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We have a EPYC Rome 7502 host running vSphere v7.0.3, and for .net applications we're seeing a performance hit of around 50%, compared to the old host which is Xeon E5-2630.

The performance hit in the .net app is all CPU based.

CPUZ suggests the host should be faster for both single and multithreaded tasks.

According to VMWare's white paper we shouldn't have to do anything. It doesn't explain how to configure the individual VMs optimally. Its really hard to understand how the physical cores (+ HT cores) are presented to the virtual machine. Should we assign 4 or 8 CPUs for one 'CCX'?

I'm assuming something is being presented to the operating system differently, which is causing either the OS scheduler, or .net's to assign a thread to a core inefficiently.

Any suggestions on how to improve the performance to get it to where it should be? How to configure the vm to use one CCX 'node' would be a great start.

BJury
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    You could look into the firmware of your host. Maybe there is an update. Just as a suggestion. – Christopher Perrin Oct 29 '21 at 12:20
  • Have you tried fixing the VM to a number of cores (a 7502's got 4 cores x 8 CCXs) using the same L3 cache? – Zac67 Oct 31 '21 at 19:25
  • @Zac67 we've tried 4 and 8 cores, but to no effect. (8 to try to include HT cores.) Is there anyway to see which cores are being used by which VMs in vSphere? – BJury Nov 01 '21 at 09:29
  • Unless you've configured otherwise, core allocation is entirely dynamic. You can see that on the vSphere Client's performance tab if nothing else is running on the host. You should try fixing the *scheduling affinity* in the VM's CPU settings to specific cores (rather logical processors). I haven't figured out the exact numbering the ESXi does, so for a 4-core VM you should test something like `0-3` or `0-1,4-5` or `0-1, 8-9` - possibly even `0,2,4,6` or `0,4,8,12` or `0,8,16,24`. You should be able to see some pattern. – Zac67 Nov 01 '21 at 10:32
  • Also, have you checked https://developer.amd.com/wp-content/resources/56779_1.0.pdf ? – Zac67 Nov 01 '21 at 15:18
  • @Zac67 thanks, but I think those recommendations were superseded when U2 was released? (See paper in the post?) – BJury Nov 02 '21 at 10:38
  • @BJury Yes, they should, but I'd test that all the same. AMD make a few good points and I'm not entirely sure VMware have addressed them all. – Zac67 Nov 02 '21 at 10:42

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