Re: PARI/GP pthread questions

[Karim Belabas <Karim.Belabas@math.u-bordeaux.fr>]

* Bill Allombert [2023-12-03 13:15]:
[...]
> > 2)
> > Under 3.4.22 nbthreads
> > ...
> > * pthread: number of threads (unlimited, default: number of cores)
> > ...
> > 
> > On 16C/32T AMD 7950X CPU I see 3200% CPU in top when starting
> > GP script with parforeach.
> > 
> > So should above doc be corrected to
> > 
> > "default: #cores * #threads_per_cor"
> > 
> > or better to be correct for multi-CPU systems:
> 
> When we wrote that, hyperthreading was only used in mainframes.
> It would be much better if the default was the number of cores instead of the number
> of hyperthreads.
> Unfortunately the GNU C library only report the number of CPU threads
> see getconf "_NPROCESSORS_CONF".
> I recommends to set nbthreads to the total number of cores.

Is there a difference with the simpler 'nproc' (from coreutils) ?

For systems with hyper-threading, one can get the number of physical cores
as follows on my laptop:

# grep '^cpu cores' /proc/cpuinfo | uniq
cpu cores	: 4

# getconf _NPROCESSORS_CONF 
8

# nproc 
8

Maybe we could include the number of physical cores in the output of
Configure --mt=pthreads ? And add to INSTALL.tex that we advise to
override the default value of nbthreads in gprc.

Not sure whether it would be wise to change the default in
  pthread.c:pari_mt_init().

Cheers,

    K.B.
-- 
Pr. Karim Belabas, U. Bordeaux, Vice-président en charge du Numérique
Institut de Mathématiques de Bordeaux UMR 5251 - (+33) 05 40 00 29 77
http://www.math.u-bordeaux.fr/~kbelabas/