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: 2022-04-06T03:14:16Z
Lists: pgsql-hackers 
Hi,

On 2022-04-05 20:00:50 -0700, David G. Johnston wrote:
> On Tue, Apr 5, 2022 at 4:16 PM Andres Freund <andres@anarazel.de> wrote:
> > 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:
> > > > 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.

I at least tried...


> On a slightly different track, I took the time to write-up a "Purpose"
> section for pgstat.c :
> 
> It may possibly be duplicating some things written elsewhere as I didn't go
> looking for similar prior art yet, I just wanted to get thoughts down.

There's very very little prior documentation in this area.


> This is the kind of preliminary framing I've been constructing in my own
> mind as I try to absorb this patch.  I haven't formed an opinion whether
> the actual user-facing documentation should cover some or all of this
> instead of the preamble to pgstat.c (which could just point to the docs for
> prerequisite reading).

>  * The PgStat namespace defines an API that facilitates concurrent access
>  * to a shared memory region where cumulative statistical data is saved.
>  * At shutdown, one of the running system workers will initiate the writing
>  * of the data to file. Then, during startup (following a clean shutdown)
> the
>  * Postmaster process will early on ensure that the file is loaded into
> memory.

I added something roughly along those lines in the version I just sent, based
on a suggestion by Melanie over IM:

 * Statistics are loaded from the filesystem during startup (by the startup
 * process), unless preceded by a crash, in which case all stats are
 * discarded. They are written out by the checkpointer process just before
 * shutting down, except when shutting down in immediate mode.



>  * Each cumulative statistic producing system must construct a PgStat_Kind
>  * datum in this file. The details are described elsewhere, but of
>  * particular importance is that each kind is classified as having either a
>  * fixed number of objects that it tracks, or a variable number.
>  *
>  * During normal operations, the different consumers of the API will have
> their
>  * accessed managed by the API, the protocol used is determined based upon
> whether
>  * the statistical kind is fixed-numbered or variable-numbered.
>  * Readers of variable-numbered statistics will have the option to locally
>  * cache the data, while writers may have their updates locally queued
>  * and applied in a batch. Thus favoring speed over freshness.
>  * The fixed-numbered statistics are faster to process and thus forgo
>  * these mechanisms in favor of a light-weight lock.

This feels a bit jumbled. Of course something using an API will be managed by
the API. I don't know what protocol reallly means?


> Additionally, both due to unclean shutdown or user
> request,
>  * statistics can be reset - meaning that their stored numeric values are
> returned
>  * to zero, and any non-numeric data that may be tracked (say a timestamp)
> is cleared.

I think this is basically covered in the above?

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.