23、CacheManager原理剖析与源码分析

一、图解

二、源码分析

###org.apache.spark.rdd/RDD.scalal
###入口

final def iterator(split: Partition, context: TaskContext): Iterator[T] = {
    if (storageLevel != StorageLevel.NONE) {
      // cacheManager相关东西
      // 如果storageLevel不为NONE，就是说，我们之前持久化过RDD，那么就不要直接去父RDD执行算子，计算新的RDD的partition了
      // 优先尝试使用CacheManager，去获取持久化的数据
      SparkEnv.get.cacheManager.getOrCompute(this, split, context, storageLevel)
    } else {
      // 进行rdd partition的计算
      computeOrReadCheckpoint(split, context)
    }
  }

###org.apache.spark/CacheManager.scala

def getOrCompute[T](
      rdd: RDD[T],
      partition: Partition,
      context: TaskContext,
      storageLevel: StorageLevel): Iterator[T] = {

    val key = RDDBlockId(rdd.id, partition.index)
    logDebug(s"Looking for partition $key")
    // 直接用BlockManager来获取数据，如果获取到了，直接返回就好了
    blockManager.get(key) match {
      case Some(blockResult) =>
        // Partition is already materialized, so just return its values
        val inputMetrics = blockResult.inputMetrics
        val existingMetrics = context.taskMetrics
          .getInputMetricsForReadMethod(inputMetrics.readMethod)
        existingMetrics.incBytesRead(inputMetrics.bytesRead)

        val iter = blockResult.data.asInstanceOf[Iterator[T]]
        new InterruptibleIterator[T](context, iter) {
          override def next(): T = {
            existingMetrics.incRecordsRead(1)
            delegate.next()
          }
        }
      // 如果从BlockManager获取不到数据，要进行后续的处理
      // 虽然RDD持久化过，但是因为未知原因，书籍即不在本地内存或磁盘，也不在远程BlockManager的本地或磁盘
      case None =>
        // Acquire a lock for loading this partition
        // If another thread already holds the lock, wait for it to finish return its results
        // 再次调用一次BlockManager的get()方法，去获取数据，如果获取到了，那么直接返回数据，如果还是没有获取数据，那么往后走
        val storedValues = acquireLockForPartition[T](key)
        if (storedValues.isDefined) {
          return new InterruptibleIterator[T](context, storedValues.get)
        }

        // Otherwise, we have to load the partition ourselves
        try {
          logInfo(s"Partition $key not found, computing it")
          // 调用computeOrReadCheckpoint()方法
          // 如果rdd之前checkpoint过，那么尝试读取它的checkpoint，如果rdd没有checkpoint过，那么只能重新使用父RDD的数据，执行算子，计算一份
          val computedValues = rdd.computeOrReadCheckpoint(partition, context)

          // If the task is running locally, do not persist the result
          if (context.isRunningLocally) {
            return computedValues
          }

          // Otherwise, cache the values and keep track of any updates in block statuses
          val updatedBlocks = new ArrayBuffer[(BlockId, BlockStatus)]
          // 由于走CacheManager，肯定意味着RDD是设置过持久化级别的，只是因为某些原因，持久化数据没有找到，才会到这
          // 所以读取了checkpoint的数据，或者是重新计算数据之后，要用putInBlockManager()方法，将数据再BlockManager中持久化一份
          val cachedValues = putInBlockManager(key, computedValues, storageLevel, updatedBlocks)
          val metrics = context.taskMetrics
          val lastUpdatedBlocks = metrics.updatedBlocks.getOrElse(Seq[(BlockId, BlockStatus)]())
          metrics.updatedBlocks = Some(lastUpdatedBlocks ++ updatedBlocks.toSeq)
          new InterruptibleIterator(context, cachedValues)

        } finally {
          loading.synchronized {
            loading.remove(key)
            loading.notifyAll()
          }
        }
    }
  }

###org.apache.spark.storage/BlockManager.scala

// 通过BlockManager获取数据的入口方法，优先从本地获取，如果本地没有，那么从远程获取
  def get(blockId: BlockId): Option[BlockResult] = {
    val local = getLocal(blockId)
    if (local.isDefined) {
      logInfo(s"Found block $blockId locally")
      return local
    }
    val remote = getRemote(blockId)
    if (remote.isDefined) {
      logInfo(s"Found block $blockId remotely")
      return remote
    }
    None
  }

###org.apache.spark.rdd/RDD.scalal

private[spark] def computeOrReadCheckpoint(split: Partition, context: TaskContext): Iterator[T] =
  {
    // Checkpointed相关先忽略
    if (isCheckpointed) firstParent[T].iterator(split, context) else compute(split, context)
  }

###org.apache.spark/CacheManager.scala

private def putInBlockManager[T](
      key: BlockId,
      values: Iterator[T],
      level: StorageLevel,
      updatedBlocks: ArrayBuffer[(BlockId, BlockStatus)],
      effectiveStorageLevel: Option[StorageLevel] = None): Iterator[T] = {

    val putLevel = effectiveStorageLevel.getOrElse(level)
    // 如果持久化级别，没有指定内存级别，仅仅是纯磁盘的级别
    if (!putLevel.useMemory) {
      /*
       * This RDD is not to be cached in memory, so we can just pass the computed values as an
       * iterator directly to the BlockManager rather than first fully unrolling it in memory.
       */
      updatedBlocks ++=
      // 直接调用blockManager的putIterator()方法，将数据写入磁盘即可
        blockManager.putIterator(key, values, level, tellMaster = true, effectiveStorageLevel)
      blockManager.get(key) match {
        case Some(v) => v.data.asInstanceOf[Iterator[T]]
        case None =>
          logInfo(s"Failure to store $key")
          throw new BlockException(key, s"Block manager failed to return cached value for $key!")
      }
    }
    // 如果指定了内存级别，往下看
    else {
      /*
       * This RDD is to be cached in memory. In this case we cannot pass the computed values
       * to the BlockManager as an iterator and expect to read it back later. This is because
       * we may end up dropping a partition from memory store before getting it back.
       *
       * In addition, we must be careful to not unroll the entire partition in memory at once.
       * Otherwise, we may cause an OOM exception if the JVM does not have enough space for this
       * single partition. Instead, we unroll the values cautiously, potentially aborting and
       * dropping the partition to disk if applicable.
       */
      // 这里会调用blockManager的unrollSafely()方法，尝试将数据写入内存
      // 如果unrollSafely()方法判断数据可以写入内存，那么就将数据写入内存
      // 如果unrollSafely()方法判断某些数据无法写入内存，那么只能写入磁盘文件
      blockManager.memoryStore.unrollSafely(key, values, updatedBlocks) match {
        case Left(arr) =>
          // We have successfully unrolled the entire partition, so cache it in memory
          updatedBlocks ++=
            blockManager.putArray(key, arr, level, tellMaster = true, effectiveStorageLevel)
          arr.iterator.asInstanceOf[Iterator[T]]
        case Right(it) =>
          // There is not enough space to cache this partition in memory
          val returnValues = it.asInstanceOf[Iterator[T]]
          // 如果有些数据实在无法写入内存，那么就判断，数据是否有磁盘级别
          // 如果有的话，那么就使用磁盘级别，将数据写入磁盘文件
          if (putLevel.useDisk) {
            logWarning(s"Persisting partition $key to disk instead.")
            val diskOnlyLevel = StorageLevel(useDisk = true, useMemory = false,
              useOffHeap = false, deserialized = false, putLevel.replication)
            putInBlockManager[T](key, returnValues, level, updatedBlocks, Some(diskOnlyLevel))
          } else {
            returnValues
          }
      }
    }
  }

###org.apache.spark.storage/MemoryStore.scala

def unrollSafely(
      blockId: BlockId,
      values: Iterator[Any],
      droppedBlocks: ArrayBuffer[(BlockId, BlockStatus)])
    : Either[Array[Any], Iterator[Any]] = {

    // Number of elements unrolled so far
    var elementsUnrolled = 0
    // Whether there is still enough memory for us to continue unrolling this block
    var keepUnrolling = true
    // Initial per-thread memory to request for unrolling blocks (bytes). Exposed for testing.
    val initialMemoryThreshold = unrollMemoryThreshold
    // How often to check whether we need to request more memory
    val memoryCheckPeriod = 16
    // Memory currently reserved by this thread for this particular unrolling operation
    var memoryThreshold = initialMemoryThreshold
    // Memory to request as a multiple of current vector size
    val memoryGrowthFactor = 1.5
    // Previous unroll memory held by this thread, for releasing later (only at the very end)
    val previousMemoryReserved = currentUnrollMemoryForThisThread
    // Underlying vector for unrolling the block
    var vector = new SizeTrackingVector[Any]

    // Request enough memory to begin unrolling
    keepUnrolling = reserveUnrollMemoryForThisThread(initialMemoryThreshold)

    if (!keepUnrolling) {
      logWarning(s"Failed to reserve initial memory threshold of " +
        s"${Utils.bytesToString(initialMemoryThreshold)} for computing block $blockId in memory.")
    }

    // Unroll this block safely, checking whether we have exceeded our threshold periodically
    try {
      while (values.hasNext && keepUnrolling) {
        vector += values.next()
        if (elementsUnrolled % memoryCheckPeriod == 0) {
          // If our vector's size has exceeded the threshold, request more memory
          val currentSize = vector.estimateSize()
          if (currentSize >= memoryThreshold) {
            val amountToRequest = (currentSize * memoryGrowthFactor - memoryThreshold).toLong
            // Hold the accounting lock, in case another thread concurrently puts a block that
            // takes up the unrolling space we just ensured here
            accountingLock.synchronized {
              if (!reserveUnrollMemoryForThisThread(amountToRequest)) {
                // If the first request is not granted, try again after ensuring free space
                // If there is still not enough space, give up and drop the partition
                val spaceToEnsure = maxUnrollMemory - currentUnrollMemory
                // 反复循环，判断，只要还有数据没有写入内存，而且可以继续尝试往内存中写
                // 那么就判断，如果内存大小够不够存放数据，调用ensureFreeSpace()方法，尝试清空一些内存空间
                if (spaceToEnsure > 0) {
                  val result = ensureFreeSpace(blockId, spaceToEnsure)
                  droppedBlocks ++= result.droppedBlocks
                }
                keepUnrolling = reserveUnrollMemoryForThisThread(amountToRequest)
              }
            }
            // New threshold is currentSize * memoryGrowthFactor
            memoryThreshold += amountToRequest
          }
        }
        elementsUnrolled += 1
      }

      if (keepUnrolling) {
        // We successfully unrolled the entirety of this block
        Left(vector.toArray)
      } else {
        // We ran out of space while unrolling the values for this block
        logUnrollFailureMessage(blockId, vector.estimateSize())
        Right(vector.iterator ++ values)
      }

    } finally {
      // If we return an array, the values returned do not depend on the underlying vector and
      // we can immediately free up space for other threads. Otherwise, if we return an iterator,
      // we release the memory claimed by this thread later on when the task finishes.
      if (keepUnrolling) {
        val amountToRelease = currentUnrollMemoryForThisThread - previousMemoryReserved
        releaseUnrollMemoryForThisThread(amountToRelease)
      }
    }
  }