From a70ccfc6de412d7ec8517594ece0777c837c375b Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Fri, 12 Aug 2005 05:05:51 +0000
Subject: [PATCH] Remove BufferBlockPointers array in favor of a base +
 (bufnum) * BLCKSZ computation.  On modern machines this is as fast if not
 faster, and we don't have to clog the CPU's L2 cache with a tens-of-KB
 pointer array. If we ever decide to adopt a more dynamic allocation method
 for shared buffers, we'll probably have to revert this patch, but in the
 meantime we might as well save a few bytes and nanoseconds.  Per Qingqing
 Zhou.

---
 src/backend/storage/buffer/buf_init.c | 23 +----------------------
 src/backend/storage/buffer/bufmgr.c   |  2 +-
 src/include/storage/bufmgr.h          |  4 ++--
 3 files changed, 4 insertions(+), 25 deletions(-)

diff --git a/src/backend/storage/buffer/buf_init.c b/src/backend/storage/buffer/buf_init.c
index 9d3c2e44ff..ed38f8d874 100644
--- a/src/backend/storage/buffer/buf_init.c
+++ b/src/backend/storage/buffer/buf_init.c
@@ -19,11 +19,9 @@
 
 
 BufferDesc *BufferDescriptors;
-Block	   *BufferBlockPointers;
+char	   *BufferBlocks;
 int32	   *PrivateRefCount;
 
-static char *BufferBlocks;
-
 /* statistics counters */
 long int	ReadBufferCount;
 long int	ReadLocalBufferCount;
@@ -154,30 +152,11 @@ InitBufferPool(void)
 void
 InitBufferPoolAccess(void)
 {
-	char	   *block;
-	int			i;
-
 	/*
 	 * Allocate and zero local arrays of per-buffer info.
 	 */
-	BufferBlockPointers = (Block *) calloc(NBuffers,
-										   sizeof(*BufferBlockPointers));
 	PrivateRefCount = (int32 *) calloc(NBuffers,
 									   sizeof(*PrivateRefCount));
-
-	/*
-	 * Construct addresses for the individual buffer data blocks.  We do
-	 * this just to speed up the BufferGetBlock() macro.  (Since the
-	 * addresses should be the same in every backend, we could inherit
-	 * this data from the postmaster --- but in the EXEC_BACKEND case
-	 * that doesn't work.)
-	 */
-	block = BufferBlocks;
-	for (i = 0; i < NBuffers; i++)
-	{
-		BufferBlockPointers[i] = (Block) block;
-		block += BLCKSZ;
-	}
 }
 
 /*
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index 65169c5386..af6cecbce3 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -54,7 +54,7 @@
 
 
 /* Note: these two macros only work on shared buffers, not local ones! */
-#define BufHdrGetBlock(bufHdr)	BufferBlockPointers[(bufHdr)->buf_id]
+#define BufHdrGetBlock(bufHdr)	((Block) (BufferBlocks + ((Size) (bufHdr)->buf_id) * BLCKSZ))
 #define BufferGetLSN(bufHdr)	(*((XLogRecPtr*) BufHdrGetBlock(bufHdr)))
 
 /* Note: this macro only works on local buffers, not shared ones! */
diff --git a/src/include/storage/bufmgr.h b/src/include/storage/bufmgr.h
index 9eea85bf9e..f0e169417c 100644
--- a/src/include/storage/bufmgr.h
+++ b/src/include/storage/bufmgr.h
@@ -33,7 +33,7 @@ extern int	bgwriter_lru_maxpages;
 extern int	bgwriter_all_maxpages;
 
 /* in buf_init.c */
-extern DLLIMPORT Block *BufferBlockPointers;
+extern DLLIMPORT char *BufferBlocks;
 extern DLLIMPORT int32 *PrivateRefCount;
 
 /* in localbuf.c */
@@ -107,7 +107,7 @@ extern DLLIMPORT int32 *LocalRefCount;
 	BufferIsLocal(buffer) ? \
 		LocalBufferBlockPointers[-(buffer) - 1] \
 	: \
-		BufferBlockPointers[(buffer) - 1] \
+		(Block) (BufferBlocks + ((Size) ((buffer) - 1)) * BLCKSZ) \
 )
 
 /*
-- 
2.39.5