spark core 2.0 BlockManager dropFromMemory

从内存中丢弃一个数据块，如果可以，可能把数据块放到磁盘上。 当内存到达限制并且需要释放空间时调用。

/**   * Drop a block from memory, possibly putting it on disk if applicable. Called when the memory   * store reaches its limit and needs to free up space.   *   * If `data` is not put on disk, it won't be created.   *   * The caller of this method must hold a write lock on the block before calling this method.   * This method does not release the write lock.   *   * @return the block's new effective StorageLevel.   */  private[storage] override def dropFromMemory[T: ClassTag](      blockId: BlockId,      data: () => Either[Array[T], ChunkedByteBuffer]): StorageLevel = {    logInfo(s"Dropping block $blockId from memory")    val info = blockInfoManager.assertBlockIsLockedForWriting(blockId)    var blockIsUpdated = false    val level = info.level    // Drop to disk, if storage level requires    if (level.useDisk && !diskStore.contains(blockId)) {      logInfo(s"Writing block $blockId to disk")      data() match {        case Left(elements) =>          diskStore.put(blockId) { fileOutputStream =>            serializerManager.dataSerializeStream(              blockId,              fileOutputStream,              elements.toIterator)(info.classTag.asInstanceOf[ClassTag[T]])          }        case Right(bytes) =>          diskStore.putBytes(blockId, bytes)      }      blockIsUpdated = true    }    // Actually drop from memory store    val droppedMemorySize =      if (memoryStore.contains(blockId)) memoryStore.getSize(blockId) else 0L    val blockIsRemoved = memoryStore.remove(blockId)    if (blockIsRemoved) {      blockIsUpdated = true    } else {      logWarning(s"Block $blockId could not be dropped from memory as it does not exist")    }    val status = getCurrentBlockStatus(blockId, info)    if (info.tellMaster) {      reportBlockStatus(blockId, status, droppedMemorySize)    }    if (blockIsUpdated) {      addUpdatedBlockStatusToTaskMetrics(blockId, status)    }    status.storageLevel  }

/**   * Return the updated storage status of the block with the given ID. More specifically, if   * the block is dropped from memory and possibly added to disk, return the new storage level   * and the updated in-memory and on-disk sizes.   */  private def getCurrentBlockStatus(blockId: BlockId, info: BlockInfo): BlockStatus = {    info.synchronized {      info.level match {        case null =>          BlockStatus.empty        case level =>          val inMem = level.useMemory && memoryStore.contains(blockId)          val onDisk = level.useDisk && diskStore.contains(blockId)          val deserialized = if (inMem) level.deserialized else false          val replication = if (inMem  || onDisk) level.replication else 1          val storageLevel = StorageLevel(            useDisk = onDisk,            useMemory = inMem,            useOffHeap = level.useOffHeap,            deserialized = deserialized,            replication = replication)          val memSize = if (inMem) memoryStore.getSize(blockId) else 0L          val diskSize = if (onDisk) diskStore.getSize(blockId) else 0L          BlockStatus(storageLevel, memSize, diskSize)      }    }  }