Thread

Re: Add os_page_num to pg_buffercache

Mircea Cadariu <cadariu.mircea@gmail.com> — 2025-07-09T09:51:16Z

Hi,

Thanks for the prompt reply!

On 09/07/2025 08:37, Bertrand Drouvot wrote:
> Yeah, I tried to avoid code duplication for the "os pages" related stuff in
> v1. I can check if more can be done (outside of the "os pages" related stuff).
>
> Might be done in a dedicated patch though, I mean I don't think that should be
> a blocker for this one.

Agreed, if there's any low-hanging fruit to address now that this file 
is cracked open, then great. Otherwise, makes sense to leave it for a 
separate dedicated patch.

If you don't mind I have some further questions on the patch, see below.

> +		if (get_call_result_type(fcinfo, NULL, &expected_tupledesc) != TYPEFUNC_COMPOSITE)
> +			elog(ERROR, "return type must be a row type");

Is this needed in the new pg_buffercache_os_pages function? I noticed 
this check also in the "original" pg_buffercache_pages. There's a 
comment there indicating that (if I understand correctly) its purpose is 
to handle upgrades from version 1.0, mentioning a field unrelated to 
this patch.

If it's needed, shall we consider adding a similar comment as 
in pg_buffercache_pages?

> +		/*
> +		 * Different database block sizes (4kB, 8kB, ..., 32kB) can be used,
> +		 * while the OS may have different memory page sizes.
> +		 *
> +		 * To correctly map between them, we need to: 1. Determine the OS
> +		 * memory page size 2. Calculate how many OS pages are used by all
> +		 * buffer blocks 3. Calculate how many OS pages are contained within
> +		 * each database block.
> +		 */
For step number 3 - should it be the other way around: database blocks 
are contained within OS pages?

Kind regards,

Mircea Cadariu