[SPARK-5073] spark.storage.memoryMapThreshold have two default value

Because major OS page sizes is about 4KB, the default value of spark.storage.memoryMapThreshold is integrated to 2 * 4096

Author: lewuathe <lewuathe@me.com>

Closes #3900 from Lewuathe/integrate-memoryMapThreshold and squashes the following commits:

e417acd [lewuathe] [SPARK-5073] Update docs/configuration
834aba4 [lewuathe] [SPARK-5073] Fix style
adcea33 [lewuathe] [SPARK-5073] Integrate memory map threshold to 2MB
fcce2e5 [lewuathe] [SPARK-5073] spark.storage.memoryMapThreshold have two default value

core/src/main/scala/org/apache/spark/storage/DiskStore.scala

@@ -31,7 +31,8 @@ import org.apache.spark.util.Utils
 private[spark] class DiskStore(blockManager: BlockManager, diskManager: DiskBlockManager)
   extends BlockStore(blockManager) with Logging {
 
-  val minMemoryMapBytes = blockManager.conf.getLong("spark.storage.memoryMapThreshold", 2 * 4096L)
+  val minMemoryMapBytes = blockManager.conf.getLong(
+    "spark.storage.memoryMapThreshold", 2 * 1024L * 1024L)
 
   override def getSize(blockId: BlockId): Long = {
     diskManager.getFile(blockId.name).length

docs/configuration.md

@@ -678,7 +678,7 @@ Apart from these, the following properties are also available, and may be useful
 </tr>
 <tr>
   <td><code>spark.storage.memoryMapThreshold</code></td>
-  <td>8192</td>
+  <td>2097152</td>
   <td>
     Size of a block, in bytes, above which Spark memory maps when reading a block from disk.
     This prevents Spark from memory mapping very small blocks. In general, memory