in virtualbox, what happens when you allocate more than one virtual core?

Question

If I'm using Oracle's virtualbox, and I assign more than one virtual core to the virtual machine, how are the actual cores assigned? Does it use both real cores in the virtual machine, or does it use something that emulates cores?

Answer 1

Your question is almost like asking: How does an operating system determine which core to run a given process/thread on? Your computer is making that type of decision all the time - it has far more processes/threads running than you have cores available. This specific answer is similar in nature but also depends on how the guest machine is configured and what support your hardware has available to accelerate the virtualization process - so this answer is certainly not definitive and I won't really touch on how the host schedules code to be executed, but lets examine two relatively simple cases:

The first would be a fully virtualized machine - this would be a machine with no or minimal acceleration enabled. The hardware presented to the guest is fully virtualized even though many CPU instructions are simply passed through and executed directly on the CPU. In cases like this, your guest VM more-or-less behaves like any process running on the host: The CPU resources are scheduled by the operating system (to be clear, the host in this case) and the processes/threads can be run on whatever cores they are allowed to. The default is typically any core that is available, though some optimizations may be present to try and keep a process on the same core to allow the L1/L2 caches to be more effective and minimize context switches. Typically you would only have a single CPU allocated to the guest operating system in these cases, and that would roughly translate to a single process running on the host.

In a slightly more complex scenario, a virtual machine is configured with all available CPU virtualization acceleration options. In Intel speak these are referred to as VT-x for AMD it is AMD-V. These primarily support privileged instructions that would normally require some binary translation / trapping to keep the host and guest protected. As such, the host operating system loses a little bit of visibility. Include in that hardware accelerated MMU support (such that memory page tables can be accessed directly without being shadowed by the virtualization software) - and the visibility drops a little more. Ultimately though it still largely behaves as the first example: It is a process running on the host and is scheduled accordingly - only that you can think of a thread being allocated to run the instructions (or pass them through) for each virtual CPU.

It is worth noting that while you can (with the right hardware support) allocate more virtual cores to the guest than you have available, it isn't a good idea. Typically this will result in decreased performance as the guest potentially thrashes the CPU and can't properly schedule the resources that are being requested - even if the the CPU is not fully taxed. I bring this up as a scenario that shares certain similarities with a multi-threaded program that spawns far more threads (that are actually busy) than there are idle CPU cores available to run them. Your performance will typically be worse than if you had used fewer threads to get the work done.

In the extreme case, VirtualBox even supports hot-plugging CPU resources - though only a few operating systems properly support it: Windows 2008 Data Center edition and certain Linux kernels. The same rules generally apply where one guest CPU core is treated as a process/thread on a logical core for the host, however it is really up to the host and hardware itself to decide which logical core will be used for the virtual core.

With all that being said - your question of how VirtualBox actually assigns those resources... well, I haven't dug through the code so I certainly can't answer definitively but it has been my experience that it generally behaves as described. If you are really curious you could experiment with tagging the VirtualBox VBoxSvc.exe and associated processes in Task Manager and choosing the "Set Affinity" option and limiting their execution to a single CPU and see if those settings are honored. It probably depends on what level of HW assist you have available if those settings are honored by the host as the guest probably isn't really running as part of those.