Horton

Re: shared-memory based stats collector - v68

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>, Ibrar Ahmed <ibrar.ahmad@gmail.com>, Fujii Masao <masao.fujii@oss.nttdata.com>, gkokolatos@protonmail.com, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2022-04-05T02:36:34Z
Lists: pgsql-hackers 
Hi,

On 2022-04-04 19:03:13 -0700, David G. Johnston wrote:
> > > (if this is true...but given this is an optimization category I'm
> > thinking
> > > maybe it doesn't actually matter...)
> >
> > It is true. Not sure what you mean with "optimization category"?
> >
> >
> I mean that distinguishing between stats that are fixed and those that are
> variable implies that fixed kinds have a better performance (speed, memory)
> characteristic than variable kinds (at least in part due to the presence of
> changecount).  If fixed kinds did not have a performance benefit then
> having the variable kind implementation simply handle fixed kinds as well
> (using the common struct header and storage in a hash table) would make the
> implementation simpler since all statistics would report through the same
> API.

Yes, fixed-numbered stats are faster.



> Coming back to this:
> """
> + /* 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,
> """
> 
> I see three "KIND_GROUP" categories here:
> PGSTAT_KIND_CLUSTER (open to a different word here though...)
> PGSTAT_KIND_DATABASE (we seem to agree on this above)
> PGSTAT_KIND_GLOBAL (already used in the code)
> 
> This single enum can replace the two booleans that, in combination, would
> define 4 unique groups (of which only three are interesting -
> database+fixed doesn't seem interesting and so is not given a name/value
> here).

The more I think about it, the less I think a split like that makes sense. The
difference between PGSTAT_KIND_CLUSTER / PGSTAT_KIND_DATABASE is tiny. Nearly
all code just deals with both together.

I think all this is going to achieve is to making code more complicated. There
is a *single* non-assert use of accessed_across_databases and now a single
assertion involving it.

What would having PGSTAT_KIND_CLUSTER and PGSTAT_KIND_DATABASE achieve?

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.