nthread no larger than spark.task.cpus

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

@@ -70,6 +70,13 @@ object XGBoost extends Serializable {
        obj: ObjectiveTrait = null, eval: EvalTrait = null): XGBoostModel = {
     val numWorkers = trainingData.partitions.length
     implicit val sc = trainingData.sparkContext
+    if (configMap.contains("nthread")) {
+      val nThread = configMap("nthread")
+      val coresPerTask = sc.getConf.get("spark.task.cpus", "1")
+      require(nThread.toString <= coresPerTask,
+        s"the nthread configuration ($nThread) must be no larger than " +
+          s"spark.task.cpus ($coresPerTask)")
+    }
     val tracker = new RabitTracker(numWorkers)
     require(tracker.start(), "FAULT: Failed to start tracker")
     val boosters = buildDistributedBoosters(trainingData, configMap,