Horton

Re: shared-memory based stats collector

Joshua D. Drake <jd@commandprompt.com>

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From: "Joshua D. Drake" <jd@commandprompt.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Andres Freund <andres@anarazel.de>, Magnus Hagander <magnus@hagander.net>, Tom Lane <tgl@sss.pgh.pa.us>, Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2018-07-06T19:46:35Z
Lists: pgsql-hackers 
On 07/06/2018 12:34 PM, Robert Haas wrote:
> On Fri, Jul 6, 2018 at 3:02 PM, Joshua D. Drake <jd@commandprompt.com> wrote:
>> Will stats, if we move toward the suggested changes be "less" accurate than
>> they are now? We already know that stats are generally not accurate but they
>> are close enough. If we move toward this change will it still be close
>> enough?
> There proposed change would have no impact at all on the long-term
> accuracy of the statistics.  It would just mean that there would be
> race conditions when reading them, so that for example you would be
> more likely to see a count of heap scans that doesn't match the count
> of index scans, because an update arrives in between when you read the
> first value and when you read the second one.  I don't see that
> mattering a whole lot, TBH, but maybe I'm missing something.

I agree that it probably isn't a big deal. Generally speaking when we 
look at stats it is to get an "idea" of what is going on. We don't care 
if we are missing an increase/decrease of 20 of any particular value 
within stats. Based on this and what Andres said, it seems like a net 
win to me.

JD

>

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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.