algorithm improvement

Author: Sun Yu

timeout.c

@@ -305,24 +305,18 @@ TIMEOUT_PUBLIC void timeouts_del(struct timeouts *T, struct timeout *to) {
 } /* timeouts_del() */
 
 
-static inline reltime_t timeout_rem(struct timeouts *T, struct timeout *to) {
-	return to->expires - T->curtime;
-} /* timeout_rem() */
-
-
-static inline int timeout_wheel(timeout_t timeout) {
-	/* must be called with timeout != 0, so fls input is nonzero */
-	return (fls(MIN(timeout, TIMEOUT_MAX)) - 1) / WHEEL_BIT;
+static inline int timeout_wheel(timeout_t curtime, timeout_t expires) {
+	/* must be called with expires > curtime, so fls input is nonzero */
+	return (fls(MIN(curtime ^ expires, TIMEOUT_MAX)) - 1) / WHEEL_BIT;
 } /* timeout_wheel() */
 
 
 static inline int timeout_slot(int wheel, timeout_t expires) {
-	return WHEEL_MASK & ((expires >> (wheel * WHEEL_BIT)) - !!wheel);
+	return WHEEL_MASK & ((expires >> (wheel * WHEEL_BIT)));
 } /* timeout_slot() */
 
 
 static void timeouts_sched(struct timeouts *T, struct timeout *to, timeout_t expires) {
-	timeout_t rem;
 	int wheel, slot;
 
 	timeouts_del(T, to);
@@ -332,14 +326,7 @@ static void timeouts_sched(struct timeouts *T, struct timeout *to, timeout_t exp
 	TO_SET_TIMEOUTS(to, T);
 
 	if (expires > T->curtime) {
-		rem = timeout_rem(T, to);
-
-		/* rem is nonzero since:
-		 *   rem == timeout_rem(T,to),
-		 *       == to->expires - T->curtime
-		 *   and above we have expires > T->curtime.
-		 */
-		wheel = timeout_wheel(rem);
+		wheel = timeout_wheel(T->curtime, expires);
 		slot = timeout_slot(wheel, to->expires);
 
 		to->pending = &T->wheel[wheel][slot];
@@ -402,34 +389,26 @@ TIMEOUT_PUBLIC void timeouts_update(struct timeouts *T, abstime_t curtime) {
 		/*
 		 * Calculate the slots expiring in this wheel
 		 *
-		 * If the elapsed time is greater than the maximum period of
+		 * Assume that current slot is empty, it's timer either expired or moved to the lower wheel.
+		 * If the elapsed time is greater or equal to the maximum period of
 		 * the wheel, mark every position as expiring.
 		 *
 		 * Otherwise, to determine the expired slots fill in all the
 		 * bits between the last slot processed and the current
-		 * slot, inclusive of the last slot. We'll bitwise-AND this
+		 * slot(exclusive, we don't need to check it again), inclusive of the last slot. We'll bitwise-AND this
 		 * with our pending set below.
-		 *
-		 * If a wheel rolls over, force a tick of the next higher
-		 * wheel.
 		 */
-		if ((elapsed >> (wheel * WHEEL_BIT)) > WHEEL_MAX) {
+		if ((elapsed >> (wheel * WHEEL_BIT)) >= WHEEL_MAX) {
 			pending = (wheel_t)~WHEEL_C(0);
 		} else {
-			wheel_t _elapsed = WHEEL_MASK & (elapsed >> (wheel * WHEEL_BIT));
+			wheel_t _elapsed;
 			int oslot, nslot;
 
-			/*
-			 * TODO: It's likely that at least one of the
-			 * following three bit fill operations is redundant
-			 * or can be replaced with a simpler operation.
-			 */
 			oslot = WHEEL_MASK & (T->curtime >> (wheel * WHEEL_BIT));
-			pending = rotl(((UINT64_C(1) << _elapsed) - 1), oslot);
-
 			nslot = WHEEL_MASK & (curtime >> (wheel * WHEEL_BIT));
-			pending |= rotr(rotl(((WHEEL_C(1) << _elapsed) - 1), nslot), _elapsed);
-			pending |= WHEEL_C(1) << nslot;
+			_elapsed = WHEEL_MASK & (WHEEL_LEN + nslot - oslot);
+
+			pending = rotr(rotl(((WHEEL_C(1) << _elapsed) - 1), nslot + 1), _elapsed);
 		}
 
 		while (pending & T->pending[wheel]) {
@@ -441,9 +420,6 @@ TIMEOUT_PUBLIC void timeouts_update(struct timeouts *T, abstime_t curtime) {
 
 		if (!(0x1 & pending))
 			break; /* break if we didn't wrap around end of wheel */
-
-		/* if we're continuing, the next wheel must tick at least once */
-		elapsed = MAX(elapsed, (WHEEL_LEN << (wheel * WHEEL_BIT)));
 	}
 
 	T->curtime = curtime;
@@ -512,8 +488,8 @@ static timeout_t timeouts_int(struct timeouts *T) {
 
 			/* ctz input cannot be zero: T->pending[wheel] is
 			 * nonzero, so rotr() is nonzero. */
-			_timeout = (ctz(rotr(T->pending[wheel], slot)) + !!wheel) << (wheel * WHEEL_BIT);
-			/* +1 to higher order wheels as those timeouts are one rotation in the future (otherwise they'd be on a lower wheel or expired) */
+			_timeout = (ctz(rotr(T->pending[wheel], slot)) + !!(T->pending[wheel] & (TIMEOUT_C(1) << slot))) << (wheel * WHEEL_BIT);
+			/* +1 when a timeout greater than TIMEOUT_MAX and wrap in to current slot of the highest wheel */
 
 			_timeout -= relmask & T->curtime;
 			/* reduce by how much lower wheels have progressed */