virtualbox.org

I have a machine with two processors, having 10 physical cores each. The host OS is Debian 9, the guest OS is Ubuntu 16.04. The VM is headless, and configured with bog-standard settings (I changed nothing except of course --cpus 20).

For some reason, when I run "stress --cpu 20" on the guest OS, the 20 VBoxHeadless threads in the host OS seem capped to 90% CPU usage (NB.: the execution cap is 100%)! They also do a bazillion of voluntary_ctx_switches (10,000 per second ).

Can anyone help me figure out what the heck is going on here? Thanks in advance.

You have a host with 20 CPU cores and want to run a guest whith 20 CPUs. Then the host doesn't have a CPU left to handle the running of the VM and needs to switch very often between letting the VM run and handling it.

Providing fewer virtual CPUs makes no difference.

Please provide a VM log file. With the VM fully shut down, right click it in GUI. Select "Show Log" and save "VBox.log" (ONLY) to a zip file. Attach the zip here.

Sure, I'll do it shortly. Just to be clear, there's no GUI: I'll paste here the contents of Logs/Vbox.log.

Edit: I am sifting through the log to obfuscate usernames, IPs, and other potentially sensitive information. In the meantime, I "solved" the problem by resorting to kvm (which I really didn't want to use, as it is remarkably more convoluted), pinning each virtual core to a physical core, and sharing the NUMA topology with the guest. I now have full CPU utilisation, no context switches, and this is reflected by the performance.

If you're obfuscating the log the please don't bother posting it.

Why? You should see what you're looking for anyway.

Thanks for your help, by the way.

Because I don't want to see a log filtered by what you think is important.