Force the size and alignment of LWLock array entries to be either 16 or 32

bytes.  This shouldn't make any difference on x86 machines, where the size
happened to be 16 bytes anyway, but on 64-bit machines and machines with
slock_t int or wider, it will speed array indexing and hopefully reduce
SMP cache contention effects.  Per recent experimentation.

Author: tglsfdc

src/backend/storage/lmgr/lwlock.c

@@ -15,7 +15,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/storage/lmgr/lwlock.c,v 1.29 2005/08/20 23:26:24 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/storage/lmgr/lwlock.c,v 1.30 2005/09/16 00:30:05 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -38,12 +38,32 @@ typedef struct LWLock
 	/* tail is undefined when head is NULL */
 } LWLock;
 
+/*
+ * All the LWLock structs are allocated as an array in shared memory.
+ * (LWLockIds are indexes into the array.)  We force the array stride to
+ * be a power of 2, which saves a few cycles in indexing, but more
+ * importantly also ensures that individual LWLocks don't cross cache line
+ * boundaries.  This reduces cache contention problems, especially on AMD
+ * Opterons.  (Of course, we have to also ensure that the array start
+ * address is suitably aligned.)
+ *
+ * LWLock is between 16 and 32 bytes on all known platforms, so these two
+ * cases are sufficient.
+ */
+#define LWLOCK_PADDED_SIZE	(sizeof(LWLock) <= 16 ? 16 : 32)
+
+typedef union LWLockPadded
+{
+	LWLock		lock;
+	char		pad[LWLOCK_PADDED_SIZE];
+} LWLockPadded;
+
 /*
  * This points to the array of LWLocks in shared memory.  Backends inherit
- * the pointer by fork from the postmaster.  LWLockIds are indexes into
- * the array.
+ * the pointer by fork from the postmaster (except in the EXEC_BACKEND case,
+ * where we have special measures to pass it down).
  */
-NON_EXEC_STATIC LWLock *LWLockArray = NULL;
+NON_EXEC_STATIC LWLockPadded *LWLockArray = NULL;
 
 /* shared counter for dynamic allocation of LWLockIds */
 static int *LWLockCounter;
@@ -135,10 +155,11 @@ LWLockShmemSize(void)
 	Size		size;
 	int			numLocks = NumLWLocks();
 
-	/* Allocate the LWLocks plus space for shared allocation counter. */
-	size = mul_size(numLocks, sizeof(LWLock));
+	/* Space for the LWLock array. */
+	size = mul_size(numLocks, sizeof(LWLockPadded));
 
-	size = add_size(size, 2 * sizeof(int));
+	/* Space for shared allocation counter, plus room for alignment. */
+	size = add_size(size, 2 * sizeof(int) + LWLOCK_PADDED_SIZE);
 
 	return size;
 }
@@ -152,23 +173,29 @@ CreateLWLocks(void)
 {
 	int			numLocks = NumLWLocks();
 	Size		spaceLocks = LWLockShmemSize();
-	LWLock	   *lock;
+	LWLockPadded *lock;
+	char	   *ptr;
 	int			id;
 
 	/* Allocate space */
-	LWLockArray = (LWLock *) ShmemAlloc(spaceLocks);
+	ptr = (char *) ShmemAlloc(spaceLocks);
+
+	/* Ensure desired alignment of LWLock array */
+	ptr += LWLOCK_PADDED_SIZE - ((unsigned long) ptr) % LWLOCK_PADDED_SIZE;
+
+	LWLockArray = (LWLockPadded *) ptr;
 
 	/*
 	 * Initialize all LWLocks to "unlocked" state
 	 */
 	for (id = 0, lock = LWLockArray; id < numLocks; id++, lock++)
 	{
-		SpinLockInit(&lock->mutex);
-		lock->releaseOK = true;
-		lock->exclusive = 0;
-		lock->shared = 0;
-		lock->head = NULL;
-		lock->tail = NULL;
+		SpinLockInit(&lock->lock.mutex);
+		lock->lock.releaseOK = true;
+		lock->lock.exclusive = 0;
+		lock->lock.shared = 0;
+		lock->lock.head = NULL;
+		lock->lock.tail = NULL;
 	}
 
 	/*
@@ -206,7 +233,7 @@ LWLockAssign(void)
 void
 LWLockAcquire(LWLockId lockid, LWLockMode mode)
 {
-	volatile LWLock *lock = LWLockArray + lockid;
+	volatile LWLock *lock = &(LWLockArray[lockid].lock);
 	PGPROC	   *proc = MyProc;
 	bool		retry = false;
 	int			extraWaits = 0;
@@ -358,7 +385,7 @@ LWLockAcquire(LWLockId lockid, LWLockMode mode)
 bool
 LWLockConditionalAcquire(LWLockId lockid, LWLockMode mode)
 {
-	volatile LWLock *lock = LWLockArray + lockid;
+	volatile LWLock *lock = &(LWLockArray[lockid].lock);
 	bool		mustwait;
 
 	PRINT_LWDEBUG("LWLockConditionalAcquire", lockid, lock);
@@ -423,7 +450,7 @@ LWLockConditionalAcquire(LWLockId lockid, LWLockMode mode)
 void
 LWLockRelease(LWLockId lockid)
 {
-	volatile LWLock *lock = LWLockArray + lockid;
+	volatile LWLock *lock = &(LWLockArray[lockid].lock);
 	PGPROC	   *head;
 	PGPROC	   *proc;
 	int			i;