v6-0002-Convert-PgStat_IO-to-pointer-to-avoid-huge-static.patch

From 50517c03c8f118e14bb31d06e4cdf7627866a668 Mon Sep 17 00:00:00 2001
From: Jakub Wartak <jakub.wartak@enterprisedb.com>
Date: Mon, 2 Mar 2026 08:20:09 +0100
Subject: [PATCH v6 2/2] Convert PgStat_IO to pointer to avoid huge static
 memory allocation if not used

---
 src/backend/utils/activity/pgstat.c    |  9 ++++++++-
 src/backend/utils/activity/pgstat_io.c | 15 ++++++++++++---
 src/include/utils/pgstat_internal.h    |  2 +-
 3 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index 11bb71cad5a..62a950987de 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -1634,10 +1634,17 @@ pgstat_write_statsfile(void)
 
 		pgstat_build_snapshot_fixed(kind);
 		if (pgstat_is_kind_builtin(kind))
-			ptr = ((char *) &pgStatLocal.snapshot) + info->snapshot_ctl_off;
+		{
+			if(kind == PGSTAT_KIND_IO)
+				ptr = (char *) pgStatLocal.snapshot.io;
+			else
+				ptr = ((char *) &pgStatLocal.snapshot) + info->snapshot_ctl_off;
+		}
 		else
 			ptr = pgStatLocal.snapshot.custom_data[kind - PGSTAT_KIND_CUSTOM_MIN];
 
+		Assert(ptr != NULL);
+
 		fputc(PGSTAT_FILE_ENTRY_FIXED, fpout);
 		pgstat_write_chunk_s(fpout, &kind);
 		pgstat_write_chunk(fpout, ptr, info->shared_data_len);
diff --git a/src/backend/utils/activity/pgstat_io.c b/src/backend/utils/activity/pgstat_io.c
index 148a2a9c7d5..b5c75e67781 100644
--- a/src/backend/utils/activity/pgstat_io.c
+++ b/src/backend/utils/activity/pgstat_io.c
@@ -19,6 +19,7 @@
 #include "executor/instrument.h"
 #include "port/pg_bitutils.h"
 #include "storage/bufmgr.h"
+#include "utils/memutils.h"
 #include "utils/pgstat_internal.h"
 
 static PgStat_PendingIO PendingIOStats;
@@ -197,7 +198,7 @@ pgstat_fetch_stat_io(void)
 {
 	pgstat_snapshot_fixed(PGSTAT_KIND_IO);
 
-	return &pgStatLocal.snapshot.io;
+	return pgStatLocal.snapshot.io;
 }
 
 /*
@@ -344,6 +345,9 @@ pgstat_io_init_shmem_cb(void *stats)
 
 	for (int i = 0; i < BACKEND_NUM_TYPES; i++)
 		LWLockInitialize(&stat_shmem->locks[i], LWTRANCHE_PGSTATS_DATA);
+
+	/* this might end up being lazily allocated in pgstat_io_snapshot_cb() */
+	pgStatLocal.snapshot.io = NULL;
 }
 
 void
@@ -371,11 +375,16 @@ pgstat_io_reset_all_cb(TimestampTz ts)
 void
 pgstat_io_snapshot_cb(void)
 {
+	/* allocate memory only when necessary on first use */
+	if (unlikely(pgStatLocal.snapshot.io == NULL))
+		pgStatLocal.snapshot.io = MemoryContextAllocZero(TopMemoryContext,
+												   sizeof(PgStat_IO));
+
 	for (int i = 0; i < BACKEND_NUM_TYPES; i++)
 	{
 		LWLock	   *bktype_lock = &pgStatLocal.shmem->io.locks[i];
 		PgStat_BktypeIO *bktype_shstats = &pgStatLocal.shmem->io.stats.stats[i];
-		PgStat_BktypeIO *bktype_snap = &pgStatLocal.snapshot.io.stats[i];
+		PgStat_BktypeIO *bktype_snap = &pgStatLocal.snapshot.io->stats[i];
 
 		LWLockAcquire(bktype_lock, LW_SHARED);
 
@@ -384,7 +393,7 @@ pgstat_io_snapshot_cb(void)
 		 * the reset timestamp as well.
 		 */
 		if (i == 0)
-			pgStatLocal.snapshot.io.stat_reset_timestamp =
+			pgStatLocal.snapshot.io->stat_reset_timestamp =
 				pgStatLocal.shmem->io.stats.stat_reset_timestamp;
 
 		/* using struct assignment due to better type safety */
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 9b8fbae00ed..407657e060c 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -600,7 +600,7 @@ typedef struct PgStat_Snapshot
 
 	PgStat_CheckpointerStats checkpointer;
 
-	PgStat_IO	io;
+	PgStat_IO	*io;
 
 	PgStat_SLRUStats slru[SLRU_NUM_ELEMENTS];
 
-- 
2.43.0