Question: 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?

[![enter image description here](https://i.stack.imgur.com/EJCLW.png)](https://i.stack.imgur.com/EJCLW.png)

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?

Answer: 
In my case, using 6 out of 8 CPUs in a 4 core 8 thread processor was faster in tests than 4 out of 8, despite VirtualBox cautioning that I may get degradation.  

So I think that VirtualBox prevents one from shooting oneself in the foot if one is a newbie and will complain about slowness, while as a superuser you may dismiss this caution.

Long answer:
============

I have [Intel-Core-i7-2820QM](https://ark.intel.com/products/52227/Intel-Core-i7-2820QM-Processor-8M-Cache-up-to-3_40-GHz) with 4 cores and 8 threads. I have Virtual Box 5.2.6 with Windows 10 host and guest.

**Setting 6 `CPU`s** out 8 in Virtual Box Guest settings gives next values in [userbenchmark](http://www.userbenchmark.com/UserRun/6954363):

```

Multi core

MC Int 307
MC Float 326
MC Mixed 315
47% 316 Pts

Quad core

QC Int 270
QC Float 272
QC Mixed 268
61% 270 Pts

```

CPU-Z:

[![enter image description here](https://i.stack.imgur.com/M2N3d.png)](https://i.stack.imgur.com/M2N3d.png)

**Setting 4 `CPU`s:**

[Userbenchmark](http://www.userbenchmark.com/UserRun/6954468)

```

Multi core

MC Int 260
MC Float 269
MC Mixed 269
40% 266 Pts

Quad core

QC Int 260
QC Float 264
QC Mixed 262
59% 262 Pts

```

CPU-Z:

[![1108 4.21 6](https://i.stack.imgur.com/veNQg.png)](https://i.stack.imgur.com/veNQg.png)

**Notes:**

No thermal throttling (which happens when 100 degree Celsius) happened during test runs:

[![997 3.37 4](https://i.stack.imgur.com/R8bwE.png)](https://i.stack.imgur.com/R8bwE.png)
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.