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Re: [Qemu-devel] TCP Segementation Offloading

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] TCP Segementation Offloading
Date: Thu, 5 May 2016 18:42:03 +0100
User-agent: Mutt/1.6.0 (2016-04-01)

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

> 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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