How hard is BHyVe's 16 vCPU limit, is it configurable under any circumstance?

Tinker tinkr at
Wed Nov 12 22:09:01 UTC 2014

On 2014-11-12 23:55, Allan Jude wrote:
> On 2014-11-12 16:39, tinkr at wrote:
>> Hi!
>> In order justify giving energy to BHyVe, I need to know if it's
>> "future-proof" in that the 16 vCPU limit can be increased?
>> Please let the world know if BHyVe's 16 vCPU limit can be lifted in 
>> some
>> way, by configuration, patch, etc. (and if you want to, why this limit
>> is in place by default today).
>> Thanks!!
>> Tinker
>> _______________________________________________
>> freebsd-virtualization at mailing list
>> To unsubscribe, send any mail to
>> "freebsd-virtualization-unsubscribe at"
> You can increase the limit by editing sys/amd64/include/vmm.h
> #define VM_MAXCPU       16
> From what I've been told, things scale badly above 24 CPUs. They plan 
> to
> solve this issue, but have not yet. If you system has enough cores to
> support using more than 16 per VM, you can modify the file and 
> recompile
> the kernel and use as many CPUs as you want, but not much testing has
> happened with bigger numbers.

Dear Allan,

Thank you very much for responding.

Aha, great!

Only for completeness, do you have any particular idea about
  * When the scaling above 24 vCPU:s will be optimized, like approx how 
many years away is it (like 1 or more than 1)?, and
  * What the technological reason for the scaling is, is it that somehow 
the BHyVe instances on the different cores need to inter-communicate, 
for instance that all disk and network IO is done via one single core 

In all cases, your response is great news, as your baseline answer that 
it's doable and only a question of optimization and tweaking of present 
functionality -

Thanks again!

More information about the freebsd-virtualization mailing list