Horton

Re: shared-memory based stats collector - v70

Greg Stark <stark@mit.edu>

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From: Greg Stark <stark@mit.edu>
To: "Drouvot, Bertrand" <bdrouvot@amazon.com>
Cc: Andres Freund <andres@anarazel.de>, 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: 2022-08-17T19:46:42Z
Lists: pgsql-hackers 
On Tue, 16 Aug 2022 at 08:49, Drouvot, Bertrand <bdrouvot@amazon.com> wrote:
>
>
> +               if (p->key.kind != PGSTAT_KIND_RELATION)
> +                       continue;

Hm. So presumably this needs to be extended. Either to let the caller
decide which types of stats to return or to somehow return all the
stats intermixed. In my monitoring code I did the latter because I
didn't think going through the hash table repeatedly would be very
efficient. But it's definitely a pretty awkward API since I need a
switch statement that explicitly lists each case and casts the result.

> > 2) When I did the function attached above I tried to avoid returning
> > the whole set and make it possible to process them as they arrive.
>
> Is it the way it has been done? (did not look at your function yet)

I did it with callbacks. It was quick and easy and convenient for my
use case. But in general I don't really like callbacks and would think
some kind of iterator style api would be nicer.

I am handling the stats entries as they turn up. I'm constructing the
text output for each in a callback and buffering up the whole http
response in a string buffer.

I think that's ok but if I wanted to avoid buffering it up and do
network i/o then I would think the thing to do would be to build the
list of entry keys and then loop over that list doing a hash lookup
for each one and generating the response for each out and writing it
to the network. That way there wouldn't be anything locked, not even
the hash table, while doing network i/o. It would mean a lot of
traffic on the hash table though.

> > -- on that note I wonder if I've done something
> > wrong because I noted a few records with InvalidOid where I didn't
> > expect it.
>
> It looks like that InvalidOid for the dbid means that the entry is for a
> shared relation.

Ah yes. I had actually found that but forgotten it.

There's also a database entry with dboid=InvalidOid which is
apparently where background workers with no database attached report
stats.

> I've in mind to add some filtering on the dbid (I think it could be
> useful for monitoring tool with a persistent connection to one database
> but that wants to pull the stats database per database).
>
> I don't think a look up through the local cache will work if the
> entry/key is related to another database the API is launched from.

Isn't there also a local hash table used to find the entries to reduce
traffic on the shared hash table? Even if you don't take a snapshot
does it get entered there? There are definitely still parts of this
I'm working on a pretty vague understanding of :/

-- 
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.