Horton

Re: pg_stat_bgwriter.buffers_backend is pretty meaningless (and more?)

Melanie Plageman <melanieplageman@gmail.com>

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From: Melanie Plageman <melanieplageman@gmail.com>
To: Andres Freund <andres@anarazel.de>
Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>, Kyotaro Horiguchi <horikyota.ntt@gmail.com>, Magnus Hagander <magnus@hagander.net>, Pg Hackers <pgsql-hackers@postgresql.org>, Lukas Fittl <lukas@fittl.com>, Thomas Munro <thomas.munro@gmail.com>
Date: 2021-09-09T01:28:38Z
Lists: pgsql-hackers

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On Fri, Aug 13, 2021 at 3:08 AM Andres Freund <andres@anarazel.de> wrote:
>
> Hi,
>
> On 2021-08-11 16:11:34 -0400, Melanie Plageman wrote:
> > On Tue, Aug 3, 2021 at 2:13 PM Andres Freund <andres@anarazel.de> wrote:
> > > > Also, I'm unsure how writing the buffer action stats out in
> > > > pgstat_write_statsfiles() will work, since I think that backends can
> > > > update their buffer action stats after we would have already persisted
> > > > the data from the BufferActionStatsArray -- causing us to lose those
> > > > updates.
> > >
> > > I was thinking it'd work differently. Whenever a connection ends, it reports
> > > its data up to pgstats.c (otherwise we'd loose those stats). By the time
> > > shutdown happens, they all need to have already have reported their stats - so
> > > we don't need to do anything to get the data to pgstats.c during shutdown
> > > time.
> > >
> >
> > When you say "whenever a connection ends", what part of the code are you
> > referring to specifically?
>
> pgstat_beshutdown_hook()
>
>
> > Also, when you say "shutdown", do you mean a backend shutting down or
> > all backends shutting down (including postmaster) -- like pg_ctl stop?
>
> Admittedly our language is very imprecise around this :(. What I meant
> is that backends would report their own stats up to the stats collector
> when the connection ends (in pgstat_beshutdown_hook()). That means that
> when the whole server (pgstat and then postmaster, potentially via
> pg_ctl stop) shuts down, all the per-connection stats have already been
> reported up to pgstat.
>

So, I realized that the patch has a problem. I added the code to send
buffer actions stats to the stats collector
(pgstat_send_buffer_actions()) to pgstat_report_stat() and this isn't
getting called when all types of backends exit.

I originally thought to add pgstat_send_buffer_actions() to
pgstat_beshutdown_hook() (as suggested), but, this is called after
pgstat_shutdown_hook(), so, we aren't able to send stats to the stats
collector at that time. (pgstat_shutdown_hook() sets pgstat_is_shutdown
to true and then in pgstat_beshutdown_hook() (called after), if we call
pgstat_send_buffer_actions(), it calls pgstat_send() which calls
pgstat_assert_is_up() which trips when pgstat_is_shutdown is true.)

After calling pgstat_send_buffer_actions() from pgstat_report_stat(), it
seems to miss checkpointer stats entirely. I did find that if I
sprinkled pgstat_send_buffer_actions() around in the various places that
pgstat_send_checkpointer() is called, I could get checkpointer stats
(see attached patch, capture_checkpointer_buffer_actions.patch), but,
that seems a little bit haphazard since pgstat_send_buffer_actions() is
supposed to capture stats for all backend types. Is there somewhere else
I can call it that is exercised by all backend types before
pgstat_shutdown_hook() is called but after they would have finished any
relevant buffer actions?

- Melanie

Commits

  1. Stabilize pg_stat_io writes test

  2. Fix flakey pg_stat_io test

  3. Suppress more compiler warnings in new pgstats code.

  4. Suppress compiler warnings in new pgstats code.

  5. Add tests for pg_stat_io

  6. Create regress_tblspc in test_setup

  7. Add pg_stat_io view, providing more detailed IO statistics

  8. pgstat: Track more detailed relation IO statistics

  9. pgstat: Infrastructure for more detailed IO statistics

  10. doc: Fix some issues in logical replication section

  11. Manual cleanup and pgindent of pgstat and bufmgr related code

  12. Have the planner consider Incremental Sort for DISTINCT

  13. Use actual backend IDs in pg_stat_get_backend_idset() and friends.

  14. Remove redundant call to pgstat_report_wal()

  15. Add BackendType for standalone backends

  16. Initialize backend status reporting during bootstrap.