Added FlushIndexTask to flush documents at index thread level.

lucene/benchmark/conf/indexing-flush-by-RAM-multithreaded.alg

@@ -53,7 +53,8 @@ log.queries=true
 
     { "Populate"
         CreateIndex
-        [{ "MAddDocs" AddDoc } : 5000] : 4
+        #[{ "MAddDocs" AddDoc } : 5000] : 4
+        [{ {{"MAddDocs" AddDoc } : 5000} FlushIndex }  ] : 20
         ForceMerge(1)
         CloseIndex
     }

lucene/benchmark/src/java/org/apache/lucene/benchmark/byTask/tasks/FlushIndexTask.java

@@ -0,0 +1,37 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.lucene.benchmark.byTask.tasks;
+
+import org.apache.lucene.benchmark.byTask.PerfRunData;
+import org.apache.lucene.index.IndexWriter;
+
+/** Flush Index Task uses flushNextBuffer() to flush documents at thread level */
+public class FlushIndexTask extends PerfTask {
+
+  public FlushIndexTask(PerfRunData runData) {
+    super(runData);
+  }
+
+  @Override
+  public int doLogic() throws Exception {
+    IndexWriter iw = getRunData().getIndexWriter();
+    if (iw != null) {
+      iw.flushNextBuffer();
+    }
+    return 1;
+  }
+}