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.

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 39c58d00fe6ca6afd04c2587e7b7e35fbb238ca1..386811389d728da581bf6c5c30b7262d783e3f64 100644 (file)
--- a/src/backend/access/transam/xlog.c
+++ b/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;
 
 /*

diff --git a/src/include/pg_config_manual.h b/src/include/pg_config_manual.h
index 1d60be2c4770bb8a42961405d1b84df8c44a3d5e..2e6aad1ca56aa8cceedf6622ba91a9aa8475c688 100644 (file)
--- a/src/include/pg_config_manual.h
+++ b/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