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Xenserver 8.0, missing AVX-512

Jorge Guerra1709161037


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I noticed the exact same thing.
Also Turbo mode is not fully working. Which might be related to the CPU 'driver' they are using. The fact is that CPU support is falling behind.

It's one thing to 'say' that CPU x is supported. It's another to make the CPU being usable with all it's functions and features.

I have been digging into this stuff, but have not been able to find a solution yet.
If you find a solution please let me know, I really need to get better performance for CAD workloads, single core and vector stuff (AVX-512).

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Hello Tobias and others,

Thank you for your reply Tobias. I have checked the link that you posted. A lot of those things I had already checked.


I think I figured out what is going on. And I believe it's working just fine. :(


Nonetheless in my case it was not related to the BIOS settings.
When I booted the server with a live CD, containing CoreFreq, I can bench the CPU and I can see that the CPU boost without any issues to the full speed 3.7 Ghz.
The live cd can be downloaded here. https://github.com/cyring/CoreFreq/wiki/Live-CD

My hardware:

  1. HP DL 380 Gen10 with Intel Xeon Gold 6132 @ 2.6 Ghz

I have tested with XenServer 7.6 Update3 and Citrix Hypervisor 8.0 (no patches)

I have created one VM, running Windows 2012 R2, 60GB and 14 vCPU's. (that's our XenApp config).


I was digging through some Intel documentation and found an interesting bit of information.

CPU's are being advertised with their base speed and max turbo speed. However this ONLY goes if you are not using more modern instruction sets like AVX2.
This documents shows three graphs, which I have included below. The first one shows the "old" SSE instructions, it's base core speed and max turbo speed for those instructions. Same goes for the second graph but instead of SSE it shows AVX2 and the third shows AVX-512 (which Xen does not support, for now).

I don't know for sure, but I think that Xen is also activly executing AVX2 instructions. Which would explain why I never see turbo boost to the full 3.7 Ghz. It only goes to 3.2
In regards to how many cores we have a load on.. We have 30vCPU's assigned, 14 for the VM and 16 for Dom0. Those are being scheduled on the Pcpu's. In a rather fast way.. Which might cause a load on all pCPU's. and therefore reduce max core speed. Makes me wonder. Would it better to reduce the number of vCPU's for this reason? That's someting i'm going to try next.. In those tests I will also try to disable HT. And check the core clock speed.

Again. We need maximum boost on single cores, for CAD applications.

Thank you for your time!

Source: https://www.intel.com/content/www/us/en/processors/xeon/scalable/xeon-scalable-spec-update.html







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On 18/06/2019 at 4:25 PM, Tobias Kreidl said:

I just found out while AVX-512 is not officially supported yet, you can enable it in CH 8.0 with the command line:

/opt/xensource/libexec/xen-cmdline --set-xen cpuid=avx512




Thank you for the command, we put it and the AVX appeared in the hypervisors and in the virtual machines after restart .
After 2 weeks since we executed the command, we had no problems with the pool.
We will leave it enabled while integrating it from citrix.
If in the future we see that it causes a problem, we will report it.
Sorry for the delay in answering and giving feedback
Thank you again

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