From c50d0f17e223e9e85421bc2bc897291383cba74d Mon Sep 17 00:00:00 2001
From: Nazir Bilal Yavuz <byavuz81@gmail.com>
Date: Mon, 31 Mar 2025 12:21:27 +0300
Subject: [PATCH v7 2/2] Count free buffers at the start of the autoprewarm

Streamified version of the autoprewarm code may do unnecessary I/O and
buffer evicting. To prevent it at least a little bit, count the number
of free buffers in the buffer pool and queue buffers up to that number
in the callback function of the autoprewarm.
---
 contrib/pg_prewarm/autoprewarm.c      |  7 +++++--
 src/backend/storage/buffer/freelist.c | 17 +++++++++++++++++
 src/include/storage/buf_internals.h   |  1 +
 3 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/contrib/pg_prewarm/autoprewarm.c b/contrib/pg_prewarm/autoprewarm.c
index 68c99664de5..df10e02358d 100644
--- a/contrib/pg_prewarm/autoprewarm.c
+++ b/contrib/pg_prewarm/autoprewarm.c
@@ -518,6 +518,7 @@ void
 autoprewarm_database_main(Datum main_arg)
 {
 	int			pos;
+	int			stop_idx;
 	BlockInfoRecord *block_info;
 	Relation	rel = NULL;
 	dsm_segment *seg;
@@ -539,11 +540,13 @@ autoprewarm_database_main(Datum main_arg)
 	BackgroundWorkerInitializeConnectionByOid(apw_state->database, InvalidOid, 0);
 	block_info = (BlockInfoRecord *) dsm_segment_address(seg);
 	pos = apw_state->prewarm_start_idx;
+	stop_idx = Min(apw_state->prewarm_stop_idx,
+				   pos + get_number_of_free_buffers());
 
 	cur_database = block_info[pos].database;
 
 	/* Loop until we run out of blocks to prewarm. */
-	while (pos < apw_state->prewarm_stop_idx)
+	while (pos < stop_idx)
 	{
 		BlockInfoRecord *blk = &block_info[pos];
 
@@ -616,7 +619,7 @@ autoprewarm_database_main(Datum main_arg)
 			nblocks_in_fork = RelationGetNumberOfBlocksInFork(rel, blk->forknum);
 			nblocks_processed = autoprewarm_prewarm_relation(rel,
 															 pos,
-															 apw_state->prewarm_stop_idx,
+															 stop_idx,
 															 nblocks_in_fork,
 															 block_info);
 
diff --git a/src/backend/storage/buffer/freelist.c b/src/backend/storage/buffer/freelist.c
index 336715b6c63..9f26e940426 100644
--- a/src/backend/storage/buffer/freelist.c
+++ b/src/backend/storage/buffer/freelist.c
@@ -180,6 +180,23 @@ have_free_buffer(void)
 		return false;
 }
 
+/*
+ * get_number_of_free_buffers -- a lockless way to get the number of free
+ *								 buffers in buffer pool.
+ *
+ * Note that result continuously changes as free buffers are moved out by other
+ * operations.
+ */
+int
+get_number_of_free_buffers(void)
+{
+	/* All the buffers are free. */
+	if (StrategyControl->firstFreeBuffer < 0)
+		return NBuffers;
+	else
+		return StrategyControl->lastFreeBuffer - StrategyControl->firstFreeBuffer;
+}
+
 /*
  * StrategyGetBuffer
  *
diff --git a/src/include/storage/buf_internals.h b/src/include/storage/buf_internals.h
index 9327f60c44c..197d2db49f5 100644
--- a/src/include/storage/buf_internals.h
+++ b/src/include/storage/buf_internals.h
@@ -445,6 +445,7 @@ extern void StrategyNotifyBgWriter(int bgwprocno);
 extern Size StrategyShmemSize(void);
 extern void StrategyInitialize(bool init);
 extern bool have_free_buffer(void);
+extern int	get_number_of_free_buffers(void);
 
 /* buf_table.c */
 extern Size BufTableShmemSize(int size);
-- 
2.43.0