Github user li-zhihui commented on a diff in the pull request:

    https://github.com/apache/spark/pull/900#discussion_r14280169
  
    --- Diff: 
yarn/common/src/main/scala/org/apache/spark/scheduler/cluster/YarnClusterSchedulerBackend.scala
 ---
    @@ -0,0 +1,39 @@
    +/*
    + * Licensed to the Apache Software Foundation (ASF) under one or more
    + * contributor license agreements.  See the NOTICE file distributed with
    + * this work for additional information regarding copyright ownership.
    + * The ASF licenses this file to You under the Apache License, Version 2.0
    + * (the "License"); you may not use this file except in compliance with
    + * the License.  You may obtain a copy of the License at
    + *
    + *    http://www.apache.org/licenses/LICENSE-2.0
    + *
    + * Unless required by applicable law or agreed to in writing, software
    + * distributed under the License is distributed on an "AS IS" BASIS,
    + * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    + * See the License for the specific language governing permissions and
    + * limitations under the License.
    + */
    +
    +package org.apache.spark.scheduler.cluster
    +
    +import org.apache.spark.SparkContext
    +import org.apache.spark.scheduler.TaskSchedulerImpl
    +import org.apache.spark.util.IntParam
    +
    +private[spark] class YarnClusterSchedulerBackend(
    +    scheduler: TaskSchedulerImpl,
    +    sc: SparkContext)
    +  extends CoarseGrainedSchedulerBackend(scheduler, sc.env.actorSystem) {
    +
    +  override def start() {
    +    super.start()
    +    var numExecutors = 2
    --- End diff --
    
    @kayousterhout The constant (default value of numExecutors) is not only 
defined in this class and  ApplicationMasterArguments. It is also defined in 
ClientArguments. 
    
    Even I guess the below default values are from same consideration like 
numExecurots.
    
https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/SparkContext.scala#L1232
    
https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/SparkContext.scala#L1232
    
https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/SparkContext.scala#L1232
    
    If we want to make the value to be constant, we should considerate all of 
them.
    So maybe we can add an object org.apache.spark.Constants to manange all 
constants.
    @tgravescs 


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