HADOOP-18118. Fix KMS Accept Queue Size default value to 500 (#3972)

diff --git a/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/http/HttpServer2.java b/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/http/HttpServer2.java
index 934848b..178f761 100644
--- a/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/http/HttpServer2.java
+++ b/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/http/HttpServer2.java
@@ -144,7 +144,7 @@
 
   public static final String HTTP_SOCKET_BACKLOG_SIZE_KEY =
       "hadoop.http.socket.backlog.size";
-  public static final int HTTP_SOCKET_BACKLOG_SIZE_DEFAULT = 128;
+  public static final int HTTP_SOCKET_BACKLOG_SIZE_DEFAULT = 500;
   public static final String HTTP_MAX_THREADS_KEY = "hadoop.http.max.threads";
   public static final String HTTP_ACCEPTOR_COUNT_KEY =
       "hadoop.http.acceptor.count";
diff --git a/hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/http/TestHttpServer.java b/hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/http/TestHttpServer.java
index 062033d..b716105 100644
--- a/hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/http/TestHttpServer.java
+++ b/hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/http/TestHttpServer.java
@@ -744,6 +744,13 @@
   }
 
   @Test
+  public void testBacklogSize2() throws Exception
+  {
+    Configuration conf = new Configuration();
+    assertEquals(500, conf.get(HttpServer2.HTTP_SOCKET_BACKLOG_SIZE_KEY));
+  }
+
+  @Test
   public void testIdleTimeout() throws Exception {
     final int idleTimeout = 1000;
     Configuration conf = new Configuration();