Horton

Re: per backend WAL statistics

Michael Paquier <michael@paquier.xyz>

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From: Michael Paquier <michael@paquier.xyz>
To: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Cc: Andres Freund <andres@anarazel.de>, Xuneng Zhou <xunengzhou@gmail.com>, pgsql-hackers@lists.postgresql.org
Date: 2025-03-10T07:46:53Z
Lists: pgsql-hackers

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On Sat, Mar 08, 2025 at 07:53:04AM +0000, Bertrand Drouvot wrote:
> That would not be an issue should we only access the struct
> fields in the code, but that's not the case as we're making use of
> pg_memory_is_all_zeros() on it.

It does not hurt to keep it as it is, honestly.

I've reviewed the last patch of the series, and noticed a couple of 
inconsistent comments across it, and some indentation issue.
 
@@ -199,7 +258,8 @@ pgstat_flush_backend(bool nowait, bits32 flags)
         return false;
 
     if (pg_memory_is_all_zeros(&PendingBackendStats,
-                               sizeof(struct PgStat_BackendPending)))
+                               sizeof(struct PgStat_BackendPending))
+        && !pgstat_backend_wal_have_pending())
         return false;

I have one issue with pgstat_flush_backend() and the early exit check
done here.  If for example we use FLUSH_WAL but there is some IO data
pending, we would lock the stats entry for nothing.  We could also
return true even if there is no pending WAL data if the lock could not
be taken, which would be incorrect because there was no data to flush
to begin with.  I think that this should be adjusted so as we limit
the entry lock depending on the flags given in input, like in the
attached.

Thoughts?
--
Michael

Commits

  1. Fix use-after-free in pgstat_fetch_stat_backend_by_pid()

  2. Remove initialization from PendingBackendStats

  3. Add WAL data to backend statistics

  4. Improve check for detection of pending data in backend statistics

  5. Fix some gaps in pg_stat_io with WAL receiver and WAL summarizer

  6. Handle auxiliary processes in SQL functions of backend statistics

  7. Invent pgstat_fetch_stat_backend_by_pid()

  8. Refactor code of pg_stat_get_wal() building result tuple

  9. Adding new PgStat_WalCounters structure in pgstat.h

  10. Remove pgstat_flush_wal()

  11. Refactor some code related to backend statistics

  12. Add backend-level statistics to pgstats