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Let's leave the memory issue for a moment, aside. If I have two 64-cores AMD EPYC processors each, how many virtual machines (servers) can I run at the same time? Is that right that I can run 64 VMs at the same time? (assuming that each vm need 2 logic processors). My question was raised during reading the following article: Selecting the Number of Processors for a Virtual Machine

Hiddai
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    There's no definitive answer to that question - it depends mostly on the workload intended for the host. – fuero Apr 13 '20 at 20:49
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    Depending on the virtualization technology you use, the number of virtual machines is not limited by the number of processors. – Piotr P. Karwasz Apr 13 '20 at 21:41
  • @fuero, but, does 'workload' rest on memory, storage and ... processing (CPU, cores)? – Hiddai Apr 16 '20 at 16:40

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If we reframe your question a bit to not be specifically about capacity planning:

The entire point of virtualization is to make more efficient use of the available resources than would be the case if each service was siloed within the physical hardware running it. Typically many supporting services each only utilize a fraction of the available compute power in a modern server. You could easily run multiple virtual servers with this kind of usage profile for each physical CPU core available in your machines.

Other use cases require a lot more available compute power at any given time. In these cases performance will suffer if you overcommit your environment - it will be a bit like individual requests for a CPU time slice get double-booked, and some of them will need to wait for the next opportunity.

What you will see in a hypervisor that has been stretched too thin is that a value called CPU Ready time begins increasing: That's basically a measure of time during which the virtual CPU was ready to do work but didn't receive tasks. Elevated such numbers are a strong indicator that you need to purchase additional hardware or decrease the number of assigned virtual CPU cores in your environment.

Mikael H
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If you want to use all the processors, you can run at least 64 VMs with stable performance for sure; you can run more than 64 VMs but you have to monitor their performance.

Anita Salerno
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