Horton

Re: shared-memory based stats collector

Kyotaro Horiguchi <horikyota.ntt@gmail.com>

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From: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
To: masao.fujii@oss.nttdata.com
Cc: gkokolatos@protonmail.com, pgsql-hackers@lists.postgresql.org
Date: 2021-03-09T07:51:10Z
Lists: pgsql-hackers 
At Sat, 6 Mar 2021 00:32:07 +0900, Fujii Masao <masao.fujii@oss.nttdata.com> wrote in 
> 
> 
> On 2021/03/05 17:18, Kyotaro Horiguchi wrote:
> > At Thu, 21 Jan 2021 12:03:48 +0900 (JST), Kyotaro Horiguchi
> > <horikyota.ntt@gmail.com> wrote in
> >> Commit 960869da08 (database statistics) conflicted with this. Rebased.
> >>
> >> I'm concerned about the behavior that pgstat_update_connstats calls
> >> GetCurrentTimestamp() every time stats update happens (with intervals
> >> of 10s-60s in this patch). But I didn't change that design since that
> >> happens with about 0.5s intervals in master and the rate is largely
> >> reduced in this patch, to make this patch simpler.
> > I stepped on my foot, and another commit coflicted. Just rebased.
> 
> Thanks for rebasing the patches!
> 
> I think that 0003 patch is self-contained and useful, for example
> which
> enables us to monitor archiver process in pg_stat_activity. So IMO
> it's worth pusing 0003 patch firstly.

I'm not sure archiver process is worth realtime monitoring, but I
agree that the patch makes it possible.  Anyway it is requried in this
patchset and I'm happy to see it committed beforehand.

Thanks for the review.

> Here are the review comments for 0003 patch.
> 
> +	/* Archiver process's latch */
> +	Latch	   *archiverLatch;
> +	/* Current shared estimate of appropriate spins_per_delay value */
> 
> The last line in the above seems not necessary.

Oops. It seems like a garbage after a past rebasing. Removed.

> In proc.h, NUM_AUXILIARY_PROCS needs to be incremented.

Right.  Increased to 5 and rewrote the comment.

>     /* ----------
>      * Functions called from postmaster
>      * ----------
>      */
>     extern int	pgarch_start(void);
> 
> In pgarch.h, the above is not necessary.

Removed.

> +extern void XLogArchiveWakeup(void);
> 
> This seems no longer necessary.
> 
> +extern void XLogArchiveWakeupStart(void);
> +extern void XLogArchiveWakeupEnd(void);
> +extern void XLogArchiveWakeup(void);
> 
> These seem also no longer necessary.

Sorry for many garbages. Removed all of them.

>  			PgArchPID = 0;
>  			if (!EXIT_STATUS_0(exitstatus))
> -				LogChildExit(LOG, _("archiver process"),
> -							 pid, exitstatus);
> -			if (PgArchStartupAllowed())
> -				PgArchPID = pgarch_start();
> +				HandleChildCrash(pid, exitstatus,
> +								 _("archiver process"));
> 
> I don't think that we should treat non-zero exit condition as a crash,
> as before. Otherwise when archive_command fails on a signal,
> archiver emits FATAL error and which leads the server restart.

Sounds reasonable. Now archiver is treated the same way to wal
receiver.  Specifically exit(1) doesn't cause server restart.

> - * walwriter, autovacuum, or background worker.
> + * walwriter, autovacuum, archiver or background worker.
>   *
>   * The objectives here are to clean up our local state about the child
>   * process, and to signal all other remaining children to quickdie.
> @@ -3609,6 +3606,18 @@ HandleChildCrash(int pid, int exitstatus, const
> char *procname)
>  		signal_child(AutoVacPID, (SendStop ? SIGSTOP : SIGQUIT));
>  	}
>  +	/* Take care of the archiver too */
> +	if (pid == PgArchPID)
> +		PgArchPID = 0;
> +	else if (PgArchPID != 0 && take_action)
> +	{
> +		ereport(DEBUG2,
> +				(errmsg_internal("sending %s to process %d",
> +								 (SendStop ? "SIGSTOP" : "SIGQUIT"),
> +								 (int) PgArchPID)));
> +		signal_child(PgArchPID, (SendStop ? SIGSTOP : SIGQUIT));
> +	}
> +
> 
> Same as above.

Mmm.  In the first place, I found that I forgot to remove existing
code to handle archiver... Removed it instead of the above, which is
added by this patch.  Since the process becomes an auxiliary process,
no reason to differentiate from other auxiliary processes in handling?

> In xlogarchive.c, "#include "storage/pmsignal.h"" is no longer
> necessary.

Removed.

> pgarch_forkexec() should be removed from pgarch.c because it's no
> longer used.

Right. Removed. EXEC_BACKEND still fails for another reason of
prototype mismatch of PgArchiverMain. Fixed it toghether.

>     /* ------------------------------------------------------------
>      * Public functions called from postmaster follow
>      * ------------------------------------------------------------
>      */
> 
> The definition of PgArchiverMain() should be placed just
> after the above comment.

The module no longer have a function called from postmaster. Now
PgArchiverMain() is placed just below "/* Main entry point...".

> exit(0) in PgArchiverMain() should be proc_exit(0)?

Yeah, proc_exit() says as it should be the only function to call
exit() directly.

By the way, the patch (0003) removes the flag "wakened". The flag was
originally added to prevent spurious wakeups (66ec2db7284). At the
time the pg_ysleep in pgarch_MainLoop was replaced with WaitLatch by
89fd72cbf26, the flag survived but with losing its effect since
WaitLatch doesn't get spurious wakeups (AFAICS).  So if the change
(removal of "wakened") is correct, it might worth another patch.

I'll send new patchset soon.

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.