Horton

Re: shared-memory based stats collector

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

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From: Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>
To: andres@anarazel.de
Cc: ah@cybertec.at, magnus@hagander.net, robertmhaas@gmail.com, tgl@sss.pgh.pa.us, pgsql-hackers@postgresql.org
Date: 2018-09-26T00:55:09Z
Lists: pgsql-hackers 
Hello. Thank you for the comments.

At Thu, 20 Sep 2018 10:37:24 -0700, Andres Freund <andres@anarazel.de> wrote in <20180920173724.5w2n2nwkxtyi4azw@alap3.anarazel.de>
> Hi,
> 
> On 2018-09-20 09:55:27 +0200, Antonin Houska wrote:
> > I've spent some time reviewing this version.
> > 
> > Design
> > ------
> > 
> > 1. Even with your patch the stats collector still uses an UDP socket to
> >    receive data. Now that the shared memory API is there, shouldn't the
> >    messages be sent via shared memory queue? [1] That would increase the
> >    reliability of message delivery.
> > 
> >    I can actually imagine backends inserting data into the shared hash tables
> >    themselves, but that might make them wait if the same entries are accessed
> >    by another backend. It should be much cheaper just to insert message into
> >    the queue and let the collector process it. In future version the collector
> >    can launch parallel workers so that writes by backends do not get blocked
> >    due to full queue.
> 
> I don't think either of these is right. I think it's crucial to get rid
> of the UDP socket, but I think using a shmem queue is the wrong
> approach. Not just because postgres' shm_mq is single-reader/writer, but
> also because it's plainly unnecessary.  Backends should attempt to
> update the shared hashtable, but acquire the necessary lock
> conditionally, and leave the pending updates of the shared hashtable to
> a later time if they cannot acquire the lock.

Ok, I just intended to avoid reading many bytes from a file and
thought that writer-side can be resolved later.

Currently locks on the shared stats table is acquired by dshash
mechanism in a partition-wise manner. The number of the
partitions is currently fixed to 2^7 = 128, but writes for the
same table confilicts each other regardless of the number of
partitions. As the first step, I'm going to add
conditional-locking capability to dsahsh_find_or_insert and each
backend holds a queue of its pending updates.

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.