add a option to print DAG

Author: KaiXinXiaoLei

core/src/main/scala/org/apache/spark/SparkContext.scala

@@ -1328,6 +1328,9 @@ class SparkContext(config: SparkConf) extends Logging with ExecutorAllocationCli
     val callSite = getCallSite
     val cleanedFunc = clean(func)
     logInfo("Starting job: " + callSite.shortForm)
+    if (conf.getBoolean("spark.rddDebug.enable", false)) {
+      logInfo("toDebugString: " + rdd.toDebugString)
+    }
     dagScheduler.runJob(rdd, cleanedFunc, partitions, callSite, allowLocal,
       resultHandler, localProperties.get)
     progressBar.foreach(_.finishAll())