guest with more vCPU than pCPU a bad idea?

Hi All,

My understanding of vmware was that a guest with high parallelism should not have more vCPU than physical cores on the host. We have a high performance SQL server VM (Windows 2008 R2 soon to be 2012) with 16 vCPU on a Esxi 5.1 host with 2x8 core processors, giving a total of 32 logical processors. it sometimes sees spikes of 100% across all vCPU.

Anyway they want to run 24vCPU and as i dont have a test enviroment to try this out on to confirm my thoughts, I have come here.

If we allocated more vCPU than physical cores to one single guest, would this mean that it would work fine for certain loads but when it went past a certain tipping point (no longer being able to have all vcpu scheduled to physical cores) it would spiral into huge wait times? I guess it just depends if Vmware can communicate to the guest which vCPU are on the same physical core.

And unfortunately we are not able to scale this guest back at the moment... we are looking at extra guests with less vCPU and replicated sql instances on the same host to overcommit the hardware and make better use of our processing power