Keep heavily-contended fields in XLogCtlInsert on different cache lines.

Performance testing shows that if the insertpos_lck spinlock and the fields
that it protects are on the same cache line with other variables that are
frequently accessed, the false sharing can hurt performance a lot. Keep
them apart by adding some padding.

src/backend/access/transam/xlog.c

@@ -408,7 +408,7 @@ typedef struct
 typedef union XLogInsertSlotPadded
 {
 	XLogInsertSlot slot;
-	char		pad[64];
+	char		pad[CACHE_LINE_SIZE];
 } XLogInsertSlotPadded;
 
 /*
@@ -428,8 +428,14 @@ typedef struct XLogCtlInsert
 	uint64		CurrBytePos;
 	uint64		PrevBytePos;
 
-	/* insertion slots, see above for details */
+	/*
-	XLogInsertSlotPadded *insertSlots;
+	 * Make sure the above heavily-contended spinlock and byte positions are
+	 * on their own cache line. In particular, the RedoRecPtr and full page
+	 * write variables below should be on a different cache line. They are
+	 * read on every WAL insertion, but updated rarely, and we don't want
+	 * those reads to steal the cache line containing Curr/PrevBytePos.
+	 */
+	char		pad[CACHE_LINE_SIZE];
 
 	/*
 	 * fullPageWrites is the master copy used by all backends to determine
@@ -455,6 +461,9 @@ typedef struct XLogCtlInsert
 	bool		exclusiveBackup;
 	int			nonExclusiveBackups;
 	XLogRecPtr	lastBackupStart;
+
+	/* insertion slots, see XLogInsertSlot struct above for details */
+	XLogInsertSlotPadded *insertSlots;
 } XLogCtlInsert;
 
 /*

src/include/pg_config_manual.h

@@ -199,6 +199,17 @@
 #define USE_PPC_LWSYNC
 #endif
 
+/*
+ * Assumed cache line size. This doesn't affect correctness, but can be
+ * used for low-level optimizations. Currently, this is only used to pad
+ * some data structures in xlog.c, to ensure that highly-contended fields
+ * are on different cache lines. Too small a value can hurt performance due
+ * to false sharing, while the only downside of too large a value is a few
+ * bytes of wasted memory. The default is 128, which should be large enough
+ * for all supported platforms.
+ */
+#define CACHE_LINE_SIZE		128
+
 /*
  *------------------------------------------------------------------------
  * The following symbols are for enabling debugging code, not for