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: pgsql-hackers@lists.postgresql.org
Date: 2025-01-15T06:11:32Z
Lists: pgsql-hackers 
On Fri, Jan 10, 2025 at 09:40:38AM +0000, Bertrand Drouvot wrote:
> Please find attached v4 taking into account 2c14037bb5.

+} PgStat_WalCounters;
+
+typedef struct PgStat_WalStats
+{
+	PgStat_WalCounters wal_counters;

I know that's a nit, but perhaps that could be a patch of its own,
pushing that to pg_stat_wal_build_tuple() to reduce the diffs in the
main patch.

- * Find or create a local PgStat_BackendPending entry for proc number.
+ * Find or create a local PgStat_BackendPendingIO entry for proc number.

Seems like you are undoing a change here.

+ * WAL pending statistics are incremented inside a critical section
+ * (see XLogWrite()), so we can't use pgstat_prep_pending_entry() and we rely on
+ * PendingBackendWalStats instead.
+ */
+extern PGDLLIMPORT PgStat_PendingWalStats PendingBackendWalStats;

Hmm.  This makes me wonder if we should rethink a bit the way pending
entries are retrieved and if we should do it beforehand for the WAL
paths to avoid allocations in some critical sections.  Isn't that also
because we avoid calling pgstat_prep_backend_pending() for the I/O
case as only backends are supported now, discarding cases like the
checkpointer where I/O could happen in a critical path?  As a whole,
the approach taken by the patch is not really consistent with the
rest.
--
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