Improve spinlock code for recent x86 processors: insert a PAUSE

instruction in the s_lock() wait loop, and use test before test-and-set
in TAS() macro to avoid unnecessary bus traffic.  Patch from Manfred
Spraul, reworked a bit by Tom.

diff --git a/src/backend/storage/lmgr/s_lock.c b/src/backend/storage/lmgr/s_lock.c
index e5e372b..fac77ae 100644 (file)
--- a/src/backend/storage/lmgr/s_lock.c
+++ b/src/backend/storage/lmgr/s_lock.c
@@ -9,7 +9,7 @@
  *
  *
  * IDENTIFICATION
- *       $PostgreSQL: pgsql/src/backend/storage/lmgr/s_lock.c,v 1.22 2003/12/23 22:15:07 tgl Exp $
+ *       $PostgreSQL: pgsql/src/backend/storage/lmgr/s_lock.c,v 1.23 2003/12/27 20:58:58 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -88,6 +88,10 @@ s_lock(volatile slock_t *lock, const char *file, int line)
 
        while (TAS(lock))
        {
+               /* CPU-specific delay each time through the loop */
+               SPIN_DELAY();
+
+               /* Block the process every SPINS_PER_DELAY tries */
                if (++spins > SPINS_PER_DELAY)
                {
                        if (++delays > NUM_DELAYS)

diff --git a/src/include/storage/s_lock.h b/src/include/storage/s_lock.h
index e1851fb..9ab24f9 100644 (file)
--- a/src/include/storage/s_lock.h
+++ b/src/include/storage/s_lock.h
@@ -24,6 +24,9 @@
  *             Tests if the lock is free. Returns TRUE if free, FALSE if locked.
  *             This does *not* change the state of the lock.
  *
+ *     void SPIN_DELAY(void)
+ *             Delay operation to occur inside spinlock wait loop.
+ *
  *     Note to implementors: there are default implementations for all these
  *     macros at the bottom of the file.  Check if your platform can use
  *     these or needs to override them.
@@ -63,7 +66,7 @@
  * Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- *       $PostgreSQL: pgsql/src/include/storage/s_lock.h,v 1.123 2003/12/23 22:15:07 tgl Exp $
+ *       $PostgreSQL: pgsql/src/include/storage/s_lock.h,v 1.124 2003/12/27 20:58:58 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -107,14 +110,28 @@ tas(volatile slock_t *lock)
 {
        register slock_t _res = 1;
 
+       /* Use a non-locking test before asserting the bus lock */
        __asm__ __volatile__(
+               "       cmpb    $0,%1   \n"
+               "       jne             1f              \n"
                "       lock                    \n"
                "       xchgb   %0,%1   \n"
+               "1: \n"
 :              "=q"(_res), "=m"(*lock)
 :              "0"(_res));
        return (int) _res;
 }
 
+#define SPIN_DELAY() spin_delay()
+
+static __inline__ void
+spin_delay(void)
+{
+       __asm__ __volatile__(
+               " rep; nop                      \n"
+               : : : "memory");
+}
+
 #endif  /* __i386__ || __x86_64__ */
 
 
@@ -708,6 +725,10 @@ extern int tas_sema(volatile slock_t *lock);
 #define S_INIT_LOCK(lock)      S_UNLOCK(lock)
 #endif  /* S_INIT_LOCK */
 
+#if !defined(SPIN_DELAY)
+#define SPIN_DELAY()   ((void) 0)
+#endif  /* SPIN_DELAY */
+
 #if !defined(TAS)
 extern int     tas(volatile slock_t *lock);            /* in port/.../tas.s, or
                                                                                                 * s_lock.c */