Github user cloud-fan commented on a diff in the pull request:
https://github.com/apache/spark/pull/1165#discussion_r17518799
--- Diff: core/src/main/scala/org/apache/spark/storage/MemoryStore.scala ---
@@ -141,6 +193,93 @@ 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
+ 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)
+
+ // 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 -
1)).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
--- End diff --
but in "reserveUnrollMemoryForThisThread", we don't ensure unrollMemory is
smaller than maxUnrollMemory. So if there is enough free memory, we can still
put a block exceed the size limit(maxUnrollMemory). Is that what we want?
---
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