Github user colorant commented on a diff in the pull request:
https://github.com/apache/spark/pull/1165#discussion_r15153652
--- Diff: core/src/main/scala/org/apache/spark/storage/MemoryStore.scala ---
@@ -141,6 +188,88 @@ 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]] = {
+
+ var count = 0 // The number of elements unrolled
so far
+ var atMemoryLimit = false // Whether we are at the memory
limit for unrolling blocks
+ var previousSize = 0L // Previous estimate of the size of
our buffer
+ val memoryRequestPeriod = 1000 // How frequently we request more
memory for our buffer
+ val memoryRequestThreshold = 100 // Before this is exceeded, request
memory every round
+
+ val threadId = Thread.currentThread().getId
+ val unrollMemoryMap = SparkEnv.get.unrollMemoryMap
+ var buffer = new SizeTrackingAppendOnlyBuffer[Any]
+
+ try {
+ while (values.hasNext && !atMemoryLimit) {
+ buffer += values.next()
+ count += 1
+ if (count % memoryRequestPeriod == 1 || count <
memoryRequestThreshold) {
+ // Calculate the amount of memory to request from the global
memory pool
+ val currentSize = buffer.estimateSize()
+ val delta = if (previousSize > 0) math.max(currentSize -
previousSize, 0) else 0
+ val memoryToRequest = currentSize + delta
+ previousSize = currentSize
+
+ // Atomically check whether there is sufficient memory in the
global pool to continue
+ unrollMemoryMap.synchronized {
+ val previouslyOccupiedMemory =
unrollMemoryMap.get(threadId).getOrElse(0L)
+ val otherThreadsMemory = unrollMemoryMap.values.sum -
previouslyOccupiedMemory
+
+ // Request memory for the local buffer and return whether
request is granted
+ def requestMemory(): Boolean = {
+ val availableMemory = freeMemory - otherThreadsMemory
+ val granted = availableMemory > memoryToRequest
+ if (granted) { unrollMemoryMap(threadId) = memoryToRequest }
+ granted
+ }
+
+ // 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
+ if (!requestMemory()) {
+ val result = ensureFreeSpace(blockId, globalBufferMemory)
--- End diff --
Is the globalBufferMemory the total memory threshold for unroll? If it is,
should this be globalBufferMemory - unrollMemoryMap.values.sum - freeMemory or
something similar?
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