Horton

Re: shared-memory based stats collector - v68

Andres Freund <andres@anarazel.de>

View thread View on postgr.es
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: 2022-04-04T21:54:14Z
Lists: pgsql-hackers 
Hi,

On 2022-04-04 14:25:57 -0700, David G. Johnston wrote:
> > You mentioned this as a restriction above - I'm not seeing it as such?  I'd
> > like to write out stats more often in the future (e.g. in the
> > checkpointer),
> > but then it'd not be written out with this function...
> >
> >
> Yeah, the idea only really works if you can implement "last one out, shut
> off the lights".  I think I was subconsciously wanting this to work that
> way, but the existing process is good.

Preserving stats more than we do today (the patch doesn't really affect that)
will require a good chunk more work. My idea for it is that we'd write the
file out as part of a checkpoint / restartpoint, with a name including the
redo-lsn. Then when recovery starts, it can use the stats file associated with
that to start from.  Then we'd loose at most 1 checkpoint's worth of stats
during a crash, not more.

There's a few non-trivial corner cases to solve, around stats objects getting
dropped concurrently with creating that serialized snapshot. Solvable, but not
trivial.


> > > + * I also am unsure, off the top of my head, whether both replication
> > > slots and subscriptions,
> > > + * which are fixed, can be reset singly (today, and/or whether this
> > patch
> > > enables that capability)
> > > + */
> >
> > FWIW, neither are implemented as fixed amount stats.
> 
> 
> That was a typo, I meant to write variable.  My point was that of these 5
> kinds that will pass the assertion test only 2 of them are actually handled
> by the function today.
> 
> + PGSTAT_KIND_DATABASE = 1, /* database-wide statistics */
> + PGSTAT_KIND_RELATION, /* per-table statistics */
> + PGSTAT_KIND_FUNCTION, /* per-function statistics */
> + PGSTAT_KIND_REPLSLOT, /* per-slot statistics */
> + PGSTAT_KIND_SUBSCRIPTION, /* per-subscription statistics */

> 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?

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.