vCPU info needed

[lvaibhavt]

Hi All,

Just wanted to understand the basics of CPU assigned to a VM. For example I have a ESXi Host with 2 physical CPU's and each CPU having 4 cores. Total cores 8.

Now I have two VM's A & B each with each 6 vCPU's. As I understand each vCPU will be aligned to a different core. Now if VM A for example is using all the CPU in the OS i.e. using all 6 cores.

If VM B has to do some CPU related job (same time) -- it will not be able to do this because as I understand till the time VM B is not getting free 6 physical cores for its 6 vCPU's it will not be able to process any CPU request. Though it has two physical cores free.

Is this right ?

Thanks

Vaibhav

[jwmarrott]

That is not far off.  VM #2 will get processing time, if requested, but VM #1 being active will cause delays in VM #2 getting CPU time. When VM #2 finally gets some cycles VM #1 will pause while VM #2 uses its slices of CPU time. It is also important to note that periodically, even if VM #2 is doing nothing but is still powered on, all vCPUs of VM #2 will check in and briefly require 6 cores. This will interrupt the processing of VM #1.

This scenario will also cause very high CPU Ready time (time when a vCPU is waiting for pCPU time).

Jon