VirtualBox - High VMM Load (if CPU cores > 1)

Question

I have Lubuntu 22.04 LTS (guest) installed on a VirtualBox VM. VirtualBox is running on a Ubuntu 20.04 LTS (host) machine.

Initially, I assigned 2 (virtual) CPU cores to the VM. I noticed that I constantly had a high VMM load although the guest was idle. The Session Information window showed a huge discrepancy between guest load and VMM load. While the guest load was at ~0%, the VMM load was anywhere between 50-90%. I then switched the numbers of assigned cores around. The interesting thing is that this discrepancy only appears when I assign 2 (virtual) cores or more to the VM as shown in the picture below.

I have a couple of questions regarding this:

1. What exactly is the VMM load?
2. What is the difference between guest load and VMM load?
3. Why is there such a huge discrepancy between guest load and VMM load (with 2 cores or more)?
4. Which of these two variables is decisive when it comes to assessing the CPU load of the guest system (i.e. whether the system is working at the limit)?

I could not find satisfactory answers to my questions. And I also don't know if my VM is having a resource/performance problem. Due to the fact that while the guest load indicates no performance issues, the VMM load values look like a cause for concern. Used RAM was always around 385 MB (2 GB total assigned to VM).

The VirtualBox versions I have tried (with their respective Guest Additions): 6.1.38, 6.1.40 and 7.0.0. All with the same issue.

My CPU: Intel Xeon E3-1231 v3 @ 3.40 GHz (4 physical cores, 8 threads).

EDIT: I made an interesting observation while doing some CPU stress tests:

• 1 assigned virtual core: guest load increases but VMM load remains at ~0% during the CPU stress test period.
• 2 assigned virtual cores: guest load increases whereas VMM load decreases during the CPU stress test period.

In summary, every time the guest system needs processing power to handle user/system tasks, the VMM load goes down or stays down. This could mean that whatever the VMM load represents, it does not (or hardly) affect the processing power needed by the user/system.

Furthermore, I have observed that even if (in the case of 2 or more allocated virtual cores) the VMM load is high, the CPU load on the host system remains low. The CPU load on the host system only increases when the guest load increases. Consequently, the VMM load represents something that does not or hardly use CPU power.

The only question that remains is why the high VMM load starts to appear with 2 assigned virtual cores. Possibly processes for coordination between the cores?

Answer 1

I have found some explanation by the developers in the post VirtualBox vs Hyper-V AMD/Intel big performance difference - page 2:

VirtualBox counts the realtime in three nanosecond counters called cNsExecuting, cNsHalted and cNsOther. The first two sum up the times when the guest OS is executed and halted respectively. The third counter sums up the times when either VirtualBox itself is doing "something else" or when VirtualBox is not active at all (and the guest OS is not in the halted state). This "something else" is difficult to grasp, because it hides in the majority of the VirtualBox code.

In the Performance Monitor, the Guest Load represents the increment of cNsExecuting, and the VMM Load represents the increment of cNsOther. Since the CPU load both in the guest OS and in the host OS seem to not reflect this VMM Load, I don't expect this issue to have a high priority for the VirtualBox development.

VMM Load is then explained as an obscure counter of times when VirtualBox is doing "something else" or is not active at all. It is said not to have any implication regarding the the CPU load in both the guest and the host and is deemed by the developer to not be of much importance.

The most I can say is that if the developer doesn't see it as very important, you should do the same.