autoscaler: count head node, don't kill below target (fixes #2317) (#2320)

Specifically, subtracts 1 from the target number of workers, taking into
account that the head node has some computational resources.

Do not kill an idle node if it would drop us below the target number of
nodes (in which case we just immediately relaunch).

python/ray/autoscaler/autoscaler.py

@@ -359,6 +359,7 @@ class StandardAutoscaler(object):
         print(self.debug_string(nodes))
         self.load_metrics.prune_active_ips(
             [self.provider.internal_ip(node_id) for node_id in nodes])
+        target_workers = self.target_num_workers()
 
         # Terminate any idle or out of date nodes
         last_used = self.load_metrics.last_used_time_by_ip
@@ -367,7 +368,7 @@ class StandardAutoscaler(object):
         for node_id in nodes:
             node_ip = self.provider.internal_ip(node_id)
             if node_ip in last_used and last_used[node_ip] < horizon and \
-                    len(nodes) - num_terminated > self.config["min_workers"]:
+                    len(nodes) - num_terminated > target_workers:
                 num_terminated += 1
                 print("StandardAutoscaler: Terminating idle node: "
                       "{}".format(node_id))
@@ -394,12 +395,12 @@ class StandardAutoscaler(object):
             print(self.debug_string(nodes))
 
         # Launch new nodes if needed
-        target_num = self.target_num_workers()
-        num_nodes = len(nodes) + num_pending
-        if num_nodes < target_num:
+        num_workers = len(nodes) + num_pending
+        if num_workers < target_workers:
             max_allowed = min(self.max_launch_batch,
                               self.max_concurrent_launches - num_pending)
-            self.launch_new_node(min(max_allowed, target_num - num_nodes))
+            num_launches = min(max_allowed, target_workers - num_workers)
+            self.launch_new_node(num_launches)
             print(self.debug_string())
 
         # Process any completed updates
@@ -453,7 +454,8 @@ class StandardAutoscaler(object):
     def target_num_workers(self):
         target_frac = self.config["target_utilization_fraction"]
         cur_used = self.load_metrics.approx_workers_used()
-        ideal_num_workers = int(np.ceil(cur_used / float(target_frac)))
+        ideal_num_nodes = int(np.ceil(cur_used / float(target_frac)))
+        ideal_num_workers = ideal_num_nodes - 1  # subtract 1 for head node
         return min(self.config["max_workers"],
                    max(self.config["min_workers"], ideal_num_workers))