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I'm trying to find out the best "bang for my buck" so to speak in regards to virtual machines, and SMP.

I have an Intel Core 2 Duo, which of course has two cores and the VT extensions, and I'm running Ubuntu Linux (host) on it with VirtualBox, which has Windows Vista (guest).

Currently I've got the guest machine setup for two processors to give Windows a chance to manage its own parallelism, but I'm not certain that it's any faster. I've tried it with just one processor, but it's hard to tell if it's any better.

Any thoughts? Should the guest have two processors setup?

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It really depends on your needs, but in your situation I would think it is better to leave the SMP enabled for the guest. I base that on the fact that when you are in the VM, you are probably trying to accomplish something specific, and you don't care about the host performance, or other VMs that might be running.

Chances are though, in typical desktop computing, you aren't doing something that really shows noticeable benefits from multi-CPU setups. And in a VM, the biggest bottleneck to performance is probably I/O. Really it is a balancing act between all the concurrent demands on your hardware: The CPU, RAM, I/O, etc.

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According to the WIKI for VirtualBox:

"SMP support (up to 32 virtual CPUs), since version 3.0"

So my guess is going to be that your emulating SMP with virutal proc's, and that is why your probably not seeing any real gains.My opinion at this point is that you can just stick with one.

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Is your application on the guest OS capable of using more than one CPU core ? As I know there isn't much parallelism in recent applications yet.

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If you only have two cores you probably shouldn't set up a VM that uses both cores. Limit the VM to one core will likely give you the best performance.

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