Use a non-locking initial test in TAS_SPIN on PPC.

Further testing convinces me that this is helpful at sufficiently high
contention levels, though it's still worrisome that it loses slightly
at lower contention levels.

Per Manabu Ori.

diff --git a/src/include/storage/s_lock.h b/src/include/storage/s_lock.h
index ff7eb1418e63b19b3396485b4725e1fc06ca8441..9b02d1f5a409c7309d4ba71aa4301b6d31d194d3 100644 (file)
--- a/src/include/storage/s_lock.h
+++ b/src/include/storage/s_lock.h
@@ -358,6 +358,9 @@ typedef unsigned int slock_t;
 
 #define TAS(lock) tas(lock)
 
+/* On PPC, it's a win to use a non-locking test before the lwarx */
+#define TAS_SPIN(lock) (*(lock) ? 1 : TAS(lock))
+
 /*
  * NOTE: per the Enhanced PowerPC Architecture manual, v1.0 dated 7-May-2002,
  * an isync is a sufficient synchronization barrier after a lwarx/stwcx loop.