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: 2025-03-07T05:42:13Z
Lists: pgsql-hackers 
On Thu, Mar 06, 2025 at 10:33:52AM +0000, Bertrand Drouvot wrote:
> Indeed, there is no reason for pgstat_backend_have_pending_cb() to return true if 
> pgstat_tracks_backend_bktype() is not satisfied.
> 
> So it deserves a dedicated patch to fix this already existing issue:
> 0001 attached.

 pgstat_backend_have_pending_cb(void)
 {
-    return (!pg_memory_is_all_zeros(&PendingBackendStats,
-                                    sizeof(struct PgStat_BackendPending)));
+    if (!pgstat_tracks_backend_bktype(MyBackendType))
+        return false;
+    else
+        return (!pg_memory_is_all_zeros(&PendingBackendStats,
+                                        sizeof(struct PgStat_BackendPending)));

So, if I understand your point correctly, it is not a problem on HEAD
because we are never going to update PendingBackendStats in the
checkpointer as pgstat_count_backend_io_op[_time]() blocks any attempt
to do so.  However, it becomes a problem with the WAL portion patch
because of the dependency to pgWalUsage which may be updated by the
checkpointer and the pending callback would happily report true in
this case.  It would also become a problem if we add in backend stats
a different portion that depends on something external.

An extra check based on pgstat_tracks_backend_bktype() makes sense in
pgstat_backend_have_pending_cb(), yes, forcing the hand of the stats
reports to not do stupid things in a process where we should not
report stats.  Good catch from the sanity check in
pgstat_report_stat(), even if this is only a problem once we begin to
use something else than PendingBackendStats for the pending stats.
This has also the merit of making pgstat_report_stat() do less work.
--
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