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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
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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