VMware Server and Multi-Core CPUs

It's always been confusing to me as to what to do with VMware Server guests running on a host that has a multi-core processor.

First, just to clarify, from the information I have gathered VMware will assign a single physical processor to each virtual processor on a guest machine. If you have a multi-core processor, and your host operating system recognises each core as a separate CPU, then each virtual CPU on a guest will be assigned to a single core on your host.

You can get really fancy with assigning specific cores by tinkering with the VMX configuration file.

I've noticed that once I change my guest machine to use multiple CPUs, the CPU usage on my host shoots up to almost 100% and seems to stay there. Curiously though, this doesn't seem to affect any of the other processes running on the host. The answer, as stated in the VMware Knowledge Base, is that the host is executing the "system idle" process on the guest.

On Windows, when your processor isn't doing anything, Windows will execute a sort of 'do nothing' command that shows up in Task Manager as the "System Idle" process. It literally does nothing, and any unused CPU cycles go to this process. (Don't ask me why, I'm just the messenger here.) Unfortunately, VMware seems to translate this as an actual process and executes it on the host, but there it also does nothing. However, the host doesn't recognise this translation as an idle process and shows the VMware process itself as using those CPU cycles. So, even though it looks like your host is doing a lot while your guest is doing nothing, neither are really doing anything.

That still doesn't make me feel comfortable when I first see high CPU usage on my systems, but as long as it isn't causing any problems then I'll just leave things be.