[SPARK-2856] Decrease initial buffer size for Kryo to 64KB.

Author: Reynold Xin <[email protected]>

Closes #1780 from rxin/kryo-init-size and squashes the following commits:

551b935 [Reynold Xin] [SPARK-2856] Decrease initial buffer size for Kryo to 64KB.

Author: rxin

core/src/main/scala/org/apache/spark/serializer/KryoSerializer.scala

@@ -47,7 +47,9 @@ class KryoSerializer(conf: SparkConf)
   with Logging
   with Serializable {
 
-  private val bufferSize = conf.getInt("spark.kryoserializer.buffer.mb", 2) * 1024 * 1024
+  private val bufferSize =
+    (conf.getDouble("spark.kryoserializer.buffer.mb", 0.064) * 1024 * 1024).toInt
+
   private val maxBufferSize = conf.getInt("spark.kryoserializer.buffer.max.mb", 64) * 1024 * 1024
   private val referenceTracking = conf.getBoolean("spark.kryo.referenceTracking", true)
   private val registrationRequired = conf.getBoolean("spark.kryo.registrationRequired", false)

docs/configuration.md

@@ -412,7 +412,7 @@ Apart from these, the following properties are also available, and may be useful
 </tr>
 <tr>
   <td><code>spark.kryoserializer.buffer.mb</code></td>
-  <td>2</td>
+  <td>0.064</td>
   <td>
     Initial size of Kryo's serialization buffer, in megabytes. Note that there will be one buffer
      <i>per core</i> on each worker. This buffer will grow up to