Spark performance tuning: RDD cache model

/**
   * Mark this RDD for persisting using the specified level.
   *
   * @param newLevel the target storage level
   * @param allowOverride whether to override any existing level with the new one
   * This method is applicable to the case where we previously set the RDD cache level, and now if we want to modify the RDD cache level, we only need to set allowOverride to true
   */
  private def persist(newLevel: StorageLevel, allowOverride: Boolean): this.type = {
    // TODO: Handle changes of StorageLevel
    //This program also explains why the first special case of the second method above will not report an error
    if (storageLevel != StorageLevel.NONE && newLevel != storageLevel && !allowOverride) {
      throw new UnsupportedOperationException(
        "Cannot change storage level of an RDD after it was already assigned a level")
    }
    // If this is the first time this RDD is marked for persisting, register it
    // with the SparkContext for cleanups and accounting. Do this only once.
    if (storageLevel == StorageLevel.NONE) {
      sc.cleaner.foreach(_.registerRDDForCleanup(this))
      sc.persistRDD(this)
    }
    storageLevel = newLevel
    this
  }

@DeveloperApi
class StorageLevel private(
    private var _useDisk: Boolean, //Whether to use disk
    private var _useMemory: Boolean, //Whether to use memory
    private var _useOffHeap: Boolean, //Whether to use off-heap memory
    private var _deserialized: Boolean, //Whether to deserialize
    private var _replication: Int = 1) //Backup factor, default is 1

object StorageLevel {
  val NONE = new StorageLevel(false, false, false, false)
  val DISK_ONLY = new StorageLevel(true, false, false, false)
  val DISK_ONLY_2 = new StorageLevel(true, false, false, false, 2)
  val MEMORY_ONLY = new StorageLevel(false, true, false, true)
  val MEMORY_ONLY_2 = new StorageLevel(false, true, false, true, 2)
  val MEMORY_ONLY_SER = new StorageLevel(false, true, false, false)
  val MEMORY_ONLY_SER_2 = new StorageLevel(false, true, false, false, 2)
  val MEMORY_AND_DISK = new StorageLevel(true, true, false, true)
  val MEMORY_AND_DISK_2 = new StorageLevel(true, true, false, true, 2)
  val MEMORY_AND_DISK_SER = new StorageLevel(true, true, false, false)
  val MEMORY_AND_DISK_SER_2 = new StorageLevel(true, true, false, false, 2)
  val OFF_HEAP = new StorageLevel(true, true, true, false, 1)