Date: Thu, 4 Mar 2021 00:40:52 +0800
From: Julien Rouhaud <rjuju123@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Georgios <gkokolatos@protonmail.com>,
	Julien Rouhaud <julien.rouhaud@free.fr>,
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>,
	"robertmhaas@gmail.com" <robertmhaas@gmail.com>
Subject: Re: Shared memory size computation oversight?
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On Wed, Mar 03, 2021 at 11:23:54AM -0500, Tom Lane wrote:
> Georgios <gkokolatos@protonmail.com> writes:
> > My limited understanding is also based in a comment in CreateSharedMemoryAndSemaphores()
> 
> >          * Size of the Postgres shared-memory block is estimated via
> >          * moderately-accurate estimates for the big hogs, plus 100K for the
> >          * stuff that's too small to bother with estimating.
> 
> Right.  That 100K slop factor is capable of hiding a multitude of sins.
> 
> I have not looked at this patch, but I think the concern is basically that
> if we have space-estimation infrastructure that misestimates what it is
> supposed to estimate, then if that infrastructure is used to estimate the
> size of any of the "big hog" data structures, we might misestimate by
> enough that the slop factor wouldn't hide it.

Exactly.  And now that I looked deeper I can see that multiple estimates are
entirely ignoring the padding alignment (e.g. ProcGlobalShmemSize()), which can
exceed the 6kB originally estimated by Robert.