Re: postgresql 18 and shm

[Marc Baudoin <babafou%babafou.eu.org@localhost>]

Greg Troxel <gdt%lexort.com@localhost> écrit :
> Marc Baudoin <babafou%babafou.eu.org@localhost> writes:
> 
> > The minimum values that make PostgreSQL 18 run are (on the system
> > I tested last year) are:
> >
> > kern.ipc.semmni=11
> > kern.ipc.semmns=187
> >
> > So I rounded up to get:
> >
> > kern.ipc.semmni=20
> > kern.ipc.semmns=200
> 
> The pgsql 18 docs give formulas in terms of adding other variables, and
> give an invocation to print them.  One can trace the formulas and see if
> that gets the same result as your experiment.

From https://www.postgresql.org/docs/18/kernel-resources.html#SYSVIPC

SEMMNI	at least ceil(num_os_semaphores / 16) plus room for other applications
SEMMNS	ceil(num_os_semaphores / 16) * 17 plus room for other applications

test@test=> show num_os_semaphores ;
 num_os_semaphores 
-------------------
 174
(1 ligne)

ceil(num_os_semaphores / 16) = ceil(10.875) = 11 (my empirical
value)

ceil(num_os_semaphores / 16) * 17 = 11 * 17 = 187 (my empirical
value again)

So it checks ;-)