Index: kern_time.c
===================================================================
--- kern_time.c	(revision 247813)
+++ kern_time.c	(working copy)
@@ -683,7 +681,7 @@ kern_getitimer(struct thread *td, u_int which, str
 		*aitv = p->p_realtimer;
 		PROC_UNLOCK(p);
 		if (timevalisset(&aitv->it_value)) {
-			getmicrouptime(&ctv);
+			microuptime(&ctv);
 			if (timevalcmp(&aitv->it_value, &ctv, <))
 				timevalclear(&aitv->it_value);
 			else
@@ -728,6 +726,7 @@ kern_setitimer(struct thread *td, u_int which, str
 {
 	struct proc *p = td->td_proc;
 	struct timeval ctv;
+	sbintime_t sbt, pr;
 
 	if (aitv == NULL)
 		return (kern_getitimer(td, which, oitv));
@@ -745,11 +744,13 @@ kern_setitimer(struct thread *td, u_int which, str
 		PROC_LOCK(p);
 		if (timevalisset(&p->p_realtimer.it_value))
 			callout_stop(&p->p_itcallout);
-		getmicrouptime(&ctv);
+		microuptime(&ctv);
 		if (timevalisset(&aitv->it_value)) {
-			callout_reset(&p->p_itcallout, tvtohz(&aitv->it_value),
-			    realitexpire, p);
+			pr = tvtosbt(aitv->it_value) >> tc_precexp;
 			timevaladd(&aitv->it_value, &ctv);
+			sbt = tvtosbt(aitv->it_value);
+			callout_reset_sbt(&p->p_itcallout, sbt, pr,
+			    realitexpire, p, C_ABSOLUTE);
 		}
 		*oitv = p->p_realtimer;
 		p->p_realtimer = *aitv;
@@ -785,7 +786,8 @@ void
 realitexpire(void *arg)
 {
 	struct proc *p;
-	struct timeval ctv, ntv;
+	struct timeval ctv;
+	sbintime_t isbt;
 
 	p = (struct proc *)arg;
 	kern_psignal(p, SIGALRM);
@@ -795,19 +797,17 @@ realitexpire(void *arg)
 			wakeup(&p->p_itcallout);
 		return;
 	}
-	for (;;) {
+	isbt = tvtosbt(p->p_realtimer.it_interval);
+	if (isbt >= sbt_timethreshold)
+		getmicrouptime(&ctv);
+	else
+		microuptime(&ctv);
+	do {
 		timevaladd(&p->p_realtimer.it_value,
 		    &p->p_realtimer.it_interval);
-		getmicrouptime(&ctv);
-		if (timevalcmp(&p->p_realtimer.it_value, &ctv, >)) {
-			ntv = p->p_realtimer.it_value;
-			timevalsub(&ntv, &ctv);
-			callout_reset(&p->p_itcallout, tvtohz(&ntv) - 1,
-			    realitexpire, p);
-			return;
-		}
-	}
-	/*NOTREACHED*/
+	} while (timevalcmp(&p->p_realtimer.it_value, &ctv, <=));
+	callout_reset_sbt(&p->p_itcallout, tvtosbt(p->p_realtimer.it_value),
+	    isbt >> tc_precexp, realitexpire, p, C_ABSOLUTE);
 }
 
 /*
@@ -822,8 +822,8 @@ itimerfix(struct timeval *tv)
 
 	if (tv->tv_sec < 0 || tv->tv_usec < 0 || tv->tv_usec >= 1000000)
 		return (EINVAL);
-	if (tv->tv_sec == 0 && tv->tv_usec != 0 && tv->tv_usec < tick)
-		tv->tv_usec = tick;
+	if (tv->tv_sec == 0 && tv->tv_usec != 0 && tv->tv_usec < tick / 16)
+		tv->tv_usec = tick / 16;
 	return (0);
 }