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: 2022-04-05T15:49:36Z
Lists: pgsql-hackers

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On Mon, Apr 4, 2022 at 7:36 PM Andres Freund <andres@anarazel.de> wrote:

>
> 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?
>

So, I decided to see what this would look like; the results are attached,
portions of it also inlined below.

I'll admit this does introduce a terminology problem - but IMO these words
are much more meaningful to the reader and code than the existing
booleans.  I'm hopeful we can bikeshed something agreeable as I'm strongly
in favor of making this change.

The ability to create defines for subsets nicely resolves the problem that
CLUSTER and DATABASE (now OBJECT to avoid DATABASE conflict in PgStat_Kind)
are generally related together - they are now grouped under the DYNAMIC
label (variable, if you want) while all of the fixed entries get associated
with GLOBAL.  Thus the majority of usages, since accessed_across_databases
is rare, end up being either DYNAMIC or GLOBAL.  The presence of any other
category should give one pause.  We could add an ALL define if we ever
decide to consolidate the API - but for now it's largely used to ensure
that stats of one type don't get processed by the other.  The boolean fixed
does that well enough but this just seems much cleaner and more
understandable to me.  Though having made up the terms and model myself,
that isn't too surprising.

The only existing usage of accessed_across_databases is in the negative
form, which translates to excluding objects, but only those from other
actual databases.

@@ -909,7 +904,7 @@ pgstat_build_snapshot(void)
  */
  if (p->key.dboid != MyDatabaseId &&
  p->key.dboid != InvalidOid &&
- !kind_info->accessed_across_databases)
+ kind_info->kind_group == PGSTAT_OBJECT)
  continue;

The only other usage of something other than GLOBAL or DYNAMIC is the
restriction on the behavior of reset_single_counter, which also has to be
an object in the current database (the later condition being enforced by
the presence of a valid object oid I presume).  The replacement for this
below is not behavior-preserving, the proposed behavior I believe we agree
is correct though.

@@ -652,7 +647,7 @@ pgstat_reset_single_counter(PgStat_Kind kind, Oid
objoid)

- Assert(!pgstat_kind_info_for(kind)->fixed_amount);
+ Assert(pgstat_kind_info_for(kind)->kind_group == PGSTAT_OBJECT);

Everything else is a straight conversion of fixed_amount to CLUSTER+OBJECT

@@ -728,7 +723,7 @@ pgstat_fetch_entry(PgStat_Kind kind, Oid dboid, Oid
objoid)

- AssertArg(!kind_info->fixed_amount);
+ AssertArg(kind_info->kind_group == PGSTAT_DYNAMIC);

and !fixed_amount to GLOBAL

@@ -825,7 +820,7 @@ pgstat_get_stat_snapshot_timestamp(bool *have_snapshot)
 bool
 pgstat_exists_entry(PgStat_Kind kind, Oid dboid, Oid objoid)
 {
- if (pgstat_kind_info_for(kind)->fixed_amount)
+ if (pgstat_kind_info_for(kind)->kind_group == PGSTAT_GLOBAL)
  return true;

  return pgstat_get_entry_ref(kind, dboid, objoid, false, NULL) != NULL;

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.