Re: per backend WAL statistics

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