Horton

Re: shared-memory based stats collector

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Magnus Hagander <magnus@hagander.net>
Cc: Kyotaro Horiguchi <horikyota.ntt@gmail.com>, Fujii Masao <masao.fujii@oss.nttdata.com>, Georgios <gkokolatos@protonmail.com>, PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>
Date: 2021-03-16T02:55:41Z
Lists: pgsql-hackers

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Hi,

On 2021-03-15 19:04:29 -0700, Andres Freund wrote:
> On 2021-03-13 10:05:21 -0800, Andres Freund wrote:
> > Cool. I'll give it a try.

Ooops, I was intending to write a bit about my attempt at that:

I did roughly the first steps of the split as I had outlined. I moved:

1) wait event related functions into utils/activity/wait_event.c /
   wait_event.h

2) "backend status" functionality (PgBackendStatus stuff) into
   utils/activity/backend_status.c

3) "progress" related functionality into
   utils/activity/backend_progress.c

I think 1 and 2 are good (albeit in need of further polish). I'm a bit
less sure about 3:
- There's dependency from backend_status.h to backend_progress.h,
  because it PGSTAT_NUM_PROGRESS_PARAM etc.
- it's a fairly small amount of code
- there's potential for confusion, because there's also
  include/commands/progress.h

On balance I think 3) is probably worth it, but I'm far from confident.

Happy to bikeshed about the names for a moment...


Questions / Points:

- I'm inclined to leave pgstat_report_{activity, tmpfile, appname,
  timestamp, ..} alone naming-wise, but to rename pgstat_bestart() to
  something like pgbestat_start()?

- I've not gone through all the files that could now remove pgstat.h,
  replacing it with wait_event.h - I'm thinking it might be worth
  waiting till just after code freeze with that (there'll new additions,
  and it's likely to cause conflicts)?

- backend_status.h needs miscadmin.h, due to BackendType. Imo that's a
  header we should try to avoid exposing in headers if possible. But
  right now there's no good place to move BackendType to. So I'd let
  this slide for now.



On 2021-03-15 19:04:29 -0700, Andres Freund wrote:
> I have a few questions about the patch:
> - Why was collect_oids() changed to a different hashtable as part of
>   this change? Seems fairly independent?
>
> - What's the point of all those cached_* stuff? There's not a single
>   comment explaining it as far as I can tell...
>
>   Several of them are never used as a cache! E.g. cached_archiverstats,
>   cached_bgwriterstats, ...
>
> - What is the idea behind pgstat_reset_shared_counters() using
>   pgstat_copy_global_stats() to reset, using StatsShmem->*_reset_offset?
>   But then still taking a lock in pgstat_fetch_stat_*?  Again, no
>   comments explaining what the goal is.
>
>   It kinda looks like you tried to make both read and write paths not
>   use the lock, but then ended up using a lock?
>
>
> Do you have some benchmarks that you used to verify performance?
>
>
> I think I'm going to try to split the storage of fixed-size stats in
> StatsShmemStruct into a separate patch. That's already a pretty large
> change, and it's pretty much unrelated to the rest.


Some more:
- pgstat_vacuum_stat() talks about using three phases, one with only a
  shared lock. That doesn't seem to exist (anymore)?

- WAIT_EVENT_PGSTAT_MAIN was renamed to WAIT_EVENT_READING_STATS_FILE. I
  assume that was intended to be used in pgstat_read_statsfile(),
  pgstat_write_statsfile()? Not sure there's much point in adding it,
  because at that stage nobody can observe wait events anyway...

- There are a lot of diffs like
@@ -1267,7 +1264,7 @@ pg_stat_get_db_xact_commit(PG_FUNCTION_ARGS)
     if ((dbentry = pgstat_fetch_stat_dbentry(dbid)) == NULL)
         result = 0;
     else
-        result = (int64) (dbentry->n_xact_commit);
+        result = (int64) (dbentry->counts.n_xact_commit);

     PG_RETURN_INT64(result);
 }

  I wonder if we should instead just return *entry->counts from
  pgstat_fetch_stat_* (adjusting the names). There's as far as I can
  tell never a reason for pgstatfuncs.c (and others if there are) to
  need the remaining members?  That'd could the patch down a lot I
  think?

  What's especially nice is that afterwards PgStat_StatEntryHeader
  wouldn't need to be exposed anymore, and in turn, atomic.h wouldn't
  need to be included anymore.

- pgstat.h had dshash_table_handle members in PgStat_StatDBEntry, I
  assume those were leftovers from an older version, not something you
  forsee needing?

- There's a pretty weird change in relcache.c:
+       /* break mutual link with stats entry */
+       pgstat_delinkstats(relation);
+
+       if (relation->rd_rel)

  did you add that if intentionally?


- Is there any reason to treat PgStat_ReplSlot as something of a dynamic
  number, instead of just having a fixed number of slots in
  StatsShmemStruct? The GUC can't change without a restart...



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.