Overcommitting CPUs with BHyve?

[Allan Jude]

On 2018-07-24 17:30, Alan Somers wrote:
> 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
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Bhyve has a command line flag, -p, to let you pin a vCPU to a physical
CPU. This might avoid some of the issues with the threads hopping around
all the time.

If you were anyone else, I'd also ask if you ensured your
vfs.zfs.arc_max was low enough to actually leave some ram for the VM to use.

-- 
Allan Jude

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