Re: Shared memory size computation oversight?

From: Julien Rouhaud <rjuju123(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Georgios <gkokolatos(at)protonmail(dot)com>, Julien Rouhaud <julien(dot)rouhaud(at)free(dot)fr>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>, "robertmhaas(at)gmail(dot)com" <robertmhaas(at)gmail(dot)com>
Subject: Re: Shared memory size computation oversight?
Date: 2021-03-03 16:40:52
Message-ID: 20210303164052.f336wina7qpchabd@nol
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On Wed, Mar 03, 2021 at 11:23:54AM -0500, Tom Lane wrote:
> Georgios <gkokolatos(at)protonmail(dot)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.

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