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

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

On 1/4/24 15:40, 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 21:36:04 -0000 (UTC)
>
>   jtocino%gmx.com@localhost (Jason Bacon) writes:
>
>   >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.
>
>   There are forms of "hyperthreading" where you have more than one
>   thread per core and with recent CPU designs, you can even have
>   different thread counts on each core.
>
>   A simple 'physical core count' isn't sufficient for anything.

A simple core count is sufficient for preventing oversubscription of
cores, in order to maximize the speed of individual threads by
eliminating a major source of contention.  This is the goal of most HPC
resource managers, and of the py-joblib developers, which triggered this
discussion (adding BSD support for auto-detection of available cores).