> 
> Most problem reports found relate to deprecated Centos6 qemu-kvm packages.
> 
> In our company we have similar or even worse problems with Centos7 hosts and guest machines.

Have haven't explained what problem you are experiencing.  If you want
help with your setup please include your QEMU command-line (ps aux |
grep qemu), the traffic pattern (ideally how to reproduce it with a
benchmarking tool), and what observation you are making (e.g. netstat
counters showing dropped packets).

> I'm going to analyze these problems next week anyway and I woud be happy to share my observation with you. (Where can I register for the wiki, or whom should I sent my reports about this topic?).

I have CCed Michael Tsirkin and Jason Wang.  They do most of the
virtio-net development.

> 
> Regards,
> 
> Ingo Krabbe
> 
> 

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