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

    https://github.com/apache/spark/pull/6394#discussion_r31953212
  
    --- Diff: 
yarn/src/main/scala/org/apache/spark/deploy/yarn/YarnAllocator.scala ---
    @@ -225,12 +243,74 @@ private[yarn] class YarnAllocator(
           logInfo(s"Will request $missing executor containers, each with 
${resource.getVirtualCores} " +
             s"cores and ${resource.getMemory} MB memory including 
$memoryOverhead MB overhead")
     
    -      for (i <- 0 until missing) {
    -        val request = createContainerRequest(resource)
    -        amClient.addContainerRequest(request)
    -        val nodes = request.getNodes
    -        val hostStr = if (nodes == null || nodes.isEmpty) "Any" else 
nodes.last
    -        logInfo(s"Container request (host: $hostStr, capability: 
$resource)")
    +      // Calculated the number of executors we expected to satisfy all the 
preferred locality tasks
    +      val localityAwareTaskCores = localityAwarePendingTaskNum * 
CPUS_PER_TASK
    +      val expectedLocalityAwareContainerNum =
    +        (localityAwareTaskCores + resource.getVirtualCores - 1) / 
resource.getVirtualCores
    +
    +      // Get the all the existed and locality matched containers
    +      val existedMatchedContainers = allocatedHostToContainersMap.filter { 
case (host, _) =>
    +        preferredLocalityToCounts.contains(host)
    +      }
    +      val existedMatchedContainerNum = 
existedMatchedContainers.values.map(_.size).sum
    +
    +      // The number of containers to allocate, divided into two groups, 
one with node locality,
    +      // and the other without locality preference.
    +      var requiredLocalityFreeContainerNum: Int = 0
    +      var requiredLocalityAwareContainerNum: Int = 0
    +
    +      if (expectedLocalityAwareContainerNum <= existedMatchedContainerNum) 
{
    +        // If the current allocated executor can satisfy all the locality 
preferred tasks,
    +        // allocate the new container with no locality preference
    +        requiredLocalityFreeContainerNum = missing
    +      } else {
    +        if (expectedLocalityAwareContainerNum - existedMatchedContainerNum 
>= missing) {
    +          // If newly requested containers cannot satisfy the locality 
preferred tasks,
    +          // allocate all the new container with locality preference
    +          requiredLocalityAwareContainerNum = missing
    +        } else {
    +          // If part of newly requested can satisfy the locality preferred 
tasks, allocate part of
    +          // the containers with locality preference, and another part 
with no locality preference
    +          requiredLocalityAwareContainerNum =
    +            expectedLocalityAwareContainerNum - existedMatchedContainerNum
    +          requiredLocalityFreeContainerNum = missing -
    +            (expectedLocalityAwareContainerNum - 
existedMatchedContainerNum)
    +        }
    +      }
    --- End diff --
    
    I think this could all be simplified to
    
    ```
    val requiredLocalityAwareContainerNum = 
      math.max(math.min(expectedLocalityAwareContainerNum - 
existedMatchedContainerNum, missing),0)
    val requiredLocalityFreeContainerNum = missing - 
requiredLocalityAwareContainerNum
    ```


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