Horton

Re: shared-memory based stats collector

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Cc: masao.fujii@oss.nttdata.com, gkokolatos@protonmail.com, pgsql-hackers@lists.postgresql.org
Date: 2021-03-13T06:20:40Z
Lists: pgsql-hackers 
Hi,

On 2021-03-11 19:22:57 -0800, Andres Freund wrote:
> I started changing the patch to address my complaints. I'll try to do
> it as an incremental patch ontop of your 0004, but it might become too
> unwieldy. Not planning to touch other patches for now (and would be
> happy if the first few were committed).  I do think we'll have to
> split 0004 a bit - it's just too large to commit as is, I think.

Far from being done (and the individual commits is just me working, not
something that is intended to survive), but I thought it might be
helpful to post my WIP git tree:
https://github.com/anarazel/postgres/commits/shmstat

I do think that the removal of the the "attach" logic, as well as the
more explicit naming differentiating between the three different hash
tables is a pretty clear improvement. It's way too easy to get turned
around otherwise.


I suspect to make all of this workable we'll have to add a few
preliminary cleanup patches. I'm currently thinking:

1) There's a bunch of functions in places that don't make sense.


/* ------------------------------------------------------------
 * Local support functions follow
 * ------------------------------------------------------------
 */

[internal stuff]
...
void
pgstat_send_archiver(const char *xlog, bool failed)
...
void
pgstat_send_bgwriter(void)
...
void
pgstat_report_wal(void)
...
bool
pgstat_send_wal(bool force)
...
[internal stuff]
...
void
pgstat_count_slru_page_zeroed(int slru_idx)

...

I think it'd make sense to separately clean that up.


2) src/backend/postmaster/pgstat.c currently contains at least two,
   effectively independent, subsystems. Arguably more:
   a) cumulative stats collection infrastructure: sending data to the
      persistent stats file, and reading from it.
   b) "content aware" cumulative statistics function

   c) "current" activity infrastructure, around PgBackendStatus (which
      basically boils down to pg_stat_activity et al.)
   d) wait events
   e) command progress stuff

   They don't actually have much to do with each other, except being
   related to stats. Even without the shared memory stats, having these
   be in one file makes pretty much no sense. Having them all under one
   common pgstat_* prefix is endlessly confusing too.

   I think before making things differently complicated with this patch,
   we need to clean this up, unfortunately. I think we should initially have
   - src/backend/postmaster/pgstat.c, for a), b) above
   - src/backend/utils/activity/backend_status.c for c)
   - src/backend/utils/activity/wait_events.c for d)
   - src/backend/utils/activity/progress.c for e)

   Not at all sure about the names, but something roughly like this
   would im make sense.


The next thing to note is that after this whole patchseries, having the
remaining functionality in src/backend/postmaster/pgstat.c doesn't make
sense. The things in postmaster/ are related to postmaster
sub-processes, not random pieces of backend infrastructure. Therefore I
am thinking that patch 0004 should be changed so it basically adds all
the changed code to two new files:
- src/backend/utils/activity/stats.c - for the stats keeping
  infrastructure (shared memory hash table, pending table, etc)
- src/backend/utils/activity/stats_report.c - for pgstat_report_*,
  pgstat_count_*, pgstat_update_*, flush_*, i.e. everything that knows
  about specific kinds of stats.

The reason for that split is that imo the two pieces of code are largely
independent. One shouldn't need to understand the way stats are stored
in any sort of detail to add a new stats field and vice versa.


Horiguchi-san, is there a chance you could add a few tests (on master)
that test/document the way stats are kept across "normal" restarts, and
thrown away after crash restarts/immediate restarts and also thrown away
graceful streaming rep shutdowns?


Greetings,

Andres Freund

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.