Re: SLRU statistics

On 2020/05/07 13:47, Fujii Masao wrote:
>
>
> On 2020/05/03 1:59, Tomas Vondra wrote:
>> On Sat, May 02, 2020 at 12:55:00PM +0200, Tomas Vondra wrote:
>>> On Sat, May 02, 2020 at 03:56:07PM +0900, Fujii Masao wrote:
>>>>
>>>> ...
>>>
>>>>>> Another thing I found is; pgstat_send_slru() should be called also by
>>>>>> other processes than backend? For example, since clog data is flushed
>>>>>> basically by checkpointer, checkpointer seems to need to send slru stats.
>>>>>> Otherwise, pg_stat_slru.flushes would not be updated.
>>>>>>
>>>>>
>>>>> Hmmm, that's a good point. If I understand the issue correctly, the
>>>>> checkpointer accumulates the stats but never really sends them because
>>>>> it never calls pgstat_report_stat/pgstat_send_slru. That's only called
>>>>> from PostgresMain, but not from CheckpointerMain.
>>>>
>>>> Yes.
>>>>
>>>>> I think we could simply add pgstat_send_slru() right after the existing
>>>>> call in CheckpointerMain, right?
>>>>
>>>> Checkpointer sends off activity statistics to the stats collector in
>>>> two places, by calling pgstat_send_bgwriter(). What about calling
>>>> pgstat_send_slru() just after pgstat_send_bgwriter()?
>>>>
>>>
>>> Yep, that's what I proposed.
>>>
>>>> In previous email, I mentioned checkpointer just as an example.
>>>> So probably we need to investigate what process should send slru stats,
>>>> other than checkpointer. I guess that at least autovacuum worker,
>>>> logical replication walsender and parallel query worker (maybe this has
>>>> been already covered by parallel query some mechanisms. Sorry I've
>>>> not checked that) would need to send its slru stats.
>>>>
>>>
>>> Probably. Do you have any other process type in mind?
>
> No. For now what I'm in mind are just checkpointer, autovacuum worker,
> logical replication walsender and parallel query worker. Seems logical
> replication worker and syncer have sent slru stats via pgstat_report_stat().
>
>> I've looked at places calling pgstat_send_* functions, and I found
>> thsese places:
>>
>> src/backend/postmaster/bgwriter.c
>>
>> - AFAIK it merely writes out dirty shared buffers, so likely irrelevant.
>>
>> src/backend/postmaster/checkpointer.c
>>
>> - This is what we're already discussing here.
>>
>> src/backend/postmaster/pgarch.c
>>
>> - Seems irrelevant.
>>
>>
>> I'm a bit puzzled why we're not sending any stats from walsender, which
>> I suppose could do various stuff during logical decoding.
>
> Not sure why, but that seems an oversight...
>
>
> Also I found another minor issue; SLRUStats has not been initialized to 0
> and which could update the counters unexpectedly. Attached patch fixes
> this issue.

This is minor issue, but basically it's better to fix that before
v13 beta1 release. So barring any objection, I will commit the patch.

+ values[8] = Int64GetDatum(stat.stat_reset_timestamp);

Also I found another small issue: pg_stat_get_slru() returns the timestamp
when pg_stat_slru was reset by using Int64GetDatum(). This works maybe
because the timestamp is also int64. But TimestampTzGetDatum() should
be used here, instead. Patch attached. Thought?

Regards,

--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION