Max vCPU per guest with respect to Host pCPU totals?

[vmproteau]

Our Hosts have 8 core total (2-quad core). In the past I've cautioned against creating 8-vCPU guests (or at leaste very limited) fearing potential adverse CPU performance across the Host.

With the newer software and CPU architectures, I am reviewing this to see if it is still a valid concern. As we try to pull in heavier workloads, there may be a need for 8-vCPU guests so, eiher, I need to be confident they can be supported with our current Host CPU count or, consider adding physical CPU.

I plan to look at some metrics in the Lab and test but, anyone have some rule of thumbs, experience, white paper, or advice regarding this. Is it still not a good idea to have a guest VM that is confiured to use every core on a given Host?

[Troy_Clavell]

I think the best place to start is below

http://www.vmware.com/files/pdf/perf-vsphere-cpu_scheduler.pdf

...and i would advise against starting at 8 vCPU's. Start low and work your way up.

[vmproteau]

Thanks that's exactly what we do, but we do occasionally get requestst to start at 4-vCPU for vendor speciified requirements. Mainly I was concerned about using all cores in a given Host and I think that still doesn't make sense or is something that should be monitored closely for CPU ready increases. I would suspect a heavily used 8-vCPU VM would take over a Host. DRS perhaps moving CPU starved VMs to alternate Hosts. Just don't really want Hosts that end up supporting only one VM.

[khughes · Accepted solution]

The vendor requests for minimum requirements on CPUs (vCPU's) is something that everyone faces. I'm in that battle right now where they say if we want to run their setup virtualized we would have to dedicate resources to the VM. I can't take the credit but Tom gave me a great idea which is have the resources available to give to that particular VM if you need to, but start low like Troy suggested. By starting low you can see if it will run just fine with less resources than requested, if it doesn't or you need to troubleshoot you have the resources to bump it up.

-- Kyle

"RParker wrote: I guess I was wrong, everything CAN be virtualized "