Horton

Re: shared-memory based stats collector

Tomas Vondra <tomas.vondra@2ndquadrant.com>

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From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>, andres@anarazel.de
Cc: magnus@hagander.net, robertmhaas@gmail.com, tgl@sss.pgh.pa.us, pgsql-hackers@postgresql.org
Date: 2018-07-10T12:52:13Z
Lists: pgsql-hackers 
On 07/10/2018 02:07 PM, Kyotaro HORIGUCHI wrote:
> Hello. Thanks for the opinions.
> 
> At Fri, 6 Jul 2018 13:10:36 -0700, Andres Freund <andres@anarazel.de> wrote in <20180706201036.awheoi6tk556x6aj@alap3.anarazel.de>
>> Hi,
>>
>> On 2018-07-06 22:03:12 +0200, Magnus Hagander wrote:
>>> *If* we can provide the snapshots view of them without too much overhead I
>>> think it's worth looking into that while *also* proviiding a lower overhead
>>> interface for those that don't care about it.
>>
>> I don't see how that's possible without adding significant amounts of
>> complexity and probably memory / cpu overhead. The current stats already
>> are quite inconsistent (often outdated, partially updated, messages
>> dropped when busy) - I don't see what we really gain by building
>> something MVCC like in the "new" stats subsystem.
>>
>>
>>> If it ends up that keeping the snapshots become too much overhead in either
>>> in performance or code-maintenance, then I agree can probably drop that.
>>> But we should at least properly investigate the cost.
>>
>> I don't think it's worthwhile to more than think a bit about it. There's
>> fairly obvious tradeoffs in complexity here. Trying to get there seems
>> like a good way to make the feature too big.
> 
> Agreed.
> 
> Well, if we allow to lose consistency in some extent for improved
> performance and smaller footprint, relaxing the consistency of
> database stats can reduce footprint further especially on a
> cluster with so many databases. Backends are interested only in
> the residing database and vacuum doesn't cache stats at all. A
> possible problem is vacuum and stats collector can go into a race
> condition. I'm not sure but I suppose it is not worse than being
> involved in an IO congestion.
> 

As someone who regularly analyzes stats collected from user systems, I 
think there's certainly some value with keeping the snapshots reasonably 
consistent. But I agree it doesn't need to be perfect, and some level of 
inconsistency is acceptable (and the amount of complexity/overhead 
needed to maintain perfect consistency seems rather excessive here).

There's one more reason why attempts to keep stats snapshots "perfectly" 
consistent are likely doomed to fail - the messages are sent over UDP, 
which does not guarantee delivery etc. So there's always some level of 
possible inconsistency even with "perfectly consistent" snapshots.


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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.