Re: Shared memory size computation oversight?

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Georgios <gkokolatos(at)protonmail(dot)com>
Cc: 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 16:23:54
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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.

regards, tom lane

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