Horton

Re: shared-memory based stats collector - v68

David G. Johnston <david.g.johnston@gmail.com>

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From: "David G. Johnston" <david.g.johnston@gmail.com>
To: Andres Freund <andres@anarazel.de>
Cc: Kyotaro Horiguchi <horikyota.ntt@gmail.com>, Melanie Plageman <melanieplageman@gmail.com>, Ibrar Ahmed <ibrar.ahmad@gmail.com>, Fujii Masao <masao.fujii@oss.nttdata.com>, gkokolatos@protonmail.com, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2022-04-04T22:24:24Z
Lists: pgsql-hackers 
On Mon, Apr 4, 2022 at 2:54 PM Andres Freund <andres@anarazel.de> wrote:

>
> > As the existing function only handles functions and relations why not
> just
> > perform a specific Kind check for them?  Generalizing to assert on
> whether
> > or not the function works on fixed or variable Kinds seems beyond its
> > present state.  Or could it be used, as-is, for databases, replication
> > slots, and subscriptions today, and we just haven't migrated those areas
> to
> > use the now generalized function?
>
> It couldn't quite be used for those, because it really only makes sense for
> objects "within a database", because it wants to reset the timestamp of the
> pg_stat_database row too (I don't like that behaviour as-is, but that's the
> topic of another thread as you know...).
>
> It will work for other per-database stats though, once we have them.
>
>
> > Even then, unless we do expand the
> > definition of the this publicly facing function is seems better to
> > precisely define what it requires as an input Kind by checking for
> RELATION
> > or FUNCTION specifically.
>
> I don't see a benefit in adding a restriction on it that we'd just have to
> lift again?
>
> How about adding a
> Assert(!pgstat_kind_info_for(kind)->accessed_across_databases)
>
> and extending the function comment to say that it's used for per-database
> stats and that it resets both the passed-in stats object as well as
> pg_stat_database?
>
>
I could live with adding that, but...

Replacing the existing assert(!kind->fixed_amount) with
assert(!kind->accessed_across_databases) produces the same result as the
later presently implies the former.

Now I start to dislike the behavioral aspect of the attribute and would
rather just name it: kind->is_cluster_scoped (or something else that is
descriptive of the stat category itself, not how it is used)

Then reorganize the Kind documentation to note and emphasize these two
primary descriptors:
variable, which can be cluster or database scoped
fixed, which are cluster scoped by definition (if this is true...but given
this is an optimization category I'm thinking maybe it doesn't actually
matter...)

+ /* cluster-scoped object stats having a variable number of entries */
+ PGSTAT_KIND_REPLSLOT = 1, /* per-slot statistics */
+ PGSTAT_KIND_SUBSCRIPTION, /* per-subscription statistics */
+ PGSTAT_KIND_DATABASE, /* database-wide statistics */ (I moved this to 3rd
spot to be closer to the database-scoped options)
+
+ /* database-scoped object stats having a variable number of entries */
+ PGSTAT_KIND_RELATION, /* per-table statistics */
+ PGSTAT_KIND_FUNCTION, /* per-function statistics */
+
+ /* cluster-scoped stats having a fixed number of entries */ (maybe these
should go first, the variable following?)
+ PGSTAT_KIND_ARCHIVER,
+ PGSTAT_KIND_BGWRITER,
+ PGSTAT_KIND_CHECKPOINTER,
+ PGSTAT_KIND_SLRU,
+ PGSTAT_KIND_WAL,

David J.

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.