WIP: use priority queue in schedular

diff --git a/buildstream/_scheduler/jobs/cachesizejob.py b/buildstream/_scheduler/jobs/cachesizejob.py
index d46fd4c..17ccb0a 100644
--- a/buildstream/_scheduler/jobs/cachesizejob.py
+++ b/buildstream/_scheduler/jobs/cachesizejob.py

@@ -39,3 +39,6 @@
 
     def child_process_data(self):
         return {}
+
+    def key(self):
+        return (100, 'cache-size')

diff --git a/buildstream/_scheduler/jobs/cleanupjob.py b/buildstream/_scheduler/jobs/cleanupjob.py
index 8bdbba0..3e860e2 100644
--- a/buildstream/_scheduler/jobs/cleanupjob.py
+++ b/buildstream/_scheduler/jobs/cleanupjob.py

@@ -32,3 +32,6 @@
     def parent_complete(self, success, result):
         if success:
             self._artifacts.set_cache_size(result)
+
+    def key(self):
+        return (0, 'cleanup')

diff --git a/buildstream/_scheduler/jobs/elementjob.py b/buildstream/_scheduler/jobs/elementjob.py
index 8ce5c06..1d6305c 100644
--- a/buildstream/_scheduler/jobs/elementjob.py
+++ b/buildstream/_scheduler/jobs/elementjob.py

@@ -22,6 +22,13 @@
 
 from .job import Job
 
+_ACTIONS = {
+    "Build": 10,
+    "Fetch": 20,
+    "Pull": 30,
+    "Push": 40,
+    "Track": 50,
+}
 
 # ElementJob()
 #
@@ -113,3 +120,6 @@
             data['workspace'] = workspace.to_dict()
 
         return data
+
+    def key(self):
+        return (_ACTIONS.get(self.action_name, 100), self._element.name)

diff --git a/buildstream/_scheduler/jobs/job.py b/buildstream/_scheduler/jobs/job.py
index 60ae0d0..ff0ea83 100644
--- a/buildstream/_scheduler/jobs/job.py
+++ b/buildstream/_scheduler/jobs/job.py

@@ -353,6 +353,10 @@
     def child_process_data(self):
         return {}
 
+    def key(self):
+        raise ImplError("Job '{kind}' does not implement key()"
+                        .format(kind=type(self).__name__))
+
     #######################################################
     #                  Local Private Methods              #
     #######################################################

diff --git a/buildstream/_scheduler/scheduler.py b/buildstream/_scheduler/scheduler.py
index b76c730..6b849f3 100644
--- a/buildstream/_scheduler/scheduler.py
+++ b/buildstream/_scheduler/scheduler.py

@@ -25,6 +25,7 @@
 import signal
 import datetime
 from contextlib import contextmanager
+from sortedcontainers import SortedList
 
 # Local imports
 from .resources import Resources, ResourceType
@@ -72,7 +73,7 @@
         # Public members
         #
         self.active_jobs = []       # Jobs currently being run in the scheduler
-        self.waiting_jobs = []      # Jobs waiting for resources
+        self.waiting_jobs = SortedList([], key=lambda job: job.key())  # Jobs waiting for resources
         self.queues = None          # Exposed for the frontend to print summaries
         self.context = context      # The Context object shared with Queues
         self.terminated = False     # Whether the scheduler was asked to terminate or has terminated
@@ -222,7 +223,7 @@
     #
     def schedule_jobs(self, jobs):
         for job in jobs:
-            self.waiting_jobs.append(job)
+            self.waiting_jobs.add(job)
 
     # job_completed():
     #

diff --git a/setup.py b/setup.py
index 76610f0..95d9204 100755
--- a/setup.py
+++ b/setup.py

@@ -343,6 +343,7 @@
           'jinja2 >= 2.10',
           'protobuf >= 3.5',
           'grpcio >= 1.10',
+          'sortedcontainers >= 1.5.7',
       ],
       entry_points=bst_install_entry_points,
       tests_require=dev_requires,