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Subject: Re: MultiXact\SLRU buffers configuration
From: Andrey Borodin <x4mmm@yandex-team.ru>
In-Reply-To: 
 <CA+hUKGKEdeVKpOqBxzRF+Z4=j0T+CA7ERrXni5De71Mm6-dBWA@mail.gmail.com>
Date: Sat, 3 Apr 2021 22:57:23 +0300
Cc: Gilles Darold <gilles@darold.net>,
 Tomas Vondra <tomas.vondra@enterprisedb.com>,
 Tomas Vondra <tomas.vondra@2ndquadrant.com>,
 Alexander Korotkov <aekorotkov@gmail.com>,
 Anastasia Lubennikova <a.lubennikova@postgrespro.ru>,
 Daniel Gustafsson <daniel@yesql.se>,
 Kyotaro Horiguchi <horikyota.ntt@gmail.com>,
 pgsql-hackers <pgsql-hackers@postgresql.org>
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To: Thomas Munro <thomas.munro@gmail.com>
Archived-At: 
 <https://www.postgresql.org/message-id/27D90FFC-B897-4BC5-B4F5-9047B9886AA3%40yandex-team.ru>
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> 1 =D0=B0=D0=BF=D1=80. 2021 =D0=B3., =D0=B2 06:40, Thomas Munro =
<thomas.munro@gmail.com> =D0=BD=D0=B0=D0=BF=D0=B8=D1=81=D0=B0=D0=BB(=D0=B0=
):
>=20
> 2.  Remove the cap of 128 buffers for xact_buffers as agreed.  We
> still need a cap though, to avoid a couple of kinds of overflow inside
> slru.c, both when computing the default value and accepting a
> user-provided number.  I introduced SLRU_MAX_ALLOWED_BUFFERS to keep
> it <=3D 1GB, and tested this on a 32 bit build with extreme block =
sizes.
BTW we do not document maximum values right now.
I was toying around with big values. For example if we set different big =
xact_buffers we can get something like
FATAL:  not enough shared memory for data structure "Notify" (72768 =
bytes requested)
FATAL:  not enough shared memory for data structure "Async Queue =
Control" (2492 bytes requested)
FATAL:  not enough shared memory for data structure "Checkpointer Data" =
(393280 bytes requested)

But never anything about xact_buffers. I don't think it's important, =
though.

>=20
> Likewise, I removed the cap of 16 buffers for commit_ts_buffers, but
> only if you have track_commit_timestamp enabled. =20
Is there a reason to leave 16 pages if commit_ts is disabled? They might =
be useful for some artefacts of previously enabled commit_ts?

> 4.  Change the default for commit_ts_buffers back to shared_buffers /
> 1024 (with a minimum of 4), because I think you might have changed it
> by a copy and paste error -- or did you intend to make the default
> higher?
I changed default due to some experiments with =
https://www.postgresql.org/message-id/flat/20210115220744.GA24457%40alvher=
re.pgsql
In fact most important part of that thread was removing the cap, which =
is done by the patchset now.

Thanks!

Best regards, Andrey Borodin.