Re: Draft for basic NUMA observability

On Thu, Apr 3, 2025 at 10:23 AM Bertrand Drouvot
<bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
>
> Hi,

Hi Bertrand,

> On Thu, Apr 03, 2025 at 09:01:43AM +0200, Jakub Wartak wrote:
[..]

> === v21-0002
> While pg_buffercache_build_tuple() is not added (pg_buffercache_save_tuple()
> is).

Fixed

> About v21-0002:
>
> === 1
>
> I can see that the pg_buffercache_init_entries() helper comments are added in
> v21-0003 but I think that it would be better to add them in v21-0002
> (where the helper is actually created).

Moved

> About v21-0003:
>
> === 2
>
> > I hear you, attached v21 / 0003 is free of float/double arithmetics
> > and uses non-float point values.
>
> + if (buffers_per_page > 1)
> + os_page_query_count = NBuffers;
> + else
> + os_page_query_count = NBuffers * pages_per_buffer;
>
> yeah, that's more elegant. I think that it properly handles the relationships
> between buffer and page sizes without relying on float arithmetic.

Cool

> === 3
>
> + if (buffers_per_page > 1)
> + {
>
> As buffers_per_page does not change, I think I'd put this check outside of the
> for (i = 0; i < NBuffers; i++) loop, something like:
>
> "
> if (buffers_per_page > 1) {
> /* BLCKSZ < PAGESIZE: one page hosts many Buffers */
> for (i = 0; i < NBuffers; i++) {
> .
> .
> .
> .
> } else {
> /* BLCKSZ >= PAGESIZE: Buffer occupies more than one OS page */
> for (i = 0; i < NBuffers; i++) {
> .
> .
> .
> "

Done.

> === 4
>
> > That _numa_prepare_ptrs() is unused and will need to be removed,
> > but we can still move some code there if necessary.
>
> Yeah I think that it can be simply removed then.

Removed.

> === 5
>
> > @Bertrand: do you have anything against pg_shm_allocations_numa
> > instead of pg_shm_numa_allocations? I don't mind changing it...
>
> I think that pg_shm_allocations_numa() is better (given the examples you just
> shared).

OK, let's go with this name then (in v22).

> === 6
>
> > I find all of those non-user friendly and I'm afraid I won't be able
> > to pull that alone in time...
>
> Maybe we could add a few words in the doc to mention the "multiple nodes per
> buffer" case? And try to improve it for say 19?

Right, we could also put it as a limitation. I would be happy to leave
it as it must be a rare condition, but Tomas stated he's not.

> Also maybe we should just focus till v21-0003 (and discard v21-0004 for 18).

Do you mean discard pg_buffercache_numa (0002+0003) and instead go
with pg_shm_allocations_numa (0004) ?

BTW: I've noticed that Tomas responded with his v22 to this after I've
solved all of the above in mine v22, so I'll drop v23 soon and then
let's continue there.

-J.