Re: Incorrect formula for SysV IPC parameters

On Wed, Feb 3, 2016 at 12:51 PM, Kyotaro HORIGUCHI
<horiguchi.kyotaro@lab.ntt.co.jp> wrote:
> Hello, I found that the formulas to calculate SEMMNI and SEMMNS
> are incorrect in 9.2 and later.
>
> http://www.postgresql.org/docs/9.5/static/kernel-resources.html
>
> All of them say that the same thing as following,
>
> | SEMMNI  Maximum number of semaphore identifiers (i.e., sets)
> |
> |   at least ceil((max_connections + autovacuum_max_workers + 4) / 16)
> |
> | SEMMNS        Maximum number of semaphores system-wide
> |
> |  ceil((max_connections + autovacuum_max_workers + 4) / 16) * 17
> |    plus room for other applications
>
> But actually the number of semaphores PostgreSQL needs is
> calculated as following in 9.4 and later.
>
>   numSemas = MaxConnections + NUM_AUXILIARY_PROCS(=4)
>   MaxConnections = max_connections + autovacuum_max_workers + 1 +
>                    max_worker_processes
>
> So, the formula for SEMMNI should be
>
> ceil((max_connections + autovacuum_max_workers + max_worker_processes + 5) / 16)
>
> and SEMMNS should have the same fix.
>
>
> In 9.3 and 9.2, the documentation says the same thing but
> actually it is calculated as following,
>
>   numSemas = MaxConnections + NUM_AUXILIARY_PROCS(=4)
>   MaxConnections = max_connections + autovacuum_max_workers + 1 +
>                    GetNumShmemAttachedBgworkers()
>
> Omitting GetNumShmemAttachedBgworkers, the actual formula is
>
> ceil((max_connections + autovacuum_max_workers + 5) / 16)
>
>
> In 9.1, NUM_AUXILIARY_PROCS is 3 so the documentations is correct.
>
>
> I attached two patches for 9.2-9.3 and 9.4-9.6dev
> respectively. patch command complains a bit on applying it on
> 9.2.

Good catch!

ISTM that you also need to change the descriptions about SEMMNI and SEMMNS
under the Table 17-1.

Regards,

-- 
Fujii Masao