fix 'else' branch and doc update

Author: scwf

core/src/main/scala/org/apache/spark/util/Utils.scala

@@ -1665,15 +1665,15 @@ private[spark] object Utils extends Logging {
       serviceName: String = "",
       maxRetries: Int = portMaxRetries): (T, Int) = {
     val serviceString = if (serviceName.isEmpty) "" else s" '$serviceName'"
-    val startPort = conf.getInt("spark.port.min", 0)
+    val startPort = conf.getInt("spark.port.min", 1024)
     val endPort = conf.getInt("spark.port.max", 65536)
     for (offset <- 0 to maxRetries) {
       // Do not increment port if port is 0, which is treated as a special port
       val tryPort = if (port == 0) {
         (startPort + Math.random() * (endPort - startPort)).toInt
       } else {
-        // If the new port wraps around, do not try a privilege port
-        ((port + offset - 1024) % (65536 - 1024)) + 1024
+        // If the new port wraps around, ensure it is in range(startPort, endPort)
+        ((port + offset - startPort) % (endPort - startPort)) + startPort
       }
       try {
         val (service, port) = startService(tryPort)

docs/configuration.md

@@ -183,6 +183,20 @@ of the most common options to set are:
     Logs the effective SparkConf as INFO when a SparkContext is started.
   </td>
 </tr>
+<tr>
+  <td><code>spark.port.min</code></td>
+  <td>1024</td>
+  <td>
+    Min port for spark(UI, HttpServer. ConnectionManager, Akka)
+  </td>
+</tr>
+<tr>
+  <td><code>spark.port.max</code></td>
+  <td>65536</td>
+  <td>
+    Max port for spark(UI, HttpServer. ConnectionManager, Akka)
+  </td>
+</tr>
 </table>
 
 Apart from these, the following properties are also available, and may be useful in some situations: