[core] Add the Utils.getGCTotalTime() method for showing how long

the JVM spent in GC. It now prints the time and percent of time in
the final output.
Also fix cases where the garbage collector implementation returns
-1 if the count or time isn't implemented.

core/src/main/java/com/yahoo/ycsb/Client.java

@@ -76,6 +76,7 @@ class StatusThread extends Thread
   private double _maxLoadAvg;
   private double _minLoadAvg = Double.MAX_VALUE;
   private long lastGCCount = 0;
+  private long lastGCTime = 0;
 
   /**
    * Creates a new StatusThread without JVM stat tracking.
@@ -274,7 +275,10 @@ class StatusThread extends Thread
      
     final long gcs = Utils.getGCTotalCollectionCount();
     _measurements.measure("GCS", (int)(gcs - lastGCCount));
+    final long gcTime = Utils.getGCTotalTime();
+    _measurements.measure("GCS_TIME", (int)(gcTime - lastGCTime));
     lastGCCount = gcs;
+    lastGCTime = gcTime;
   }
   
   /** @return The maximum threads running during the test. */
@@ -693,6 +697,9 @@ public class Client
       exporter.write("OVERALL", "Throughput(ops/sec)", throughput);
       
       exporter.write("TOTAL_GCs", "Count", Utils.getGCTotalCollectionCount());
+      final long gcTime = Utils.getGCTotalTime();
+      exporter.write("TOTAL_GC_TIME", "Time(ms)", gcTime);
+      exporter.write("TOTAL_GC_TIME_%", "Time(%)", ((double)gcTime / runtime) * (double)100);
       if (statusthread != null && statusthread.trackJVMStats()) {
         exporter.write("MAX_MEM_USED", "MBs", statusthread.getMaxUsedMem());
         exporter.write("MIN_MEM_USED", "MBs", statusthread.getMinUsedMem());

core/src/main/java/com/yahoo/ycsb/Utils.java

@@ -219,8 +219,25 @@ public class Utils
             ManagementFactory.getGarbageCollectorMXBeans();
         long count = 0;
         for (final GarbageCollectorMXBean bean : gcBeans) {
+          if (bean.getCollectionCount() < 0) {
+            continue;
+          }
           count += bean.getCollectionCount();
         }
         return count;
       }
+      
+      /** @return The total time, in milliseconds, spent in GC. */ 
+      public static long getGCTotalTime() {
+        final List<GarbageCollectorMXBean> gcBeans = 
+            ManagementFactory.getGarbageCollectorMXBeans();
+        long time = 0;
+        for (final GarbageCollectorMXBean bean : gcBeans) {
+          if (bean.getCollectionTime() < 0) {
+            continue;
+          }
+          time += bean.getCollectionTime();
+        }
+        return time;
+      }
 }

core/src/test/java/com/yahoo/ycsb/TestUtils.java

@@ -110,6 +110,8 @@ public class TestUtils {
     Utils.getSystemLoadAverage();
     // This will probably be zero but should never be negative.
     assertTrue(Utils.getGCTotalCollectionCount() >= 0);
+    // Could be zero similar to GC total collection count
+    assertTrue(Utils.getGCTotalTime() >= 0);
   }
   
   /**