Horton

Re: shared-memory based stats collector

Antonin Houska <ah@cybertec.at>

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From: Antonin Houska <ah@cybertec.at>
To: Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>
Cc: andres@anarazel.de, magnus@hagander.net, robertmhaas@gmail.com, tgl@sss.pgh.pa.us, pgsql-hackers@postgresql.org
Date: 2018-10-29T14:10:10Z
Lists: pgsql-hackers 
Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp> wrote:

> This is more saner version of previous v5-0008, which didn't pass
> regression test. v6-0008 to v6-0010 are attached and they are
> applied on top of v5-0001-0007.
> 
> - stats collector has been removed.
> 
> - modified dshash further so that deletion is allowed during
>   sequential scan.
> 
> - I'm not sure about the following existing comment at the
>   beginning of pgstat.c
> 
>   *	- Add a pgstat config column to pg_database, so this
>   *	  entire thing can be enabled/disabled on a per db basis.

Following is the next handful of my comments:

* If you remove the stats collector, I think the remaining code in pgstat.c
  does no longer fit into the backend/postmaster/ directory.

* I'm not sure it's o.k. to call pgstat_write_statsfiles() from
  postmaster.c:reaper(): the function can raise ERROR (I see at least one code
  path: pgstat_write_statsfile() -> get_dbstat_filename()) and, as reaper() is
  a signal handler, it's hard to imagine the consequences. Maybe a reason to
  leave some functionality in a separate worker, although the "stats
  collector" would have to be changed.

* Question still remains whether all the statistics should be loaded into
  shared memory, see the note on paging near the bottom of [1].

* if dshash_seq_init() is passed consistent=false, shouldn't we call
  ensure_valid_bucket_pointers() also from dshash_seq_next()? If the scan
  needs to access the next partition and the old partition lock got released,
  the table can be resized before the next partition lock is acquired, and
  thus the backend-local copy of buckets becomes obsolete.

* Neither snapshot_statentry_all() nor backend_snapshot_all_db_entries() seems
  to be used in the current patch version.

* pgstat_initstats(): I think WaitLatch() should be used instead of sleep().

* pgstat_get_db_entry(): "return false" should probably be "return NULL".

* Is the PGSTAT_TABLE_WRITE flag actually used? Unlike PGSTAT_TABLE_CREATE, I
  couldn't find a place where it's value is tested.

* dshash_seq_init(): does it need to be called with consistent=true from
  pgstat_vacuum_stat() when the the entries returned by the scan are just
  dropped?

	dshash_seq_init(&dshstat, db_stats, true, true);

I suspect this is a thinko because another call from the same function looks
like

	dshash_seq_init(&dshstat, dshtable, false, true);

* I'm not sure about usefulness of dshash_get_num_entries(). It passes
  consistent=false to dshash_seq_init(), so the number of entries can change
  during the execution. And even if the function requested a "consistent
  scan", entries can be added / removed as soon as the scan is over.

  If the returned value is used to decide whether the hashtable should be
  scanned or not, I think you don't break anything if you simply start the
  scan unconditionally and see if you find some entries.

  And if you really need to count the entries, I suggest that you use the
  per-partition counts (dshash_partition.count) instead of scanning individual
  entries.


[1] https://www.postgresql.org/message-id/CA+TgmobQVbz4K_+RSmiM9HeRKpy3vS5xnbkL95gSEnWijzprKQ@mail.gmail.com

-- 
Antonin Houska
Cybertec Schönig & Schönig GmbH
Gröhrmühlgasse 26, A-2700 Wiener Neustadt
Web: https://www.cybertec-postgresql.com

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.