diff --git a/gluon/scheduler.py b/gluon/scheduler.py
index 254c442f..91ae2e92 100644
--- a/gluon/scheduler.py
+++ b/gluon/scheduler.py
@@ -87,11 +87,17 @@ if 'WEB2PY_PATH' not in os.environ:
     os.environ['WEB2PY_PATH'] = path
 
 try:
-    from gluon.contrib.simplejson import loads, dumps
-except:
+    # try external module
     from simplejson import loads, dumps
+except ImportError:
+    try:
+        # try stdlib (Python >= 2.6)
+        from json import loads, dumps
+    except:
+        # fallback to pure-Python module
+        from gluon.contrib.simplejson import loads, dumps
 
-IDENTIFIER = "%s#%s" % (socket.gethostname(),os.getpid())
+IDENTIFIER = "%s#%s" % (socket.gethostname(), os.getpid())
 
 logger = logging.getLogger('web2py.scheduler.%s' % IDENTIFIER)
 
@@ -160,6 +166,7 @@ class TaskReport(object):
     def __str__(self):
         return '<TaskReport: %s>' % self.status
 
+
 class JobGraph(object):
     """Experimental: with JobGraph you can specify
     dependencies amongs tasks"""
@@ -170,7 +177,9 @@ class JobGraph(object):
 
     def add_deps(self, task_parent, task_child):
         """Creates a dependency between task_parent and task_child"""
-        self.db.scheduler_task_deps.insert(task_parent=task_parent, task_child=task_child, job_name=self.job_name)
+        self.db.scheduler_task_deps.insert(task_parent=task_parent,
+                                           task_child=task_child,
+                                           job_name=self.job_name)
 
     def validate(self, job_name):
         """Validates if all tasks job_name can be completed, i.e. there
@@ -195,16 +204,18 @@ class JobGraph(object):
         try:
             rtn = []
             for k, v in nested_dict.items():
-                v.discard(k) # Ignore self dependencies
+                v.discard(k)  # Ignore self dependencies
             extra_items_in_deps = reduce(set.union, nested_dict.values()) - set(nested_dict.keys())
             nested_dict.update(dict((item, set()) for item in extra_items_in_deps))
             while True:
-                ordered = set(item for item,dep in nested_dict.items() if not dep)
+                ordered = set(item for item, dep in nested_dict.items() if not dep)
                 if not ordered:
                     break
                 rtn.append(ordered)
-                nested_dict = dict((item, (dep - ordered)) for item, dep in nested_dict.items()
-                    if item not in ordered)
+                nested_dict = dict(
+                    (item, (dep - ordered)) for item, dep in nested_dict.items()
+                    if item not in ordered
+                    )
             assert not nested_dict, "A cyclic dependency exists amongst %r" % nested_dict
             db.commit()
             return rtn
@@ -212,6 +223,7 @@ class JobGraph(object):
             db.rollback()
             return None
 
+
 def demo_function(*argv, **kwargs):
     """ test function """
     for i in range(argv[0]):
@@ -268,7 +280,7 @@ def executor(queue, task, out):
         def write(self, data):
             self.out_queue.put(data)
 
-    W2P_TASK = Storage({'id' : task.task_id, 'uuid' : task.uuid})
+    W2P_TASK = Storage({'id': task.task_id, 'uuid': task.uuid})
     stdout = LogOutput(out)
     try:
         if task.app:
@@ -293,7 +305,7 @@ def executor(queue, task, out):
                 raise NameError(
                     "name '%s' not found in scheduler's environment" % f)
             #Inject W2P_TASK into environment
-            _env.update({'W2P_TASK' : W2P_TASK})
+            _env.update({'W2P_TASK': W2P_TASK})
             #Inject W2P_TASK into current
             from gluon import current
             current.W2P_TASK = W2P_TASK
@@ -357,8 +369,7 @@ class MetaScheduler(threading.Thread):
 
             start = time.time()
 
-            while p.is_alive() and (
-                    not task.timeout or time.time() - start < task.timeout):
+            while p.is_alive() and (not task.timeout or time.time() - start < task.timeout):
                 if tout:
                     try:
                         logger.debug(' partial output saved')
@@ -568,7 +579,7 @@ class Scheduler(MetaScheduler):
                 queue=0,
                 distribution=None,
                 workers=0)
-        ) #dict holding statistics
+        )  # dict holding statistics
 
         from gluon import current
         current._scheduler = self
@@ -740,7 +751,7 @@ class Scheduler(MetaScheduler):
         contention and retries `assign_task` after 0.5 seconds
         """
         logger.debug('Assigning tasks...')
-        db.commit()  #db.commit() only for Mysql
+        db.commit()  # db.commit() only for Mysql
         x = 0
         while x < 10:
             try:
@@ -761,7 +772,7 @@ class Scheduler(MetaScheduler):
         contention and retries `pop_task` after 0.5 seconds
         """
         db = self.db
-        db.commit() #another nifty db.commit() only for Mysql
+        db.commit()  # another nifty db.commit() only for Mysql
         x = 0
         while x < 10:
             try:
@@ -1077,6 +1088,8 @@ class Scheduler(MetaScheduler):
         #build workers as dict of groups
         wkgroups = {}
         for w in all_workers:
+            if w.worker_stats['status'] == 'RUNNING':
+                continue
             group_names = w.group_names
             for gname in group_names:
                 if gname not in wkgroups:
@@ -1090,8 +1103,9 @@ class Scheduler(MetaScheduler):
         db(
             (st.status.belongs((QUEUED, ASSIGNED))) &
             (st.stop_time < now)
-            ).update(status=EXPIRED)
+        ).update(status=EXPIRED)
 
+        #calculate dependencies
         deps_with_no_deps = db(
             (sd.can_visit == False) &
             (~sd.task_child.belongs(
@@ -1163,7 +1177,7 @@ class Scheduler(MetaScheduler):
                     if not task.task_name:
                         d['task_name'] = task.function_name
                     db(
-                        (st.id==task.id) &
+                        (st.id == task.id) &
                         (st.status.belongs((QUEUED, ASSIGNED)))
                         ).update(**d)
                     wkgroups[gname]['workers'][myw]['c'] += 1
@@ -1207,8 +1221,8 @@ class Scheduler(MetaScheduler):
         else:
             for group in group_names:
                 workers = self.db(
-                   (ws.group_names.contains(group)) &
-                   (~ws.status.belongs(exclusion))
+                    (ws.group_names.contains(group)) &
+                    (~ws.status.belongs(exclusion))
                     )._select(ws.id, limitby=(0,limit))
                 self.db(ws.id.belongs(workers)).update(status=action)
 
@@ -1339,7 +1353,7 @@ class Scheduler(MetaScheduler):
             **dict(orderby=orderby,
                    left=left,
                    limitby=(0, 1))
-             ).first()
+            ).first()
         if row and output:
             row.result = row.scheduler_run.run_result and \
                 loads(row.scheduler_run.run_result,