diff --git a/waflib/Runner.py b/waflib/Runner.py
index 3f183eda..12cbb931 100644
--- a/waflib/Runner.py
+++ b/waflib/Runner.py
@@ -105,8 +105,11 @@ class Parallel(object):
 		self.outstanding = Utils.deque()
 		"""List of :py:class:`waflib.Task.Task` that may be ready to be executed"""
 
-		self.frozen = Utils.deque()
-		"""List of :py:class:`waflib.Task.Task` that are not ready yet"""
+		self.frozen = set()
+		"""Set of :py:class:`waflib.Task.Task` that need other tasks to complete first"""
+
+		self.incomplete = Utils.deque()
+		"""List of :py:class:`waflib.Task.Task` with incomplete dependencies"""
 
 		self.ready = Queue(0)
 		"""List of :py:class:`waflib.Task.Task` ready to be executed by consumers"""
@@ -134,6 +137,11 @@ class Parallel(object):
 		Flag that indicates that the build cache must be saved when a task was executed
 		(calls :py:meth:`waflib.Build.BuildContext.store`)"""
 
+		self.revdeps = Utils.defaultdict(list)
+		"""
+		The reverse dependency graph of dependencies obtained from Task.run_after
+		"""
+
 		self.spawner = Spawner(self)
 		"""
 		Coordinating daemon thread that spawns thread consumers
@@ -151,16 +159,16 @@ class Parallel(object):
 
 	def postpone(self, tsk):
 		"""
-		Adds the task to the list :py:attr:`waflib.Runner.Parallel.frozen`.
+		Adds the task to the list :py:attr:`waflib.Runner.Parallel.incomplete`.
 		The order is scrambled so as to consume as many tasks in parallel as possible.
 
 		:param tsk: task instance
 		:type tsk: :py:class:`waflib.Task.Task`
 		"""
 		if random.randint(0, 1):
-			self.frozen.appendleft(tsk)
+			self.incomplete.appendleft(tsk)
 		else:
-			self.frozen.append(tsk)
+			self.incomplete.append(tsk)
 
 	def refill_task_list(self):
 		"""
@@ -172,7 +180,7 @@ class Parallel(object):
 		while not self.outstanding:
 			if self.count:
 				self.get_out()
-			elif self.frozen:
+			elif self.incomplete:
 				try:
 					cond = self.deadlock == self.processed
 				except AttributeError:
@@ -180,36 +188,71 @@ class Parallel(object):
 				else:
 					if cond:
 						msg = 'check the build order for the tasks'
-						for tsk in self.frozen:
+						for tsk in self.incomplete:
 							if not tsk.run_after:
 								msg = 'check the methods runnable_status'
 								break
 						lst = []
-						for tsk in self.frozen:
+						for tsk in self.incomplete:
 							lst.append('%s\t-> %r' % (repr(tsk), [id(x) for x in tsk.run_after]))
 						raise Errors.WafError('Deadlock detected: %s%s' % (msg, ''.join(lst)))
 				self.deadlock = self.processed
 
-			if self.frozen:
-				self.outstanding.extend(self.frozen)
-				self.frozen.clear()
+			if self.incomplete:
+				self.outstanding.extend(self.incomplete)
+				self.incomplete.clear()
 			elif not self.count:
-				self.outstanding.extend(next(self.biter))
+				tasks = next(self.biter)
+				ready, waiting = self.prio_and_split(tasks)
+				# We cannot use a priority queue because the implementation
+				# must be able to handle incomplete dependencies
+				self.outstanding.extend(ready)
+				self.frozen.update(waiting)
 				self.total = self.bld.total()
 				break
 
+	def insert_with_prio(self, tsk):
+		# TODO the deque interface has insert in python 3.5 :-/
+		if self.outstanding and tsk.prio >= self.outstanding[0].prio:
+			self.outstanding.appendleft(tsk)
+		else:
+			self.outstanding.append(tsk)
+
 	def add_more_tasks(self, tsk):
 		"""
 		If a task provides :py:attr:`waflib.Task.Task.more_tasks`, then the tasks contained
 		in that list are added to the current build and will be processed before the next build group.
 
+		Assume that the task is done, so that task priorities do not need
+		to be re-calculated
+
 		:param tsk: task instance
 		:type tsk: :py:attr:`waflib.Task.Task`
 		"""
 		if getattr(tsk, 'more_tasks', None):
-			self.outstanding.extend(tsk.more_tasks)
+			ready, waiting = self.prio_and_split(tsk.tasks)
+			for k in ready:
+				# TODO could be better, but we will have 1 task in general?
+				self.insert_with_prio(k)
+			self.frozen.update(waiting)
 			self.total += len(tsk.more_tasks)
 
+	def mark_finished(self, tsk):
+		# we assume that frozen tasks will be consumed as the build goes
+		if tsk in self.revdeps:
+			for x in self.revdeps[tsk]:
+				# ancestors are likely to be frozen
+				if x in self.frozen:
+					# TODO remove dependencies to free some memory?
+					# x.run_after.remove(tsk)
+					for k in x.run_after:
+						if not k.hasrun:
+							break
+					else:
+						self.frozen.remove(x)
+						self.insert_with_prio(x)
+			del self.revdeps[tsk]
+
 	def get_out(self):
 		"""
 		Waits for a Task that task consumers add to :py:attr:`waflib.Runner.Parallel.out` after execution.
@@ -220,6 +263,8 @@ class Parallel(object):
 		tsk = self.out.get()
 		if not self.stop:
 			self.add_more_tasks(tsk)
+		self.mark_finished(tsk)
+
 		self.count -= 1
 		self.dirty = True
 		return tsk
@@ -238,12 +283,14 @@ class Parallel(object):
 		Mark a task as skipped/up-to-date
 		"""
 		tsk.hasrun = Task.SKIPPED
+		self.mark_finished(tsk)
 
 	def cancel(self, tsk):
 		"""
 		Mark a task as failed because of unsatisfiable dependencies
 		"""
 		tsk.hasrun = Task.CANCELED
+		self.mark_finished(tsk)
 
 	def error_handler(self, tsk):
 		"""
@@ -360,3 +407,85 @@ class Parallel(object):
 		self.ready.put(None)
 		assert (self.count == 0 or self.stop)
 
+	def prio_and_split(self, tasks):
+		"""
+		Label input tasks with priority values, and return a pair containing
+		the tasks that are ready to run and the tasks that are necessarily
+		waiting for other tasks to complete.
+
+		The priority system is really meant as an optional layer for optimization:
+		dependency cycles are found more quickly, and build should be more efficient
+
+		:return: A pair of task lists
+		:rtype: tuple
+		"""
+		# to disable:
+		#return tasks, []
+		for x in tasks:
+			x.visited = 0
+
+		reverse = self.revdeps
+
+		for x in tasks:
+			for k in x.run_after:
+				reverse[k].append(x)
+
+		# the priority number is not the tree size
+		def visit(n):
+			if n.visited == 0:
+				n.visited = 1
+				if n in reverse:
+					rev = reverse[n]
+					n.prio = n.priority() + len(rev) + sum(visit(k) for k in rev)
+				else:
+					n.prio = n.priority()
+				n.visited = 2
+			elif n.visited == 1:
+				raise Errors.WafError('Dependency cycle found!')
+			return n.prio
+
+		for x in tasks:
+			if x.visited != 0:
+				# must visit all to detect cycles
+				continue
+			try:
+				visit(x)
+			except Errors.WafError:
+				self.debug_cycles(tasks, reverse)
+
+		ready = []
+		waiting = []
+		for x in tasks:
+			for k in x.run_after:
+				if not k.hasrun:
+					waiting.append(x)
+					break
+			else:
+				ready.append(x)
+
+		ready.sort(key=lambda x: x.prio, reverse=True)
+		return (ready, waiting)
+
+	def debug_cycles(self, tasks, reverse):
+		# TODO display more than one cycle?
+		tmp = {}
+		for x in tasks:
+			tmp[x] = 0
+
+		def visit(n, acc):
+			if tmp[n] == 0:
+				tmp[n] = 1
+				for k in reverse.get(n, []):
+					visit(k, [n] + acc)
+				tmp[n] = 2
+			elif tmp[n] == 1:
+				lst = []
+				for tsk in acc:
+					lst.append(repr(tsk))
+					if tsk is n:
+						# exclude prior nodes, we want the minimum cycle
+						break
+				raise Errors.WafError('Task dependency cycle in "run_after" constraints: %s' % ''.join(lst))
+		for x in tasks:
+			visit(x, [])
+
diff --git a/waflib/Task.py b/waflib/Task.py
index e42e09b9..9df9e0fd 100644
--- a/waflib/Task.py
+++ b/waflib/Task.py
@@ -209,6 +209,15 @@ class Task(evil):
 			x = '"%s"' % x
 		return x
 
+	def priority(self):
+		"""
+		The default priority for this task instance
+
+		:return: a positive numeric value representing the urgency of running this task early
+		:rtype: int
+		"""
+		return getattr(self, 'weight', 0)
+
 	def split_argfile(self, cmd):
 		"""
 		Splits a list of process commands into the executable part and its list of arguments