Horton

Re: shared-memory based stats collector - v70

Drouvot, Bertrand <bdrouvot@amazon.com>

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From: "Drouvot, Bertrand" <bdrouvot@amazon.com>
To: Andres Freund <andres@anarazel.de>, Greg Stark <stark@mit.edu>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Melanie Plageman <melanieplageman@gmail.com>, Kyotaro Horiguchi <horikyota.ntt@gmail.com>, Justin Pryzby <pryzby@telsasoft.com>, Thomas Munro <thomas.munro@gmail.com>, "David G. Johnston" <david.g.johnston@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2022-08-18T06:39:02Z
Lists: pgsql-hackers 
Hi,

On 8/18/22 1:30 AM, Andres Freund wrote:
> Hi,
>
> On 2022-08-17 15:46:42 -0400, Greg Stark wrote:
>> Isn't there also a local hash table used to find the entries to reduce
>> traffic on the shared hash table? Even if you don't take a snapshot
>> does it get entered there? There are definitely still parts of this
>> I'm working on a pretty vague understanding of :/
> Yes, there is. But it's more about code that generates stats, rather than
> reporting functions. While there's backend local pending stats we need to have
> a refcount on the shared stats item so that the stats item can't be dropped
> and then revived when those local stats are flushed.

What do you think about something along those lines for the reporting 
part only?

Datum
pgstat_fetch_all_tables_stats(PG_FUNCTION_ARGS)
{
     int         dbid = PG_ARGISNULL(0) ? -1 : (int) PG_GETARG_OID(0);
     ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
     dshash_seq_status hstat;
     PgStatShared_HashEntry *p;
     PgStatShared_Common *stats_data;

     /* Only members of pg_read_all_stats can use this function */
     if (!has_privs_of_role(GetUserId(), ROLE_PG_READ_ALL_STATS))
     {
         aclcheck_error(ACLCHECK_NO_PRIV, OBJECT_FUNCTION, 
"pgstat_fetch_all_tables_stats");
     }

     pgstat_assert_is_up();

     SetSingleFuncCall(fcinfo, 0);

     dshash_seq_init(&hstat, pgStatLocal.shared_hash, false);
     while ((p = dshash_seq_next(&hstat)) != NULL)
     {
         Datum           values[PG_STAT_GET_ALL_TABLES_STATS_COLS];
         bool            nulls[PG_STAT_GET_ALL_TABLES_STATS_COLS];
         PgStat_StatTabEntry * tabentry = NULL;

         MemSet(values, 0, sizeof(values));
         MemSet(nulls, false, sizeof(nulls));

         /* If looking for specific dbid, ignore all the others */
         if (dbid != -1 && p->key.dboid != (Oid) dbid)
             continue;

         /* If the entry is not of kind relation then ignore it */
         if (p->key.kind != PGSTAT_KIND_RELATION)
             continue;

         /* If the entry has been dropped then ignore it */
         if (p->dropped)
             continue;

         stats_data = dsa_get_address(pgStatLocal.dsa, p->body);
         LWLockAcquire(&stats_data->lock, LW_SHARED);
         tabentry = pgstat_get_entry_data(p->key.kind, stats_data);

         values[0] = ObjectIdGetDatum(p->key.dboid);
         values[1] = ObjectIdGetDatum(p->key.objoid);
         values[2]= DatumGetInt64(tabentry->tuples_inserted);
         values[3]= DatumGetInt64(tabentry->tuples_updated);
         values[4]= DatumGetInt64(tabentry->tuples_deleted);
         .
         .
         .
         tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc, 
values, nulls);

         LWLockRelease(&stats_data->lock);
     }
     dshash_seq_term(&hstat);

     return (Datum) 0;
}

I also tried to make use of pgstat_get_entry_ref() but went into a 
failed assertion: pgstat_get_entry_ref -> dshash_find -> 
ASSERT_NO_PARTITION_LOCKS_HELD_BY_ME(hash_table) due to lock acquired by 
dshash_seq_next().

Regards,

-- 

Bertrand Drouvot
Amazon Web Services: https://aws.amazon.com

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.