Github user mateiz commented on a diff in the pull request:
https://github.com/apache/spark/pull/1165#discussion_r15331032
--- 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)
+ // Memory currently reserved by this thread (bytes)
+ var memoryThreshold = initialMemoryThreshold
+ // Memory to request as a multiple of current vector size
+ val memoryGrowthFactor = 1.5
+
+ val threadId = Thread.currentThread().getId
+ val unrollMemoryMap = SparkEnv.get.unrollMemoryMap
+ var vector = new SizeTrackingVector[Any]
+
+ // Request memory for our vector and return whether request is granted.
+ // This involves synchronizing across all threads and can be expensive
if called frequently.
+ def requestMemory(memoryToRequest: Long): Boolean = {
+ unrollMemoryMap.synchronized {
--- End diff --
Can we access unrollMemoryMap using putLock as well? Otherwise there's a
risk of deadlocks if some methods grab the locks in one order, and some grab
them in the other order. I don't think that happens here but it would be better
to just remove the risk.
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