Horton

Re: shared-memory based stats collector

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Cc: masao.fujii@oss.nttdata.com, gkokolatos@protonmail.com, pgsql-hackers@lists.postgresql.org
Date: 2021-03-22T23:17:37Z
Lists: pgsql-hackers 
Hi,

On 2021-03-22 12:02:39 +0900, Kyotaro Horiguchi wrote:
> At Mon, 22 Mar 2021 09:55:59 +0900 (JST), Kyotaro Horiguchi <horikyota.ntt@gmail.com> wrote in
> > At Thu, 18 Mar 2021 01:47:20 -0700, Andres Freund <andres@anarazel.de> wrote in
> > > Hi,
> > >
> > > On 2021-03-18 16:56:02 +0900, Kyotaro Horiguchi wrote:
> > > > At Tue, 16 Mar 2021 10:27:55 +0900 (JST), Kyotaro Horiguchi <horikyota.ntt@gmail.com> wrote in
> > > > Rebased and fixed two bugs.  Not addressed received comments in this
> > > > version.
> > >
> > > Since I am heavily editing the code, could you submit "functional"
> > > changes (as opposed to fixing rebase issues) as incremental patches?
> >
> > Oh.. please wait for.. a moment, maybe.
>
> This is that.

Thanks! That change shouldn't be necessary on my branch - I did
something to fix this kind of problem too. I decided that there's no
point in doing hash table lookups for the database: It's not going to
change in the life of a backend. So there's now two static "pending"
entries: One for the current DB, one for the shared DB.  There's only
one place that needed to change,
pgstat_report_checksum_failures_in_db(), which now reports the changes
directly instead of going via pending.

I suspect we should actually do that with a number of other DB specific
functions. Things like recovery conflicts, deadlocks, checksum failures
imo should really not be delayed till later. And you should never have
enough of them to make contention a concern.


You can see a somewhat sensible list of changes from your v52 at
https://github.com/anarazel/postgres/compare/master...shmstat-before-split-2021-03-22
(I did fix some of damage from rebase in a non-incremental way, of course)

My branch: https://github.com/anarazel/postgres/tree/shmstat

It would be cool if you could check if there any relevant things between
v52 and v56 that I should include.


I think a lot of the concerns I had with the patch are addressed at the
end of my series of changes.  Please let me know what you think.


My next step is going to be to squash all my changes into the base
patch, and try to extract all the things that I think can be
independently committed, and to reduce unnecessary diff noise.  Once
that's done I plan to post that series to the list.


TODO:

- explain the design at the top of pgstat.c

- figure out a way to deal with the different demands on stats
  consistency / efficiency

- see how hard it'd be to not need collect_oids()

- split pgstat.c

- consider removing PgStatTypes and replacing it with the oid of the
  table the type of stats reside in. So PGSTAT_TYPE_DB would be
  DatabaseRelationId, PGSTAT_TYPE_TABLE would be RelationRelationId, ...

  I think that'd make the system more cleanly extensible going forward?

- I'm not yet happy with the naming schemes in use in pgstat.c. I feel
  like I improved it a bunch, but it's not yet there.

- the replication slot stuff isn't quite right in my branch

- I still don't quite like the reset_offset stuff - I wonder if we can
  find something better there. And if not, whether we can deduplicate
  the code between functions like pgstat_fetch_stat_checkpointer() and
  pgstat_report_checkpointer().

  At the very least it'll need a lot better comments.

- bunch of FIXMEs / XXXs

Greetings,

Andres Freund

Commits

  1. Improve comment about dropped entries in pgstat.c

  2. Fix temporary memory leak in system table index scans

  3. pgstat: set timestamps of fixed-numbered stats after a crash.

  4. pgstat: Update docs to match the shared memory stats reality.

  5. pgstat: Hide instability in stats.spec with -DCATCACHE_FORCE_RELEASE.

  6. pgstat: add/extend tests for resetting various kinds of stats.

  7. Add minimal tests for recovery conflict handling.

  8. pgstat: test stats interactions with physical replication.

  9. pgstat: add tests for handling of restarts, including crashes.

  10. pgstat: add tests for transaction behaviour, 2PC, function stats.

  11. pgstat: add pg_stat_have_stats() test helper.

  12. pgstat: add pg_stat_force_next_flush(), use it to simplify tests.

  13. pgstat: move pgstat.c to utils/activity.

  14. pgstat: store statistics in shared memory.

  15. pgstat: remove stats_temp_directory.

  16. pgstat: rename STATS_COLLECTOR GUC group to STATS_CUMULATIVE.

  17. pgstat: revise replication slot API in preparation for shared memory stats.

  18. pgstat: scaffolding for transactional stats creation / drop.

  19. pgstat: introduce PgStat_Kind enum.

  20. pgstat: prepare APIs used by pgstatfuncs for shared memory stats.

  21. pgstat: add pgstat_copy_relation_stats().

  22. pgstat: rename some pgstat_send_* functions to pgstat_report_*.

  23. pgstat: stats collector references in comments.

  24. pgstat: move transactional code into pgstat_xact.c.

  25. dsm: allow use in single user mode.

  26. dshash: revise sequential scan support.

  27. pgstat: remove some superflous comments from pgstat.h.

  28. pgstat: reorder pgstat.[ch] contents.

  29. pgstat: split different types of stats into separate files.

  30. pgstat: introduce pgstat_relation_should_count().

  31. pgstat: separate "xact level" handling out of relation specific functions.

  32. pgstat: rename pgstat_initstats() to pgstat_relation_init().

  33. pgstat: split out WAL handling from pgstat_{initialize,report_stat}.

  34. pgstat: run pgindent on pgstat.c/h.

  35. pgstat: split relation, database handling out of pgstat_report_stat().

  36. Move code around in StartupXLOG().

  37. pgstat: Prepare to use mechanism for truncated rels also for droppped rels.

  38. pgstat: Split out relation stats handling from AtEO[Sub]Xact_PgStat() etc.

  39. pgstat: Schedule per-backend pgstat shutdown via before_shmem_exit().

  40. Schedule ShutdownXLOG() in single user mode using before_shmem_exit().

  41. Make parallel worker shutdown complete entirely via before_shmem_exit().

  42. pgstat: Bring up pgstat in BaseInit() to fix uninitialized use of pgstat by AV.

  43. pgstat: split reporting/fetching of bgwriter and checkpointer stats.

  44. Split backend status and progress related functionality out of pgstat.c.

  45. Split wait event related code from pgstat.[ch] into wait_event.[ch].

  46. Make archiver process an auxiliary process.

  47. Force to send remaining WAL stats to the stats collector at walwriter exit.

  48. Add pg_stat_database counters for sessions and session time

  49. Collect statistics about SLRU caches

  50. Don't run atexit callbacks in quickdie signal handlers.

  51. Create a "fast path" for acquiring weak relation locks.