allow the user to specify the worker number and avoid unnecessary shuffle

jvm-packages/xgboost4j-spark/src/main/scala/ml/dmlc/xgboost4j/scala/spark/XGBoost.scala

@@ -43,7 +43,16 @@ object XGBoost extends Serializable {
       rabitEnv: mutable.Map[String, String],
       numWorkers: Int, round: Int, obj: ObjectiveTrait, eval: EvalTrait): RDD[Booster] = {
     import DataUtils._
-    trainingData.repartition(numWorkers).mapPartitions {
+    val partitionedData = {
+      if (numWorkers > trainingData.partitions.length) {
+        trainingData.repartition(numWorkers)
+      } else if (numWorkers < trainingData.partitions.length) {
+        trainingData.coalesce(numWorkers)
+      } else {
+        trainingData
+      }
+    }
+    partitionedData.mapPartitions {
       trainingSamples =>
         rabitEnv.put("DMLC_TASK_ID", TaskContext.getPartitionId().toString)
         Rabit.init(rabitEnv.asJava)
@@ -60,6 +69,8 @@ object XGBoost extends Serializable {
    * @param trainingData the trainingset represented as RDD
    * @param configMap Map containing the configuration entries
    * @param round the number of iterations
+   * @param nWorkers the number of xgboost workers, 0 by default which means that the number of
+   *                 workers equals to the partition number of trainingData RDD
    * @param obj the user-defined objective function, null by default
    * @param eval the user-defined evaluation function, null by default
    * @throws ml.dmlc.xgboost4j.java.XGBoostError when the model training is failed
@@ -67,8 +78,7 @@ object XGBoost extends Serializable {
    */
   @throws(classOf[XGBoostError])
   def train(trainingData: RDD[LabeledPoint], configMap: Map[String, Any], round: Int,
-       obj: ObjectiveTrait = null, eval: EvalTrait = null): XGBoostModel = {
-    val numWorkers = trainingData.partitions.length
+      nWorkers: Int = 0, obj: ObjectiveTrait = null, eval: EvalTrait = null): XGBoostModel = {
     implicit val sc = trainingData.sparkContext
     if (configMap.contains("nthread")) {
       val nThread = configMap("nthread")
@@ -77,6 +87,13 @@ object XGBoost extends Serializable {
         s"the nthread configuration ($nThread) must be no larger than " +
           s"spark.task.cpus ($coresPerTask)")
     }
+    val numWorkers = {
+      if (nWorkers > 0) {
+        nWorkers
+      } else {
+        trainingData.partitions.length
+      }
+    }
     val tracker = new RabitTracker(numWorkers)
     require(tracker.start(), "FAULT: Failed to start tracker")
     val boosters = buildDistributedBoosters(trainingData, configMap,

jvm-packages/xgboost4j-spark/src/test/scala/ml/dmlc/xgboost4j/scala/spark/XGBoostSuite.scala

@@ -35,7 +35,7 @@ import ml.dmlc.xgboost4j.scala.{DMatrix, EvalTrait}
 class XGBoostSuite extends FunSuite with BeforeAndAfter {
 
   private implicit var sc: SparkContext = null
-  private val numWorker = 2
+  private val numWorkers = 4
 
   private class EvalError extends EvalTrait {
 
@@ -114,10 +114,10 @@ class XGBoostSuite extends FunSuite with BeforeAndAfter {
 
   private def buildTrainingRDD(sparkContext: Option[SparkContext] = None): RDD[LabeledPoint] = {
     val sampleList = readFile(getClass.getResource("/agaricus.txt.train").getFile)
-    sparkContext.getOrElse(sc).parallelize(sampleList, numWorker)
+    sparkContext.getOrElse(sc).parallelize(sampleList, numWorkers)
   }
 
-  test("build RDD containing boosters") {
+  test("build RDD containing boosters with the specified worker number") {
     val trainingRDD = buildTrainingRDD()
     val testSet = readFile(getClass.getResource("/agaricus.txt.test").getFile).iterator
     import DataUtils._
@@ -127,13 +127,13 @@ class XGBoostSuite extends FunSuite with BeforeAndAfter {
       List("eta" -> "1", "max_depth" -> "2", "silent" -> "0",
         "objective" -> "binary:logistic").toMap,
       new scala.collection.mutable.HashMap[String, String],
-      numWorker, 2, null, null)
+      numWorkers = 2, round = 5, null, null)
     val boosterCount = boosterRDD.count()
-    assert(boosterCount === numWorker)
+    assert(boosterCount === 2)
     val boosters = boosterRDD.collect()
     for (booster <- boosters) {
       val predicts = booster.predict(testSetDMatrix, true)
-      assert(new EvalError().eval(predicts, testSetDMatrix) < 0.1)
+      assert(new EvalError().eval(predicts, testSetDMatrix) < 0.17)
     }
   }
 
@@ -157,13 +157,12 @@ class XGBoostSuite extends FunSuite with BeforeAndAfter {
   }
 
   test("nthread configuration must be equal to spark.task.cpus") {
-    // close the current Spark context
     sc.stop()
     sc = null
-    // start another app
-    val sparkConf = new SparkConf().setMaster("local[*]").set("spark.task.cpus", "4").
-      setAppName("test1")
+    val sparkConf = new SparkConf().setMaster("local[*]").setAppName("XGBoostSuite").
+      set("spark.task.cpus", "4")
     val customSparkContext = new SparkContext(sparkConf)
+    // start another app
     val trainingRDD = buildTrainingRDD(Some(customSparkContext))
     val paramMap = List("eta" -> "1", "max_depth" -> "2", "silent" -> "0",
       "objective" -> "binary:logistic", "nthread" -> 6).toMap