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I have an Intel Core i7 3930K with 6 CPU cores. Now, I understand that every of that cores can handle 2 separate threads. That makes 12 virtual cores - which I can see using my system monitor (see screenshot below).

But when I create a virtual machine using the VirtualBox software from Oracle, I suddenly can allocate 24 CPUs to that machine. How so?

12 or 24 cores

So, what does that mean? Are there actually 24 cores - or is VirtualBox wrong?

And what happens if I allocate for example 4 cores - how many are really added to the virtual machine?

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up vote 6 down vote accepted

One core, with or without hyper-threading, can simulate any number of virtual cores, simply by timeslicing. However, this produces all kinds of problems because the operating systems expect that code running at the same time will actually interleave. Overprovisioning can lead to performance drops so great the system can appear frozen with operations literally taking thousands of times longer than they should.

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Thanks alot, +1 you already answered my follow-up question, why my Windows 8 now always freezes! :) – 5chdn Jan 23 '13 at 14:54
The "extra" cores will be simulated (well, all of them are simulated, if you look at it that way). It works just fine. The cumulative performance will of course not be greater than that of the actual hardware. Lockups and problems are most likely bugs in the VM software that are probably fixed in newer version. I just started a 6 core VM on a 4 core HW and it works just fine, loading all 6 virtual cores (using VirtualBox 4.3.14 with Windows). VmWare also supports this. – David Balažic Nov 15 '14 at 19:08
@DavidBalažic It doesn't work "just fine". It can cause horrific performance losses. The OS will sometimes think that code is running in another core (when it actually isn't) and wait for it, using up the very resources that code would need to unblock it. It really can cause catastrophic performance losses. The OS needs to know whether a thread is running or not to behave sensibly. – David Schwartz yesterday
@DavidSchwartz It may cause bad performance. That is 99% dependent on the VM implementation. About the OS wait you mentioned: if you mean the guest OS, then it will work fine, as the VM will "break" the "thread" that is waiting and run the other thread, irrelevant of the number of actual physical cores (vmware is more flexible here, but that again depends on the version used). Maybe you can show an actual case where the problems you mention happen. – David Balažic 22 hours ago
@DavidBalažic It will not do so because it has no way to know what the thread is waiting for. It does in fact happen. OSes do in fact spin when a resource they want is held by another thread that they believe is running on a core. It really does reduce performance by factors of thousands for an OS not to know that a thread is not running. – David Schwartz 21 hours ago

The processor i7 3930K has VT-x.

Intel® Virtualization Technology (VT-x) allows one hardware platform to function as multiple “virtual” platforms. It offers improved manageability by limiting downtime and maintaining productivity by isolating computing activities into separate partitions.

That means you're able to simulate several more CPUs than the threads it can handle, thus seeing a higher number of CPUs available when using a Virtual Machine.

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If you allocate 4 cores you get 4 cores in the VM. However almost all the time you're better off just allocating 1 because you add a lot of overhead every time you add another core.

You can have a maximum of 5 cores in your case, if you go over that into the red area your virtual machine will become unstable.

Virtual box reccomends the maximum number of cores you use should be n-1 of your physical cores. Hyper-Threading does not count.

This thread on the virtual box forums supports this.

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But why does VirtualBox tell me that even 12 cores are "ok" ? (see screenshot.) I just tried 8 and it keeps freezing... – 5chdn Jan 23 '13 at 15:43
It doesn't do this on new versions of VirtualBox by the way. – Tek Aug 7 '14 at 13:22

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