Horton

Re: shared-memory based stats collector - v70

Greg Stark <stark@mit.edu>

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From: Greg Stark <stark@mit.edu>
To: Andres Freund <andres@anarazel.de>
Cc: 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: 2022-07-21T15:07:55Z
Lists: pgsql-hackers 
On Wed, 20 Jul 2022 at 15:09, Andres Freund <andres@anarazel.de> wrote:
>
> 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.

Fascinating. I'm surprised this didn't raise issues previously for
people with millions of tables. I wonder if it wasn't causing issues
and we just didn't hear about them because there were other bigger
issues :)


> >All that said -- having all objects loaded in shared memory makes my
> >work way easier.
>
> What are your trying to do?

I'm trying to implement an exporter for prometheus/openmetrics/etc
that dumps directly from shared memory without going through the SQL
backend layer. I believe this will be much more reliable, lower
overhead, safer, and consistent than writing SQL queries.

Ideally I would want to dump out the stats without connecting to each
database. I suspect that would run into problems where the schema
really adds a lot of information (such as which table each index is on
or which table a toast relation is for. There are also some things
people think of as stats that are maintained in the catalog such as
reltuples and relpages. So I'm imagining this won't strictly stay true
in the end.

It seems like just having an interface to iterate over the shared hash
table and return entries one by one without filtering by database
would be fairly straightforward and I would be able to do most of what
I want just with that. There's actually enough meta information in the
stats entries to be able to handle them as they come instead of trying
to process look up specific stats one by one.


-- 
greg

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.