Understanding CPU and memory usage in Bhyve

[Roman Bogorodskiy]

  Peter Grehan wrote:

> Hi Roman,
> 
> > For the CPU, there's some info provided by bhyvectl, e.g.:
> >
> > /usr/sbin/bhyvectl --vm=bhyve --get-stats --cpu=1
> >
> > in the field 'vcpu total runtime', for example:
> >
> > vcpu total runtime                              8178870653
> >
> > Two question about that:
> >
> >   1. What are the units used here? Looks like it's ticks, so if I want to
> >   convert this number, to, say, nanoseconds, would it be right to do
> >   something like:
> >
> >       X * 1000**3 / kern.clockrate.stathz
> >
> >   (X beeing the value returned by bhyvectl and kern.clockrate is a
> >   sysctl name)?
> 
>   It's in TSC units so you can convert to nsecs using the 
> machdep.tsc_freq sysctl node.
> 
> >   2. This value shows only vcpu time, without hypervisor time?
> 
>   That's correct. To get the hypervisor and vCPU time together, you can 
> use existing CPU usage reporting (e.g. that used by top et al).


Thanks for the clarification, it's very helpful. However, I need some
more help with that.

I'm trying to implement some domain stats reporting for the bhyve driver
in libvirt and it looks like I choose a wrong approach initially. 

It's described here:

https://www.redhat.com/archives/libvir-list/2014-April/msg00912.html

But the summary is: is there a way to figure out how much CPU time
bhyve and the guest spends on host CPUs N (N = 0, 1, ...)?

> > For the memory stats, it's less obvious. Does guest always use an amount
> > of memory specified at startup with '-m' (i.e. bhyve -m 1G) or it could
> > use less?
> 
>   That's the maximum it can use. We currently don't expose the incore 
> amount from the vmspace representing the guest's physical - that should 
> be added to bhyvectl soon.

Looking forward to that.

> > What is the format of bhyvectl --get-{low,high}mem output?
> 
>   The number reported there isn't to do with guest usage, but how the 
> memory is divided between < 4G and above 4G. For example, the default 
> for bhyve is to use up to 3G below 4G, provide 1GB for PCI MMIO decode, 
> and then put the remainder of guest memory above 4G. Here's an example 
> for an 8G VM, with 3G of guest mem below 4G (starting address 0), and 5G 
> above 4G (starting address 4G):
> 
> lowmem          0x0000000000000000/3221225472
> highmem         0x0000000100000000/5368709120
> 
> > PS I found it a little confusing that bhyvectl displays vcpu0 stats by
> > default if --cpu is not provided, expected it provide info of all vcpus
> > in the guest by default.
> 
>   Yes, that's a relic of the days when there was only a single vCPU 
> supported :(
> 
> > Also, didn't find a way to get a number of vcpus in a running guest.
> 
>   Until the vCPU state is exposed by bhyvectl, or we provide a sysctl, 
> you can use heuristics: the number of vCPU threads for the bhyve 
> process, or scan all vCPUs and only count those that have a non-zero RIP.
> 
> later,
> 
> Peter.
> 

Thanks,

Roman Bogorodskiy
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