Update slice method comment and some responding docs.

Author: advancedxy

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

@@ -520,10 +520,12 @@ class SparkContext(config: SparkConf) extends Logging with ExecutorAllocationCli
 
   /** Distribute a local Scala collection to form an RDD.
    *
-   * @note Parallelize acts lazily. If `seq` is a mutable collection and is
-   * altered after the call to parallelize and before the first action on the
-   * RDD, the resultant RDD will reflect the modified collection. Pass a copy of
-   * the argument to avoid this.
+   * @note Parallelize acts lazily. If `seq` is a mutable collection and is altered after the call
+   * to parallelize and before the first action on the RDD, the resultant RDD will reflect the
+   * modified collection. Pass a copy of the argument to avoid this.
+   *
+   * @note When splitting Range, the sub Range is exclusive. However the last slice for inclusive
+   * Range is inclusive.
    */
   def parallelize[T: ClassTag](seq: Seq[T], numSlices: Int = defaultParallelism): RDD[T] = {
     new ParallelCollectionRDD[T](this, seq, numSlices, Map[Int, Seq[String]]())

core/src/main/scala/org/apache/spark/rdd/ParallelCollectionRDD.scala

@@ -111,7 +111,8 @@ private object ParallelCollectionRDD {
   /**
    * Slice a collection into numSlices sub-collections. One extra thing we do here is to treat Range
    * collections specially, encoding the slices as other Ranges to minimize memory cost. This makes
-   * it efficient to run Spark over RDDs representing large sets of numbers.
+   * it efficient to run Spark over RDDs representing large sets of numbers. And if the collection
+   * is an inclusive Range, we use inclusive range for the last slice.
    */
   def slice[T: ClassTag](seq: Seq[T], numSlices: Int): Seq[Seq[T]] = {
     if (numSlices < 1) {
@@ -129,7 +130,7 @@ private object ParallelCollectionRDD {
     seq match {
       case r: Range => {
         positions(r.length, numSlices).zipWithIndex.map({ case ((start, end), index) =>
-          // If the range is inclusive, include the last element in the last slice
+          // If the range is inclusive, use inclusive range for the last slice
           if (r.isInclusive && index == numSlices - 1) {
             new Range.Inclusive(r.start + start * r.step, r.end, r.step)
           }