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/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