Horton

Re: shared-memory based stats collector - v70

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Greg Stark <stark@mit.edu>, Tom Lane <tgl@sss.pgh.pa.us>
Cc: 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: 2022-07-20T19:08:49Z
Lists: pgsql-hackers 
Hi, 

On July 20, 2022 8:41:53 PM GMT+02:00, Greg Stark <stark@mit.edu> wrote:
>On Wed, 20 Jul 2022 at 12:08, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>
>> AFAIR, the previous stats collector implementation had no such provision
>> either: it'd just keep adding hashtable entries as it received info about
>> new objects.  The only thing that's changed is that now those entries are
>> in shared memory instead of process-local memory.  We'd be well advised to
>> be sure that memory can be swapped out under pressure, but otherwise I'm
>> not seeing that things have gotten worse.
>
>Just to be clear I'm not looking for ways things have gotten worse.
>Just trying to understand what I'm reading and I guess I came in with
>assumptions that led me astray.
>
>But... adding entries as it received info about new objects isn't the
>same as having info on everything. I didn't really understand how the
>old system worked but if you had a very large schema but each session
>only worked with a small subset did the local stats data ever absorb
>info on the objects it never touched?

Each backend only had stats for things it touched. But the stats collector read all files at startup into hash tables and absorbed all generated stats into those as well.


>All that said -- having all objects loaded in shared memory makes my
>work way easier.

What are your trying to do? 

>It actually seems feasible to dump out all the
>objects from shared memory and including objects from other databases
>and if I don't need a consistent snapshot it even seems like it would
>be possible to do that without having a copy of more than one stats
>entry at a time in local memory. I hope that doesn't cause huge
>contention on the shared hash table to be doing that regularly.

The stats accessors now default to not creating a full snapshot of stats data at first access (but that's configurable). So yes, that behavior is possible. E.g. autovac now uses a single object access like you describe.

Andres


-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.

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.