Horton

Re: shared-memory based stats collector - v69

David G. Johnston <david.g.johnston@gmail.com>

View thread View on postgr.es
From: "David G. Johnston" <david.g.johnston@gmail.com>
To: Andres Freund <andres@anarazel.de>
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: 2022-04-06T03:00:50Z
Lists: pgsql-hackers 
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.
>
>
Thank you.

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.
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).

David J.

 * Purpose:

 * 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.
 *
 * 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.
 *
 * Cumulative in this context means that processes must, for numeric data,
send
 * a delta (or change) value via the API which will then be added to the
 * stored value in memory. The system does not track individual changes,
only
 * their net effect. 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.

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.