From: Julien Rouhaud <rjuju123@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Georgios <gkokolatos@protonmail.com>
Cc: Julien Rouhaud <julien.rouhaud@free.fr>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Cc: robertmhaas@gmail.com <robertmhaas@gmail.com>
Subject: Re: Shared memory size computation oversight?
Date: Thu, 4 Mar 2021 00:40:52 +0800
Message-ID: <20210303164052.f336wina7qpchabd@nol> (raw)
In-Reply-To: <1183451.1614788634@sss.pgh.pa.us>
References: <20210227080815.jryyhy6lrn6gahkm@nol>
<SeVx6SxBxMHIm_66gE4M8rzAn5vqXw8akcyJ-26bJxHeNC_wW9fN-xDr_G3AGiSH328-rxMzXbNf6H4TsaNIG-OJFE8LvmdflG9Zb-yIKq4=@protonmail.com>
<1183451.1614788634@sss.pgh.pa.us>
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.
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To: pgsql-hackers@postgresql.org
Cc: rjuju123@gmail.com, tgl@sss.pgh.pa.us, gkokolatos@protonmail.com, julien.rouhaud@free.fr, pgsql-hackers@lists.postgresql.org, robertmhaas@gmail.com
Subject: Re: Shared memory size computation oversight?
In-Reply-To: <20210303164052.f336wina7qpchabd@nol>
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