Tasks

In Spark, a Task (aka command) is the smallest individual unit of execution that corresponds to a RDD partition. Tasks are launched on executors.

spark rdd partitions job stage tasks.png
Figure 1. Tasks correspond to partitions in RDD

In other (more technical) words, a task is a computation on a data partition in a stage of a RDD in a Spark job.

The Task contract expects that custom tasks define runTask method.

runTask(context: TaskContext): T
Note
 T is the type defined when a Task is created.
Table 1. Task Internal Registries and Counters
Name Description

metrics

Used when ???

taskMemoryManager

TaskMemoryManager that manages the memory allocated by the task.

Used when ???

context

Used when ???

_killed

Used when ???

_executorDeserializeTime

Used when ???

_executorDeserializeCpuTime

Used when ???

taskThread

Used when ???

epoch

Set for a Task when TaskSetManager is created and later used when TaskRunner runs and when DAGScheduler handles a ShuffleMapTask successful completion.

A task can only belong to one stage and operate on a single partition. All tasks in a stage must be completed before the stages that follow can start.

Tasks are spawned one by one for each stage and partition.

Caution
 FIXME What are stageAttemptId and taskAttemptId?

A task in Spark is represented by the Task abstract class with two concrete implementations:

  • ShuffleMapTask that executes a task and divides the task’s output to multiple buckets (based on the task’s partitioner).

  • ResultTask that executes a task and sends the task’s output back to the driver application.

The very last stage in a Spark job consists of multiple ResultTasks, while earlier stages can only be ShuffleMapTasks.

Caution
 FIXME You could have a Spark job with ShuffleMapTask being the last.

Creating Task Instance

Task[T](
  val stageId: Int,
  val stageAttemptId: Int,
  val partitionId: Int,
  var localProperties: Properties = new Properties,
  serializedTaskMetrics: Array[Byte] =
    SparkEnv.get.closureSerializer.newInstance().serialize(TaskMetrics.registered).array(),
  val jobId: Option[Int] = None,
  val appId: Option[String] = None,
  val appAttemptId: Option[String] = None)
extends Serializable

Task Attributes

A Task instance is uniquely identified by the following task attributes:

  • stageId - there can be many stages in a job. Every stage has its own unique stageId that the task belongs to.

  • stageAttemptId - a stage can be re-attempted for execution in case of failure. stageAttemptId represents the attempt id of a stage that the task belongs to.

  • partitionId - a task is a unit of work on a partitioned distributed dataset. Every partition has its own unique partitionId that a task processes.

  • metrics - an instance of TaskMetrics for the task.

  • localProperties - local private properties of the task.

Running Task Thread — run Method

run(
  taskAttemptId: Long,
  attemptNumber: Int,
  metricsSystem: MetricsSystem): T

run registers task attempt id to the executor’s BlockManager and creates a TaskContextImpl that in turn gets set as the thread local TaskContext.

If the task has been killed before the task runs it is killed (with interruptThread flag disabled).

The task runs.

Caution
 FIXME Describe catch and finally blocks.
Note
 When run is called from TaskRunner.run, the Task has just been deserialized from taskBytes that were sent over the wire to an executor. localProperties and TaskMemoryManager are already assigned.

Task States

A task can be in one of the following states:

  • LAUNCHING

  • RUNNING when the task is being started.

  • FINISHED when the task finished with the serialized result.

  • FAILED when the task fails, e.g. when FetchFailedException (see FetchFailedException), CommitDeniedException or any Throwable occur

  • KILLED when an executor kills a task.

  • LOST

States are the values of org.apache.spark.TaskState.

Note
 Task status updates are sent from executors to the driver through ExecutorBackend.

Task is finished when it is in one of FINISHED, FAILED, KILLED, LOST

LOST and FAILED states are considered failures.

Tip
 Task states correspond to org.apache.mesos.Protos.TaskState.

Collect Latest Values of Accumulators — collectAccumulatorUpdates Method

collectAccumulatorUpdates(taskFailed: Boolean = false): Seq[AccumulableInfo]

collectAccumulatorUpdates collects the latest values of accumulators used in a task (and returns the values as a collection of AccumulableInfo).

Note
 It is used in TaskRunner to send a task’s final results with the latest values of accumulators used.

When taskFailed is true it filters out accumulators with countFailedValues disabled.

Caution
 FIXME Why is the check context != null?
Note
 It uses context.taskMetrics.accumulatorUpdates().
Caution
 FIXME What is context.taskMetrics.accumulatorUpdates() doing?

Killing Task — kill Method

kill(interruptThread: Boolean)

kill marks the task to be killed, i.e. it sets the internal _killed flag to true.

It calls TaskContextImpl.markInterrupted when context is set.

If interruptThread is enabled and the internal taskThread is available, kill interrupts it.

Caution
 FIXME When could context and interruptThread not be set?

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