Github user MLnick commented on a diff in the pull request:

    https://github.com/apache/spark/pull/16774#discussion_r110606615
  
    --- Diff: 
mllib/src/main/scala/org/apache/spark/ml/tuning/ValidatorParams.scala ---
    @@ -67,6 +71,39 @@ private[ml] trait ValidatorParams extends HasSeed with 
Params {
       /** @group getParam */
       def getEvaluator: Evaluator = $(evaluator)
     
    +  /**
    +   * param to control the number of models evaluated in parallel
    +   * Default: 1
    +   *
    +   * @group param
    +   */
    +  val numParallelEval: IntParam = new IntParam(this, "numParallelEval",
    +    "max number of models to evaluate in parallel, 1 for serial 
evaluation",
    +    ParamValidators.gtEq(1))
    +
    +  /** @group getParam */
    +  def getNumParallelEval: Int = $(numParallelEval)
    +
    +  /**
    +   * Creates a execution service to be used for validation, defaults to a 
thread-pool with
    +   * size of `numParallelEval`
    +   */
    +  protected var executorServiceFactory: (Int) => ExecutorService = {
    +    (requestedMaxThreads: Int) => ThreadUtils.newDaemonCachedThreadPool(
    +      s"${this.getClass.getSimpleName}-thread-pool", requestedMaxThreads)
    +  }
    +
    +  /**
    +   * Sets a function to get an execution service to be used for validation
    +   *
    +   * @param getExecutorService function to get an ExecutorService given a 
requestedMaxThread size
    +   */
    +  @Experimental
    +  @InterfaceStability.Unstable
    +  def setExecutorService(getExecutorService: (Int) => ExecutorService): 
Unit = {
    --- End diff --
    
    I think I prefer a simpler API: 
    
    ```scala
     def setExecutorService(executorService: ExecutorService): Unit = {
    ```
    
    Not sure the function version will work nicely with Java.
    
    I think the idea should be that setting `numParallelEval` will specify the 
number of threads for the default thread-pool, while a custom one can be set 
with this method (advanced usage). If set, the custom one will override.


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