Add a new "non-portable" mutex type, PTHREAD_MUTEX_ADAPTIVE_NP. This

is also implemented in glibc and is used by a number of existing
applications (mysql, firefox, etc).

This mutex type is a default mutex with the additional property that
it spins briefly when attempting to acquire a contested lock, doing
trylock operations in userland before entering the kernel to block if
eventually unsuccessful.

The expectation is that applications requesting this mutex type know
that the mutex is likely to be only held for very brief periods, so it
is faster to spin in userland and probably succeed in acquiring the
mutex, than to enter the kernel and sleep, only to be woken up almost
immediately.  This can help significantly in certain cases when
pthread mutexes are heavily contended and held for brief durations
(such as mysql).

Spin up to 200 times before entering the kernel, which represents only
a few us on modern CPUs.  No performance degradation was observed with
this value and it is sufficient to avoid a large performance drop in
mysql performance in the heavily contended pthread mutex case.

The libkse implementation is a NOP.

Reviewed by:      jeff
MFC after:        3 days

1 files changed, 2 insertions, 0 deletions

diff --git a/include/pthread.h b/include/pthread.h
index ee864fd..361b45c 100644
--- a/include/pthread.h
+++ b/include/pthread.h
@@ -98,6 +98,7 @@
  * Static initialization values. 
  */
 #define PTHREAD_MUTEX_INITIALIZER	NULL
+#define PTHREAD_ADAPTIVE_MUTEX_INITIALIZER_NP	NULL
 #define PTHREAD_COND_INITIALIZER	NULL
 #define PTHREAD_RWLOCK_INITIALIZER	NULL
 
@@ -128,6 +129,7 @@ enum pthread_mutextype {
 	PTHREAD_MUTEX_ERRORCHECK	= 1,	/* Default POSIX mutex */
 	PTHREAD_MUTEX_RECURSIVE		= 2,	/* Recursive mutex */
 	PTHREAD_MUTEX_NORMAL		= 3,	/* No error checking */
+	PTHREAD_MUTEX_ADAPTIVE_NP	= 4,	/* Adaptive mutex, spins briefly before blocking on lock */
 	PTHREAD_MUTEX_TYPE_MAX
 };