Horton

Re: shared-memory based stats collector - v70

Anton A. Melnikov <a.melnikov@postgrespro.ru>

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From: "Anton A. Melnikov" <a.melnikov@postgrespro.ru>
To: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, Michael Paquier <michael@paquier.xyz>
Cc: Andres Freund <andres@anarazel.de>, "Drouvot, Bertrand" <bdrouvot@amazon.com>, Greg Stark <stark@mit.edu>, Tom Lane <tgl@sss.pgh.pa.us>, Melanie Plageman <melanieplageman@gmail.com>, Kyotaro Horiguchi <horikyota.ntt@gmail.com>, Justin Pryzby <pryzby@telsasoft.com>, Thomas Munro <thomas.munro@gmail.com>, "David G. Johnston" <david.g.johnston@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2024-12-07T09:31:46Z
Lists: pgsql-hackers

Attachments

Hi!

On 04.12.2024 18:24, Bertrand Drouvot wrote:
> Thanks! I've the feeling that something has to be fixed, see my comments in
> [1]. It might be that the failed assertion does not handle a "valid" scenario.
> 
> [1]: https://www.postgresql.org/message-id/Z1BzI/eMTCOKA%2Bj6%40ip-10-97-1-34.eu-west-3.compute.internal


On 05.12.2024 08:43, Michael Paquier wrote:
> It's really a case that should never be reached because it points to
> an inconsistency in the interactions between the local entry cache in
> a process and the central dshash it attempts to point to, so I don't
> think that there is anything to change here.  As Andres has mentioned,
> it has a lot of value by acting as a safety guard in assert builds
> without being annoying for production deployments.

Thanks a lot for for the detailed clarification!
Everything here became clear for me.

On 05.12.2024 11:13, Michael Paquier wrote:
> On Thu, Dec 05, 2024 at 07:37:27AM +0000, Bertrand Drouvot wrote:
>> That said, I think that's worth to update the comment a bit (like in the
>> attached?) as I think that answers a legitimate question someone could have while
>> reading this code.
> 
> Perhaps this should provide some details, like the fact that we don't
> expect the server to still have references to entries that are dropped
> at shutdown when writing the stats file as all the backends and/or
> auxiliary processes should have done this cleanup before they are
> gone.

Completely agree that the original comment needs to be revised,
since it implies that it is normal for deleted entries to be here,
but it is not the case.

On 05.12.2024 17:13, Bertrand Drouvot wrote:
> Okay, attached a more elaborated comment.

Looks good for me. Detailed and clear.
Will help to avoid unnecessary questions when reading this code.

Maybe it's worth adding a warning as well,
similar to the one a few lines below in the code?

Like in the patch attached?


With the best regards,

-- 
Anton A. Melnikov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company

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.