Re: kern/57816: Add sysctl support for physical cores

[Jason Bacon <jtocino%gmx.com@localhost>]

On 1/4/24 10:05, Michael van Elst wrote:
> The following reply was made to PR kern/57816; it has been noted by GNATS.
>
> From: mlelstv%serpens.de@localhost (Michael van Elst)
> To: gnats-bugs%netbsd.org@localhost
> Cc:
> Subject: Re: kern/57816: Add sysctl support for physical cores
> Date: Thu, 4 Jan 2024 16:03:54 -0000 (UTC)
>
>   jtocino%gmx.com@localhost writes:
>
>   >Darwin reports actual cores as hw.physicalcpu, and FreeBSD as kern.smp.cores.  Both report hyperthreads for hw.ncpy, compatible with NetBSD.
>
>
>   That information is currently available using the cpuctl identify command,
>   but it's machine specific and probably delivers the information directly
>   only for x86.
>
>   For testing I had added sysctls to dump the cpu information in a machine
>   independent way, e.g.:
>
>   % sysctl kern.cpu
>   kern.cpu.cpu0.package = 0
>   kern.cpu.cpu0.core = 0
>   kern.cpu.cpu0.smt = 0
>   kern.cpu.cpu1.package = 0
>   kern.cpu.cpu1.core = 0
>   kern.cpu.cpu1.smt = 1
>   kern.cpu.cpu2.package = 0
>   kern.cpu.cpu2.core = 1
>   kern.cpu.cpu2.smt = 0
>   kern.cpu.cpu3.package = 0
>   kern.cpu.cpu3.core = 1
>   kern.cpu.cpu3.smt = 1
>
>
>   But that's not very useful on machines with many CPUs, it also
>   misses newer information about the topology (caches, numa, big.little, ...).

I think it would be useful to have another variable for the number of
physical cores.  It could produce the same value as hw.ncpu on most
architectures, which don't have hyperthreading.

Or, if there were a boolean to indicate whether hyperthreading is active
(which would always be 0 on most architectures), it would be simple
enough to divide hw.ncpu by 2.

Or, a threads_per_core variable, which would always be 1 on most archs,
and 2 where hyperthreading is available and active.