Horton

Re: shared-memory based stats collector - v68

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>, Ibrar Ahmed <ibrar.ahmad@gmail.com>, Fujii Masao <masao.fujii@oss.nttdata.com>, gkokolatos@protonmail.com, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2022-04-05T17:30:06Z
Lists: pgsql-hackers 
Hi,

On 2022-04-05 08:49:36 -0700, David G. Johnston wrote:
> On Mon, Apr 4, 2022 at 7:36 PM Andres Freund <andres@anarazel.de> wrote:
> 
> >
> > I think all this is going to achieve is to making code more complicated.
> > There
> > is a *single* non-assert use of accessed_across_databases and now a single
> > assertion involving it.
> >
> > What would having PGSTAT_KIND_CLUSTER and PGSTAT_KIND_DATABASE achieve?
> >
> 
> So, I decided to see what this would look like; the results are attached,
> portions of it also inlined below.

> I'll admit this does introduce a terminology problem - but IMO these words
> are much more meaningful to the reader and code than the existing
> booleans.  I'm hopeful we can bikeshed something agreeable as I'm strongly
> in favor of making this change.

Sorry, I just don't agree. I'm happy to try to make it look better, but this
isn't it.

Do you think it should be your way strongly enough that you'd not want to get
it in the current way?



> The ability to create defines for subsets nicely resolves the problem that
> CLUSTER and DATABASE (now OBJECT to avoid DATABASE conflict in PgStat_Kind)
> are generally related together - they are now grouped under the DYNAMIC
> label (variable, if you want) while all of the fixed entries get associated
> with GLOBAL.  Thus the majority of usages, since accessed_across_databases
> is rare, end up being either DYNAMIC or GLOBAL.

FWIW, as-is DYNAMIC isn't correct:

> +typedef enum PgStat_KindGroup
> +{
> +	PGSTAT_GLOBAL = 1,
> +	PGSTAT_CLUSTER,
> +	PGSTAT_OBJECT
> +} PgStat_KindGroup;
> +
> +#define PGSTAT_DYNAMIC (PGSTAT_CLUSTER | PGSTAT_OBJECT)

Oring PGSTAT_CLUSTER = 2 with PGSTAT_OBJECT = 3 yields 3 again. To do this
kind of thing the different values need to have power-of-two values, and then
the tests need to be done with &.

Nicely demonstrated by the fact that with the patch applied initdb doesn't
pass...


> @@ -909,7 +904,7 @@ pgstat_build_snapshot(void)
>  		 */
>  		if (p->key.dboid != MyDatabaseId &&
>  			p->key.dboid != InvalidOid &&
> -			!kind_info->accessed_across_databases)
> +			kind_info->kind_group == PGSTAT_OBJECT)
>  			continue;
>  
>  		if (p->dropped)

Imo this is far harder to interpret - !kind_info->accessed_across_databases
tells you why we're skipping in clear code. Your alternative doesn't.


> @@ -938,7 +933,7 @@ pgstat_build_snapshot(void)
>  	{
>  		const PgStat_KindInfo *kind_info = pgstat_kind_info_for(kind);
>  
> -		if (!kind_info->fixed_amount)
> +		if (kind_info->kind_group == PGSTAT_DYNAMIC)

These all would have to be kind_info->kind_group & PGSTAT_DYNAMIC, or even
(kind_group & PGSTAT_DYNAMIC) != 0, depending on the case.


> @@ -1047,8 +1042,8 @@ pgstat_delete_pending_entry(PgStat_EntryRef *entry_ref)
>  	void	   *pending_data = entry_ref->pending;
>  
>  	Assert(pending_data != NULL);
> -	/* !fixed_amount stats should be handled explicitly */
> -	Assert(!pgstat_kind_info_for(kind)->fixed_amount);
> +	/* global stats should be handled explicitly : why?*/
> +	Assert(pgstat_kind_info_for(kind)->kind_group == PGSTAT_DYNAMIC);

The pending data infrastructure doesn't provide a way of dealing with fixed
amount stats, and there's no PgStat_EntryRef for them (since they're not in
the hashtable).


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.