Github user mateiz commented on a diff in the pull request: https://github.com/apache/spark/pull/1165#discussion_r15331245 --- Diff: core/src/main/scala/org/apache/spark/storage/MemoryStore.scala --- @@ -141,6 +189,104 @@ private class MemoryStore(blockManager: BlockManager, maxMemory: Long) } /** + * Unroll the given block in memory safely. + * + * The safety of this operation refers to avoiding potential OOM exceptions caused by + * unrolling the entirety of the block in memory at once. This is achieved by periodically + * checking whether the memory restrictions for unrolling blocks are still satisfied, + * stopping immediately if not. This check is a safeguard against the scenario in which + * there is not enough free memory to accommodate the entirety of a single block. + * + * This method returns either an array with the contents of the entire block or an iterator + * containing the values of the block (if the array would have exceeded available memory). + */ + 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 = conf.getLong("spark.storage.unrollMemoryThreshold", 1024 * 1024) + // How often to check whether we need to request more memory. Exposed for testing. + val memoryCheckPeriod = conf.getLong("spark.storage.unrollCheckPeriod", 16) --- End diff -- BTW this would also make the tests slightly more realistic in that there will be fewer differences between them and real-world operation.
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