Date: Sat, 2 May 2020 18:59:05 +0200
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Fujii Masao <masao.fujii@oss.nttdata.com>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>,
	Masahiko Sawada <masahiko.sawada@2ndquadrant.com>,
	"tsunakawa.takay@fujitsu.com" <tsunakawa.takay@fujitsu.com>,
	"pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Subject: Re: SLRU statistics
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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?
>

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.


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services