Horton

Re: shared-memory based stats collector

Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>

View thread View on postgr.es
From: Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>
To: tgl@sss.pgh.pa.us
Cc: robertmhaas@gmail.com, pgsql-hackers@postgresql.org
Date: 2018-07-05T03:04:23Z
Lists: pgsql-hackers 
Hello.

At Wed, 04 Jul 2018 17:23:51 -0400, Tom Lane <tgl@sss.pgh.pa.us> wrote in <67470.1530739431@sss.pgh.pa.us>
> Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp> writes:
> > At Mon, 2 Jul 2018 14:25:58 -0400, Robert Haas <robertmhaas@gmail.com> wrote in <CA+TgmoYQhr30eAcgJCi1v0FhA+3RP1FZVnXqSTLe=6fHy9e5oA@mail.gmail.com>
> >> Copying the whole hash table kinds of sucks, partly because of the
> >> time it will take to copy it, but also because it means that memory
> >> usage is still O(nbackends * ntables).  Without looking at the patch,
> >> I'm guessing that you're doing that because we need a way to show each
> >> transaction a consistent snapshot of the data, and I admit that I
> >> don't see another obvious way to tackle that problem.  Still, it would
> >> be nice if we had a better idea.
> 
> > The consistency here means "repeatable read" of an object's stats
> > entry, not a snapshot covering all objects. We don't need to copy
> > all the entries at once following this definition. The attached
> > version makes a cache entry only for requested objects.
> 
> Uh, what?  That's basically destroying the long-standing semantics of
> statistics snapshots.  I do not think we can consider that acceptable.
> As an example, it would mean that scan counts for indexes would not
> match up with scan counts for their tables.

The current stats collector mechanism sends at most 8 table stats
in a single message. Split messages from multiple transactions
can reach to collector in shuffled order. The resulting snapshot
can be "inconsistent" if INQUIRY message comes between such split
messages.  Of course a single meesage would be enough for common
transactions but not for all.

Even though the inconsistency happens more frequently with this
patch, I don't think users expect such strict consistency of
table stats, especially on a busy system. And I believe it's a
good thing if users saw more "useful" information for the relaxed
consistency. (The actual meaning of "useful" is out of the
current focus:p)


Meanwhile, if we should keep the practical-consistency, a giant
lock is out of the question. So we need a transactional stats of
any shape. It can be a whole-image snapshot or a regular MMVC
table, or maybe the current dshash with UNDO logs. Since there
are actually many states, it is inevitable to require storage to
reproduce each state.

I think the consensus is that the whole-image snapshot takes
too-much memory. MMVC is apparently too-much for the purpose.

UNDO logs seems a bit promising. If we looking stats in a long
transaction, the required memory for UNDO information easily
reaches to the same amount with the whole-image snapshot. But I
expect that it is not so common.

I'll consider that apart from the current patch.

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.