Horton

Re: shared-memory based stats collector - v66

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Cc: melanieplageman@gmail.com, ibrar.ahmad@gmail.com, masao.fujii@oss.nttdata.com, gkokolatos@protonmail.com, pgsql-hackers@lists.postgresql.org
Date: 2022-04-05T20:40:19Z
Lists: pgsql-hackers 
Hi,

On 2022-04-02 01:16:48 -0700, Andres Freund wrote:
> I just noticed that the code doesn't appear to actually work like that right
> now. Whenever the timeout is reached, pgstat_report_stat() is called with
> force = true.
> 
> And even if the backend is busy running queries, once there's contention, the
> next invocation of pgstat_report_stat() will return the timeout relative to
> pendin_since, which then will trigger a force report via a very short timeout
> soon.
> 
> It might actually make sense to only ever return PGSTAT_RETRY_MIN_INTERVAL
> (with a slightly different name) from pgstat_report_stat() when blocked
> (limiting the max reporting delay for an idle connection) and to continue
> calling pgstat_report_stat(force = true).  But to only trigger force
> "internally" in pgstat_report_stat() when PGSTAT_MAX_INTERVAL is reached.
> 
> I think that'd mean we'd report after max PGSTAT_RETRY_MIN_INTERVAL in an idle
> connection, and try reporting every PGSTAT_RETRY_MIN_INTERVAL (increasing up
> to PGSTAT_MAX_INTERVAL when blocked) on busy connections.
> 
> Makes sense?

I tried to come up with a workload producing a *lot* of stats (multiple
function calls within a transaction, multiple transactions pipelined) and ran
it with 1000 clients (on a machine with 2 x (10 cores / 20 threads)). To
reduce overhead I set
  default_transaction_isolation=repeatable read
  track_activities=false
MVCC Snapshot acquisition is the clear bottleneck otherwise, followed by
pgstat_report_activity() (which, as confusing as it may sound, is independent
of this patch).

I do see a *small* amount of contention if I lower PGSTAT_MIN_INTERVAL to
1ms. Too small to ever be captured in pg_stat_activity.wait_event, but just
about visible in a profiler.


Which leads me to conclude we can simplify the logic significantly. Here's my
current comment explaining the logic:

 * Unless called with 'force', pending stats updates are flushed happen once
 * per PGSTAT_MIN_INTERVAL (1000ms). When not forced, stats flushes do not
 * block on lock acquisition, except if stats updates have been pending for
 * longer than PGSTAT_MAX_INTERVAL (60000ms).
 *
 * Whenever pending stats updates remain at the end of pgstat_report_stat() a
 * suggested idle timeout is returned. Currently this is always
 * PGSTAT_IDLE_INTERVAL (10000ms). Callers can use the returned time to set up
 * a timeout after which to call pgstat_report_stat(true), but are not
 * required to to do so.

Comments?

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.