Measuring topology performance. Fixing a defect that caused us to run w/o accumulator when config file was not present.

Change-Id: I5ad538b8a441cd6ff2aefea49a0def10b8e0f4d5
diff --git a/core/api/src/main/java/org/onlab/onos/net/topology/Topology.java b/core/api/src/main/java/org/onlab/onos/net/topology/Topology.java
index 70e392f..182db10 100644
--- a/core/api/src/main/java/org/onlab/onos/net/topology/Topology.java
+++ b/core/api/src/main/java/org/onlab/onos/net/topology/Topology.java
@@ -22,17 +22,23 @@
  */
 public interface Topology extends Provided {
 
-    // FIXME: Following is not true right now. It is actually System.nanoTime(),
-    // which has no relation to epoch time, wall clock, etc.
     /**
-     * Returns the time, specified in milliseconds since start of epoch,
-     * when the topology became active and made available.
+     * Returns the time, specified in system nanos of when the topology
+     * became available.
      *
-     * @return time in milliseconds since start of epoch
+     * @return time in system nanos
      */
     long time();
 
     /**
+     * Returns the time, specified in system nanos of how long the topology
+     * took to compute.
+     *
+     * @return elapsed time in system nanos
+     */
+    long computeCost();
+
+    /**
      * Returns the number of SCCs (strongly connected components) in the
      * topology.
      *