fixes from review comments

fix sync of aggs,
fix NPE in sink.isFull,
RealtimeTuningConfig lower the bufferSize to 256m

processing/src/main/java/io/druid/segment/incremental/IncrementalIndex.java

@@ -130,7 +130,8 @@ public class IncrementalIndex implements Iterable<Row>, Closeable
     int currAggSize = 0;
     for (int i = 0; i < metrics.length; i++) {
       final AggregatorFactory agg = metrics[i];
-      aggs[i] = agg.factorizeBuffered(
+      aggs[i] = new ThreadSafeAggregator(
+          agg.factorizeBuffered(
               new ColumnSelectorFactory()
               {
                 @Override
@@ -270,6 +271,7 @@ public class IncrementalIndex implements Iterable<Row>, Closeable
                   };
                 }
               }
+          )
       );
       aggPositionOffsets[i] = currAggSize;
       currAggSize += agg.getMaxIntermediateSize();
@@ -458,10 +460,8 @@ public class IncrementalIndex implements Iterable<Row>, Closeable
     in.set(row);
     int rowOffset = facts.get(key);
     for (int i = 0; i < aggs.length; i++) {
-      synchronized (aggs[i]) {
         aggs[i].aggregate(bufferHolder.get(), getMetricPosition(rowOffset, i));
     }
-    }
     in.set(null);
     return numEntries.get();
   }
@@ -473,7 +473,7 @@ public class IncrementalIndex implements Iterable<Row>, Closeable
 
   /**
    *
-   * @return true if the underlying buffer for IncrementalIndex is full and cannot accomodate more rows.
+   * @return true if the underlying buffer for IncrementalIndex is full and cannot accommodate more rows.
    */
   public boolean isFull()
   {
@@ -861,4 +861,45 @@ public class IncrementalIndex implements Iterable<Row>, Closeable
       }
     }
   }
+
+  private static class ThreadSafeAggregator implements BufferAggregator
+  {
+
+    private final BufferAggregator delegate;
+
+    public ThreadSafeAggregator(BufferAggregator delegate)
+    {
+      this.delegate = delegate;
+    }
+
+    @Override
+    public synchronized void init(ByteBuffer buf, int position)
+    {
+      delegate.init(buf, position);
+    }
+
+    @Override
+    public synchronized void aggregate(ByteBuffer buf, int position)
+    {
+      delegate.aggregate(buf, position);
+    }
+
+    @Override
+    public synchronized Object get(ByteBuffer buf, int position)
+    {
+      return delegate.get(buf, position);
+    }
+
+    @Override
+    public synchronized float getFloat(ByteBuffer buf, int position)
+    {
+      return delegate.getFloat(buf, position);
+    }
+
+    @Override
+    public synchronized void close()
+    {
+      delegate.close();
+    }
+  }
 }

server/src/main/java/io/druid/segment/indexing/RealtimeTuningConfig.java

@@ -36,7 +36,7 @@ import java.io.File;
  */
 public class RealtimeTuningConfig implements TuningConfig
 {
-  private static final int defaultBufferSize = 512 * 1024 * 1024;
+  private static final int defaultBufferSize = 256 * 1024 * 1024;
   private static final Period defaultIntermediatePersistPeriod = new Period("PT10M");
   private static final Period defaultWindowPeriod = new Period("PT10M");
   private static final File defaultBasePersistDirectory = Files.createTempDir();

server/src/main/java/io/druid/segment/realtime/plumber/Sink.java

@@ -136,7 +136,7 @@ public class Sink implements Iterable<FireHydrant>
   public boolean isFull()
   {
     synchronized (currHydrant){
-      return currHydrant.getIndex().isFull();
+      return currHydrant != null && currHydrant.getIndex().isFull();
     }
   }