Overcommitting CPUs with BHyve?

[Alan Somers]

What are people's experiences with overcommitting CPUs in BHyve?  I have an
8-core machine that often runs VMs totalling up to 5 allocated CPUs without
problems.  But today I got greedy.  I assigned 8 cores to one VM for a big
build job.  Obviously, some of those were shared with the host.  I also
assigned it 8GB of RAM (out of 16 total).  Build performance fell through
the floor, even though the host was idle.  Eventually I killed the build
and restarted it with a more modest 2 make jobs (but the VM still had 8
cores).  Performance improved.  But eventually the system seemed to be
mostly hung, while I had a build job running on the host as well as in the
VM.  I killed both build jobs, which resolved the hung processes.  Then I
restarted the host's build alone, and my system completely hung, with
top(1) indicating that many processes were in the pfault state.

So my questions are:
1) Is it a known problem to overcommit CPUs with BHyve?
2) Could this be related to the pfault hang, even though the guest was idle
at the time?

-Alan