Re: Move SLRU_PAGES_PER_SEGMENT to pg_config_manual.h

From: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
To: Andres Freund <andres(at)anarazel(dot)de>
Cc: "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>, Maxim Orlov <orlovmg(at)gmail(dot)com>, Daniil Davydov <3danissimo(at)gmail(dot)com>, Daniel Gustafsson <daniel(at)yesql(dot)se>
Subject: Re: Move SLRU_PAGES_PER_SEGMENT to pg_config_manual.h
Date: 2025-11-10 16:24:23
Message-ID: a4d195d1-b1ea-4c16-ad65-99555e37bb7a@iki.fi
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On 10/11/2025 18:11, Andres Freund wrote:
> Hi,
>
> On 2025-11-10 13:29:08 +0200, Heikki Linnakangas wrote:
>> At the "make mxidoff 64 bits" thread [1], we're discussing moving
>> SLRU_PAGES_PER_SEGMENT to pg_config_manual.h, to make it accessible from
>> pg_upgrade code. It's currently defined in slru.h, which cannot be included
>> in frontend code. There are many ways we could fix that, but moving
>> SLRU_PAGES_PER_SEGMENT to pg_config_manual.h seems best to me. Patch
>> attached.
>
> I'm not convinced that pg_config_manual.h is a good place - that suggests it
> makes sense to change the value for some installations, which I don't see any
> reason for. Wouldn't an slrudefs.h or such be more appropriate?

The comment in pg_config_manual.h says about settings defined there:

> In all cases, changing them is only useful in very rare situations
> or for developers.
That seems fitting for SLRU_PAGES_PER_SEGMENT. I don't feel strongly
either way though, I'm happy to revert and move to slrudefs.h if that's
the consensus.

I think the control file compatibility check is nice to have in any case
and we should keep that.

- Heikki

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