Opened 4 years ago

#19500 new defect

VirtualBox should now prohibit assigning all physical CPUs to a VM and/or fix VirtualBox CPU assignment manual

Reported by: adrelanos Owned by:
Component: other Version:
Keywords: Cc:
Guest type: other Host type: all


Quote VirtualBox manual

You should not configure virtual machines to use more CPU cores than are available physically.

So I concluded "i have 4 physical CPUs so let's assign 4 virtual CPUs to VM.

As I've learned today, VirtualBox forum moderators disagree.


You have 4 CPUs and you assign all of them to your guest. Expect turbulence and bumpy flight.


You should not assign all of them to the VM however - the host OS needs CPU time to run host apps, particularly VirtualBox!

Therefore, for example, if there are 4 physical CPUs, the maximum CPUs one can assign should be limited to 3.

In any case, please update VirtualBox manual accordingly. Or if these forums moderators are in fact wrong, please kindly point that out too.

Minor details if you agree with above:

Maybe you could allow the user to ignore that warning but I you will know best.

Please consider not breaking existing installations of VirtualBox with lets say 4 physical CPUs and 4 virtual CPUs assigned to VMs. Please either:

  • leave it as is (user might notice one day looking into settings), or
  • automatically reduce number of virtual CPUs to 3 (if that isn't considered intrusive)
  • a popup that asks the user if that reduction should be applied before next start of VM (ugly?)

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