Stage level scheduling

Versions: Apache Spark 3.1.1

The idea of writing this blog post came to me when I was analyzing Kubernetes changes in Apache Spark 3.1.1. Starting from this version we can use stage level scheduling, so far available only for YARN. Even though it's probably a very low level feature, it intrigued me enough to write a few words here!

New ebook 🔥

Learn 84 ways to solve common data engineering problems with cloud services.

👉 I want my Early Access edition

Definition

Stage level scheduling defines the resources needed by each stage of an Apache Spark job. It's a relatively new feature because it got implemented only in version 3.1.1 for 2 cluster managers, YARN and Kubernetes. As of this writing (20/07/2021), it's only available in the RDD API with the Dynamic Resource Allocation enabled. What an execution graph with the stage level scheduling enabled looks like? You can see an example just below:

The feature is interesting for any heterogeneous workloads, such as an initial data reduction stage requiring a lot of resources and the next step working on the reduced dataset requiring much less compute power. The author of the feature, Thomas Graves, also quotes a resources optimization example of a ML pipeline preceded by an ETL. The ML part of the job needs GPU, but the ETL doesn't, so enabling it at the beginning of the pipeline would waste this precious resource.

Resource profiles

At the code level, you will find the stage level scheduling under the name of a resource profile. Each resource profile contains the executor and task resources requested for the given stage...:

class ResourceProfile(
    val executorResources: Map[String, ExecutorResourceRequest],
    val taskResources: Map[String, TaskResourceRequest])

...and among the types of resources, you will find:

object ResourceProfile extends Logging {
  // task resources
  val CPUS = "cpus"
  
  private[spark] def getCustomTaskResources(
      rp: ResourceProfile): Map[String, TaskResourceRequest] = {
    rp.taskResources.filterKeys(k => !k.equals(ResourceProfile.CPUS)).toMap
  }

  // Executor resources
  val CORES = "cores"
  val MEMORY = "memory"
  val OFFHEAP_MEM = "offHeap"
  val OVERHEAD_MEM = "memoryOverhead"
  val PYSPARK_MEM = "pyspark.memory"
  def allSupportedExecutorResources: Array[String] =
    Array(CORES, MEMORY, OVERHEAD_MEM, PYSPARK_MEM, OFFHEAP_MEM)

  private[spark] def getCustomExecutorResources(
      rp: ResourceProfile): Map[String, ExecutorResourceRequest] = {
    rp.executorResources.
      filterKeys(k => !ResourceProfile.allSupportedExecutorResources.contains(k)).toMap
  }

To attach a resource profile to the job, you have to call the withResources(rp: ResourceProfile) method of RDD. Each RDD keeps only one profile, so if you call this method multiple times, it will save the last called one. And this link with the RDD is quite important for the scheduling part. Before submitting the stage, DAGScheduler reads all resource profiles defined in the stage RDDs. If there are multiple profiles, the scheduler merges them by taking the max value for each ExecutorResourceRequest and TaskResourceRequest:

  private[scheduler] def mergeResourceProfiles(r1: ResourceProfile, r2: ResourceProfile): ResourceProfile = {
    val mergedExecKeys = r1.executorResources ++ r2.executorResources
    val mergedExecReq = mergedExecKeys.map { case (k, v) =>
        val larger = r1.executorResources.get(k).map( x =>
          if (x.amount > v.amount) x else v).getOrElse(v)
        k -> larger
    }
// Same for tasks just below, omitted for brevity
    new ResourceProfile(mergedExecReq, mergedTaskReq)
  }

But the merge doesn't happen every time, even for multiple resource profiles in a stage! By default the configuration controlling its execution (spark.scheduler.resource.profileMergeConflicts) is disabled. You should then get an IllegalArgumentException:

       throw new IllegalArgumentException("Multiple ResourceProfiles specified in the RDDs for " +
          "this stage, either resolve the conflicting ResourceProfiles yourself or enable " +
          s"${config.RESOURCE_PROFILE_MERGE_CONFLICTS.key} and understand how Spark handles " +
          "the merging them.")

Let's assume that the exception didn't happen. What will be the next step? DAGScheduler creates the Stage with the associated resource profile. That's where it stops and passes the torch to the cluster manager.

Stage level on Kubernetes

The stage level scheduling on Kubernetes relies on the ExecutorPodsSnapshotsStore which is an in-memory store with events management capacity. Apache Spark uses an ExecutorPodsWatcher to listen for all pod-related events. Whenever something happens about the pod specification, it automatically saves this information to the snapshots store:

  private class ExecutorPodsWatcher extends Watcher[Pod] {
    override def eventReceived(action: Action, pod: Pod): Unit = {
      val podName = pod.getMetadata.getName
      logDebug(s"Received executor pod update for pod named $podName, action $action")
      snapshotsStore.updatePod(pod)
    }

Since the snapshot store is also a notifier, it updates the pod information in all subscribed listeners. One of them is ExecutorPodsAllocator:

private[spark] class ExecutorPodsAllocator(    conf: SparkConf,
    // ...
snapshotsStore: ExecutorPodsSnapshotsStore,
    clock: Clock) extends Logging {
  def start(applicationId: String): Unit = {
    snapshotsStore.addSubscriber(podAllocationDelay) {
      onNewSnapshots(applicationId, _)
    }
  }

The allocator will run every podAllocationDelay (1 second by default) and execute the onNewSnapshots method. Inside this method you will find the logic responsible for starting and stopping the executors, including the ones managed by different resource profiles. Before showing you this in action, some important things from the documentation to keep in mind:

Knowing all that it's time to see the stage level scheduling in action:

Stage level scheduling is a relatively new feature in Apache Spark, and it still has some room for improvement, especially the support of the Dataset API. But despite that, it's an interesting evolution that adds some controlled flexibility to the jobs!