Re: Draft for basic NUMA observability

On 4/4/25 09:35, Jakub Wartak wrote:
> On Fri, Apr 4, 2025 at 8:50 AM Bertrand Drouvot
> <bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
>>
>> Hi,
>>
>> On Thu, Apr 03, 2025 at 08:53:57PM +0200, Tomas Vondra wrote:
>>> On 4/3/25 15:12, Jakub Wartak wrote:
>>>> On Thu, Apr 3, 2025 at 1:52 PM Tomas Vondra <tomas(at)vondra(dot)me> wrote:
>>>>
>>>>> ...
>>>>>
>>>>> So unless someone can demonstrate a use case where this would matter,
>>>>> I'd not worry about it too much.
>>>>
>>>> OK, fine for me - just 3 cols for pg_buffercache_numa is fine for me,
>>>> it's just that I don't have cycles left today and probably lack skills
>>>> (i've never dealt with arrays so far) thus it would be slow to get it
>>>> right... but I can pick up anything tomorrow morning.
>>>>
>>>
>>> OK, I took a stab at reworking/simplifying this the way I proposed.
>>> Here's v24 - needs more polishing, but hopefully enough to show what I
>>> had in mind.
>>>
>>> It does these changes:
>>>
>>> 1) Drops 0002 with the pg_buffercache refactoring, because the new view
>>> is not "extending" the existing one.
>>
>> I think that makes sense. One would just need to join on the pg_buffercache
>> view to get more information about the buffer if needed.
>>
>> The pg_buffercache_numa_pages() doc needs an update though as I don't think that
>> "+ The <function>pg_buffercache_numa_pages()</function> provides the same
>> information as <function>pg_buffercache_pages()</function>" is still true.
>>
>>> 2) Reworks pg_buffercache_num to return just three columns, bufferid,
>>> page_num and node_id. page_num is a sequence starting from 0 for each
>>> buffer.
>>
>> +1 on the idea
>>
>>> 3) It now builds an array of records, with one record per buffer/page.
>>>
>>> 4) I realized we don't really need to worry about buffers_per_page very
>>> much, except for logging/debugging. There's always "at least one page"
>>> per buffer, even if an incomplete one, so we can do this:
>>>
>>> os_page_count = NBuffers * Max(1, pages_per_buffer);
>>>
>>> and then
>>>
>>> for (i = 0; i < NBuffers; i++)
>>> {
>>> for (j = 0; j < Max(1, pages_per_buffer); j++)
>>
>> That's a nice simplification as we always need to take care of at least one page
>> per buffer.
>>
>>> and everything just works fine, I think.
>>
>> I think the same.
>>
>>> Opinions? I personally find this much cleaner / easier to understand.
>>
>> I agree that's easier to understand and that that looks correct.
>>
>> A few random comments:
>>
>> === 1
>>
>> It looks like that firstNumaTouch is not set to false anymore.
>>
>> === 2
>>
>> + pfree(os_page_status);
>> + pfree(os_page_ptrs);
>>
>> Not sure that's needed, we should be in a short-lived memory context here
>> (ExprContext or such).
>>
>> === 3
>>
>> + ro_volatile_var = *(uint64 *)ptr
>>
>> space missing before "ptr"?
>>
>
> +my feedback as I've noticed that Bertrand already provided a review.
>
> Right, the code is now simple , and that Max() is brilliant. I've
> attached some review findings as .txt
>
> 0001 100%LGTM
> 0002 doc fix + pgident + Tomas, you should take Authored-by yourself
> there for sure, I couldn't pull this off alone in time! So big thanks!
> 0003 fixes elog UINT64_FORMAT for ming32 (a little bit funny to have
> NUMA on ming32...:))
>

OK

> When started with interleave=all on serious hardware, I'm getting (~5s
> for s_b=64GB) from pg_buffercache_numa
>
> node_id | count
> ---------+---------
> 3 | 2097152
> 0 | 2097152
> 2 | 2097152
> 1 | 2097152
>
> so this is valid result (2097152*4 numa nodes*8192 buffer
> size/1024/1024/1024 = 64GB)
>
> Also with pgbench -i -s 20 , after ~8s:
> select c.relname, n.node_id, count(*) from pg_buffercache_numa n
> join pg_buffercache b on (b.bufferid = n.bufferid)
> join pg_class c on (c.relfilenode = b.relfilenode)
> group by c.relname, n.node_id order by count(*) desc;
> relname | node_id | count
> -----------------------------------------------+---------+-------
> pgbench_accounts | 2 | 8217
> pgbench_accounts | 0 | 8190
> pgbench_accounts | 3 | 8189
> pgbench_accounts | 1 | 8187
> pg_statistic | 2 | 32
> pg_operator | 2 | 14
> pg_depend | 3 | 14
> [..]
>
> pg_shm_allocations_numa also looks good.
>
> I think v24+tiny fixes is good enough to go in.
>

OK.

Do you have any suggestions regarding the column names in the new view?
I'm not sure I like node_id and page_num.

regards

--
Tomas Vondra