Some changes in the kernel side of the semaphores.
Mwait is used to reduce the pressure of the spinlock
on the memory. Semwakeup changed to avoid retries.

Reference: /n/patches.lsub.org/patch/syssemmwait
Date: Wed Jul  4 12:19:34 CES 2012
Signed-off-by: esoriano@lsub.org

--- /sys/src/nix/port/syssem.c	Fri Jun  8 11:35:47 2012
+++ /sys/src/nix/port/syssem.c	Wed Jul  4 12:06:00 2012
@@ -6,7 +6,7 @@
 #include	"../port/error.h"
 
 /*
- * Sems are represented by two structures, Sem (user space) and
+ * Sems are characterized by two structures, Sem (user space) and
  * Ksem (kernel space). A Sem includes a spin lock (userlock) to
  * protect its three counters:
  *
@@ -22,6 +22,7 @@
 	Semtrytimes	= 100,
 	Locktrytimes	= 10,
 	Maxaltsems	= 300,
+	Lockmagic	= 0xdeaddead,	/* libc and kernel*/
 };
 
 /*
@@ -34,19 +35,19 @@
 	int try;
 	n = 0;
 
-
 	try = Locktrytimes;
-	while(tas32(&s->sem->userlock.val)){
+	while(TAS(&s->sem->userlock.val)){
 		if(s->state == Semdead)
 			error(Esemdead);
-		if(++n % 100000 == 0){
-			iprint("syssem: warning, userlock busy\n");
+		if(++n % 10000 == 0){
+			iprint("syssem: warning userlock busy\n");
 			if(--try == 0){
 				iprint("syssem: the userlock of ksem %p is dead\n", s);
 				s->state = Semdead;
 				error(Esemtimeout);
 			}
 		}
+		waitwhile(&s->sem->userlock.val, Lockmagic);
 	}
 	return;
 }
@@ -123,12 +124,8 @@
 semwakeup(Ksem *s)
 {
 	Proc* p;
-	int retries = 0;
 
 	DBG("semwakeup up %#p sem %#p\n", up, s->sem);
-retry:
-	if(retries != 0 &&  retries % 10 == 0)
-		print("semwakeup: %d retries\n", retries);
 
 	rlock(s->semaltlock);
 	lock(s);
@@ -154,7 +151,7 @@
 		_userlock(s);
 		s->sem->waiting--;
 		_userunlock(s);
-		if(tas32(&p->semawaken) == 0){
+		if(TAS(&p->semawaken) == 0){
 			poperror();
 			p->waitsem = s;
 			unlock(s);
@@ -173,15 +170,10 @@
 	if(s->sem->going > 0){
 		_userunlock(s);
 		poperror();
+		s->nowait++;
 		unlock(s);
 		runlock(s->semaltlock);
-		retries++;
-		/*
-		 * take a break?
-		 */
-		if(retries % 3 == 0)
-			sched();
-		goto retry;
+		return;
 	}
 	
 	/*
@@ -214,6 +206,12 @@
 	s->sem->going--;
 	_userunlock(s);
 	poperror();
+	if(s->nowait > 0){
+		s->nowait--;
+		unlock(s);
+		runlock(s->semaltlock);
+		return;
+	}
 	up->semawaken = 0;
 	up->waitsem = nil;
 	semqueue(s, up);
--- /sys/src/nix/port/portdat.h	Tue Jul  3 10:40:24 2012
+++ /sys/src/nix/port/portdat.h	Wed Jul  4 11:52:54 2012
@@ -474,6 +474,7 @@
 	int 	state;
 	Proc	**q;
 	int	nq;
+	int	nowait;
 	RWlock	*semaltlock;	/* located in the Segment */
 	Ksem	*next;		/* in list of semaphores for this Segment */
 };