Epyc VM to VM networking slow

planedrop

@olivierlambert This is great, thanks for letting us know! I'll give this a shot in my lab as soon as I can.

gduperrey

The fix, which was proposed as a test to resolve some of the issues encountered, has been integrated into an official update candidate which will be released to everyone next time we publish updates. For more information on this update, you can consult the following post: https://xcp-ng.org/forum/post/96135

ksyblast

Hello!
First of all thank you very much for providing the fix!

Following the blog post https://xcp-ng.org/blog/2025/09/01/september-2025-maintenance-update-for-xcp-ng-8-3/:

The change only affects Linux guests. To make it effective, their kernel must support the feature which enables this fix. Linux distributions that have recent enough kernels or apply fixes from the mainline LTS kernels support it. Older ones don't (example: Ubuntu 20.04). Some currently supported LTS distros don't have the required patch yet, notably RHEL 8 and 9 and their derivatives. This might change if we can convince them to apply the patch to their kernel.

Where can we find the list of the supported OS and kernels to run some?

olivierlambert

Because that would be a pretty big list. Which distros you have in mind?

ksyblast

@olivierlambert we are using Oracle Linux, is OEL8/9/10 supported? As I understand OEL8 and 9 do contain a fix. We can also try with any other OS suggested by you. Thanks!

olivierlambert

Question for @Team-Hypervisor-Kernel

stormi

OEL 8 & 9 wouldn't contain the fix unless they applied extra patches for this to the RHEL 8 & 9 kernel(s). I'll let the hypervisor team check the current status.

dinhngtu

kernel-4.18.0-553.71.1.0.1.el8_10 (OL8) and kernel-5.14.0-570.37.1.0.1.el9_6 (OL9) do not contain the fix. kernel-6.12.0-55.29.1.0.1.el10_0 (OL10) does.

olivierlambert

Thanks @dinhngtu and @stormi

Forza

Tested the new updates on my prod EPYC 7402P pool with iperf3. Seems like quite a good uplift

Ubuntu 24.04 VM (6 cores) -> bare metal server (6 cores) over a 2x25Gbit LACP link.

Pre-patch

iperf3 -P1 : 9.72Gbit/s
iperf3 -P6 : 14.6GBis/s

Post Patch

iperf3 -P1 : 11.3GBit/s
iperf3 -P6 : 24.2GBit/s

Ubuntu 24.04 VM (6 cores) -> Ubuntu 24.04 VM (6 cores) on the same host

Pre Patch

Forgot to test this...

Post Patch

iperf3 -P1 : 13.7GBit/s
iperf3 -P6 : 30.8GBit/s
iperf3 -P24 : 40.4GBit/s

Our servers have Last-Level Cache (LLC) as NUMA Node enabled as most our VMs do not have huge amount of vCPUs assigned. This means for the EPYC 7402P (24c/48t) we have 8 NUMA nodes. We however do not use xl cpupool-numa-split.

olivierlambert

That's nice! It means it scales relatively well with many threads, it's a good result