Horton

Re: shared-memory based stats collector - v70

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: "David G. Johnston" <david.g.johnston@gmail.com>
Cc: Kyotaro Horiguchi <horikyota.ntt@gmail.com>, Melanie Plageman <melanieplageman@gmail.com>, Justin Pryzby <pryzby@telsasoft.com>, Thomas Munro <thomas.munro@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2022-04-09T19:06:55Z
Lists: pgsql-hackers 
Hi,

On 2022-04-07 21:39:55 -0700, David G. Johnston wrote:
> On Thu, Apr 7, 2022 at 8:59 PM Andres Freund <andres@anarazel.de> wrote:
> 
> >
> >   <para>
> >    Cumulative statistics are collected in shared memory. Every
> >    <productname>PostgreSQL</productname> process collects statistics
> > locally
> >    then updates the shared data at appropriate intervals.  When a server,
> >    including a physical replica, shuts down cleanly, a permanent copy of
> > the
> >    statistics data is stored in the <filename>pg_stat</filename>
> > subdirectory,
> >    so that statistics can be retained across server restarts.  In contrast,
> >    when starting from an unclean shutdown (e.g., after an immediate
> > shutdown,
> >    a server crash, starting from a base backup, and point-in-time
> > recovery),
> >    all statistics counters are reset.
> >   </para>
> >
> 
> I like this.  My comment regarding using "i.e.," here stands though.

Argh. I'd used in e.g., but not i.e..


> >
> >    <para>
> >     The cumulative statistics system is active during recovery. All scans,
> >     reads, blocks, index usage, etc., will be recorded normally on the
> >     standby. However, WAL replay will not increment relation and database
> >     specific counters. I.e. replay will not increment pg_stat_all_tables
> >     columns (like n_tup_ins), nor will reads or writes performed by the
> >     startup process be tracked in the pg_statio views, nor will associated
> >     pg_stat_database columns be incremented.
> >    </para>
> >
> >
> I like this too.  The second part with three nors is a bit rough.  Maybe:

Agreed. I tried to come up with a smoother formulation, but didn't (perhaps
because I was a tad tired).


> ... specific counters.  In particular, replay will not increment
> pg_stat_database or pg_stat_all_tables columns, and the startup process
> will not report reads and writes for the pg_statio views.
> 
> It would helpful to give at least one specific example of what is being
> recorded normally, especially since we give three of what is not.

The second sentence is a set of examples - or do you mean examples for what
actions by the startup process are counted?

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.