Subject: bug#65697: numproc, add flag to get numer of available cores

From: Simon Heimberg <[email-masked]>
Date: Sat, 02 Sep 2023 11:38:46 +0200

Hello

Depending on what a process is mainly limited by, the ideal number of
processes to run concurrently varies.
A frequent use case is to run one process on each _physical_ core
(which is available) than on each logical processing unit.
I propose to introduce a flag --core which returns the number of
physical cores available to the process (or of all cores with --all).

I made a draft in python using getaffinity(), assuming the threads of a
core are listed after each other:
https://github.com/giampaolo/psutil/issues/1968#issuecomment-1688687251

Looking at the code in gnulib/lib/nproc.c this could get hard to
implement for all cases. (Many cases. No list for physical cores exists
yet.)

Maybe it is better to extend the doc and explain what the number really
means (with and without --all). Or add a link to more details about
processors and cores and ... . Extended doc could improve:
* As #42044 complains that nproc does not include the cores which go
online automatically. (The current help text _could_ match this.)
* As I complain that nproc can not report (available) _physical_ cores.
* I just found out (partially) why --all shows more cpus than are in my
system. (It returns the _configured_ ones. And those get set ...:
https://unix.stackexchange.com/questions/609555/how-does-linux-detect-number-of-possible-cpus
) But the help writes "installed processors", not configured ones. (On
my system, processors which are configured only are not listed by ls -l
/sys/devices/system/cpu/cpu*)

About better using one process for each core, depending on ...:
https://github.com/giampaolo/psutil/issues/1968
https://stackoverflow.com/questions/27404532/optimal-number-of-processes
https://stackoverflow.com/questions/36958661/openmp-dont-use-hyperthreading-cores-half-num-threads-w-hyperthreading