Horton

Re: shared-memory based stats collector - v69

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: "David G. Johnston" <david.g.johnston@gmail.com>
Cc: Kyotaro Horiguchi <horikyota.ntt@gmail.com>, Melanie Plageman <melanieplageman@gmail.com>, Justin Pryzby <pryzby@telsasoft.com>, Thomas Munro <thomas.munro@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2022-04-05T23:16:28Z
Lists: pgsql-hackers 
On 2022-04-05 14:43:49 -0700, David G. Johnston wrote:
> On Tue, Apr 5, 2022 at 2:23 PM Andres Freund <andres@anarazel.de> wrote:
> 
> >
> > On 2022-04-05 13:51:12 -0700, David G. Johnston wrote:
> >
> > >, but rather add to the shared queue
> >
> > Queue? Maybe you mean the hashtable?
> >
> 
> Queue implemented by a list...?  Anyway, I think I mean this:

> /*
>  * List of PgStat_EntryRefs with unflushed pending stats.
>  *
>  * Newly pending entries should only ever be added to the end of the list,
>  * otherwise pgstat_flush_pending_entries() might not see them immediately.
>  */
> static dlist_head pgStatPending = DLIST_STATIC_INIT(pgStatPending);

That's not in shared memory, but backend local...


> > >, and so the cache is effectively read-only.  It is also
> > transaction-scoped
> > >based upon the GUC and the nature of stats vis-a-vis transactions.
> >
> > No, that's not right. I think you might be thinking of
> > pgStatLocal.snapshot.stats?
> >
> >
> Probably...
> 
> 
> > I guess I should add a paragraph about snapshots / fetch consistency.
> >
> 
> I apparently confused/combined the two concepts just now so that would help.

Will add.


> >
> > > Even before I added the read-only and transaction-scoped I got a bit hung
> > > up on reading:
> > > "The shared hashtable only needs to be accessed when no prior reference
> > to
> > > the shared hashtable exists."
> >
> > > Thinking in terms of key seems to make more sense than value in this
> > > sentence - even if there is a one-to-one correspondence.
> >
> > Maybe "prior reference to the shared hashtable exists for the key"?
> >
> 
> I specifically dislike having two mentions of the "shared hashtable" in the
> same sentence, so I tried to phrase the second half in terms of the local
> hashtable.

You left two mentions of "shared hashtable" in the sentence prior though
:). I'll try to rephrase. But it's not the end if this isn't the most elegant
prose...


> > Was that comma after e.g. intentional?
> >
> 
> It is.  That is the style I was taught, and that we seem to adhere to in
> user-facing documentation.  Source code is a mixed bag with no enforcement,
> but while we are here...

Looks a bit odd to me. But I guess I'll add it then...


> > >   *
> > > @@ -19,19 +19,21 @@
> > >   *
> > >   * All variable-numbered stats are addressed by PgStat_HashKey while
> > running.
> > >   * It is not possible to have statistics for an object that cannot be
> > > - * addressed that way at runtime. A wider identifier can be used when
> > > + * addressed that way at runtime. A alternate identifier can be used
> > when
> > >   * serializing to disk (used for replication slot stats).
> >
> > Not sure this improves things.
> >
> >
> It just seems odd that width is being mentioned when the actual struct is a
> combination of three subcomponents.  I do feel I'd need to understand
> exactly what replication slot stats are doing uniquely here, though, to
> make any point beyond that.

There's no real numeric identifier for replication slot stats. So I'm using
the "index" used in slot.c while running. But that can change during
start/stop.

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.