Horton

Re: shared-memory based stats collector

Kyotaro Horiguchi <horikyota.ntt@gmail.com>

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From: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
To: andres@anarazel.de
Cc: magnus@hagander.net, masao.fujii@oss.nttdata.com, gkokolatos@protonmail.com, pgsql-hackers@lists.postgresql.org
Date: 2021-03-16T07:44:38Z
Lists: pgsql-hackers 
At Mon, 15 Mar 2021 19:04:29 -0700, Andres Freund <andres@anarazel.de> wrote in 
> I have a few questions about the patch:
> 
> - Why was collect_oids() changed to a different hashtable as part of
>   this change? Seems fairly independent?

Right. It is changed at the time I used simplehash for some other
stuff. 

> - What's the point of all those cached_* stuff? There's not a single
>   comment explaining it as far as I can tell...
> 
>   Several of them are never used as a cache! E.g. cached_archiverstats,
>   cached_bgwriterstats, ...

They're just local copies of them on shared memory. That works to
allow early release of locks on shared objects.  And they're to avoid
repeated copying of the same stats entry and to offer a consistent
values. For example, pgstat_fetch_stat_tabentry() is called repeatedly
in a single view.

About cached_archiverstats... cached_archvierstats_is_valid is utterly
useless. Since we don't offer in-transaction consistency for the
values, I think *_is_valid are useless.

> - What is the idea behind pgstat_reset_shared_counters() using
>   pgstat_copy_global_stats() to reset, using StatsShmem->*_reset_offset?
>   But then still taking a lock in pgstat_fetch_stat_*?  Again, no
>   comments explaining what the goal is.
> 
>   It kinda looks like you tried to make both read and write paths not
>   use the lock, but then ended up using a lock?

Mmm. That is a kind of inconsistent between the view from upper side
and from lower side.  As you say, I tried to get rid of *the*
StatsLock in pgstat_reset_shared_counters and eventually I forgot to
remove the locking.  I think the lock is no longer necessary.

> Do you have some benchmarks that you used to verify performance?

https://www.postgresql.org/message-id/20201008.160326.2246946707652981235.horikyota.ntt%40gmail.com

It is using pgbench, with 800 clients with 20 threads.

Graphs of performance gain/loss from the master for the following
benchmarks are attached.

> - Fetching 1 tuple from 1 of 100 tables from 100 to 800 clients.
> - Fetching 1 tuple from 1 of 10 tables from 100 to 800 clients.

v36 showed about 60% of degradation (TPS redued to 1/3 of master) at
600 clients, but it has been dissapeared as of v39. The graphs are of
v39. I'm asking for the script that was used for the benchark and will
send them later.

> I think I'm going to try to split the storage of fixed-size stats in
> StatsShmemStruct into a separate patch. That's already a pretty large
> change, and it's pretty much unrelated to the rest.

I'm not sure the ratio between the "Global stats structs" (aka
fixed-size stats?) and other stuff but agreed that they are almost
unrelated to each other.

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center

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.