Re: MultiXact\SLRU buffers configuration

From: Andrey Borodin <x4mmm(at)yandex-team(dot)ru>
To: Thomas Munro <thomas(dot)munro(at)gmail(dot)com>
Cc: Gilles Darold <gilles(at)darold(dot)net>, Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, Alexander Korotkov <aekorotkov(at)gmail(dot)com>, Anastasia Lubennikova <a(dot)lubennikova(at)postgrespro(dot)ru>, Daniel Gustafsson <daniel(at)yesql(dot)se>, Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: MultiXact\SLRU buffers configuration
Date: 2021-04-03 19:57:23
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> 1 апр. 2021 г., в 06:40, Thomas Munro <thomas(dot)munro(at)gmail(dot)com> написал(а):
> 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 <= 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.

> Likewise, I removed the cap of 16 buffers for commit_ts_buffers, but
> only if you have track_commit_timestamp enabled.
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
In fact most important part of that thread was removing the cap, which is done by the patchset now.


Best regards, Andrey Borodin.

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