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Is it possible to run more VMs than cores available on the computer?

Why is it not possible to allocate more cores to a single virtual machine then number is available on the host computer?

I am using virtualbox.

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Is it possible to run more VMs then cores

Yes. Just the same way you can run many processes.

Why is it not possible to allocate more than the cores to a single VM, then what is available on the computer?

Because that is how the hyper-visor was developed. Some virtualization programs will fully emulate everything including the CPU in software (see binary translation). Others make use of hardware features on your system like VT-x (AMD-V), which is much faster then full emulation, but includes some limitations. Since you are depending on VM features from built into the CPU, you cannot emulate more CPU then actually exists on the system.

As far as I know, no desktop hypervisor is doing full emulation for x86/x64. Occasionally you will see a hypervisor that does full emulation so you can get a PPC on x86, or ARM, on x86.

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Regarding emulating more cores then are on the computer, another reason is because it can only make the VM slower, so why bother. If you need a ton of cores, use amazon cloud or something. – David Mar 25 '13 at 21:36
Ok the info is good, but here is what I got out of it...if you run more vms than cores, the processes containing the threads will just be swapped among themselves. However you cannot allocate more cores than the cpu allows because the threads cannot run in parallel without having to alternate between themselves, thus reducing speed and is pointless. – agz Mar 26 '13 at 1:09

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