Github user mateiz commented on a diff in the pull request:
https://github.com/apache/spark/pull/1165#discussion_r15435454
--- Diff: core/src/main/scala/org/apache/spark/storage/MemoryStore.scala ---
@@ -141,6 +193,104 @@ 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
+ 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
--- End diff --
Maybe add a "Note that during this call to unrollSafely(), nothing else in
this thread can be releasing memory, so it's okay to remember the value here".
At first I thought there might be a concurrency bug if someone else changes
unrollMemoryMap(thisThread).
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