Re: Draft for basic NUMA observability

Hi,

On Thu, Apr 03, 2025 at 09:01:43AM +0200, Jakub Wartak wrote:
> On Wed, Apr 2, 2025 at 6:40 PM Tomas Vondra <tomas(at)vondra(dot)me> wrote:
>
> Hi Tomas,
>
> > OK, so you agree the commit messages are complete / correct?
>
> Yes.

Not 100% sure on my side.

=== v21-0002

Says:

"
This introduces three new functions:

- pg_buffercache_init_entries
- pg_buffercache_build_tuple
- get_buffercache_tuple
"

While pg_buffercache_build_tuple() is not added (pg_buffercache_save_tuple()
is).

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).

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.

=== 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++) {
.
.
.
"

=== 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.

=== 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).

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

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com