Github user rxin commented on a diff in the pull request:
https://github.com/apache/spark/pull/1083#discussion_r13779767
--- Diff: core/src/main/scala/org/apache/spark/CacheManager.scala ---
@@ -128,4 +76,89 @@ private[spark] class CacheManager(blockManager:
BlockManager) extends Logging {
}
}
}
+
+ /**
+ * Acquire a loading lock for the partition identified by the given
block ID.
+ *
+ * If the lock is free, just acquire it and return None. Otherwise,
another thread is already
+ * loading the partition, so we wait for it to finish and return the
values loaded by the thread.
+ */
+ private def acquireLockForPartition(id: RDDBlockId):
Option[Iterator[Any]] = {
+ loading.synchronized {
+ if (!loading.contains(id)) {
+ // If the partition is free, acquire its lock and begin computing
its value
+ loading.add(id)
+ None
+ } else {
+ // Otherwise, wait for another thread to finish and return its
result
+ logInfo(s"Another thread is loading $id, waiting for it to
finish...")
+ while (loading.contains(id)) {
+ try {
+ loading.wait()
+ } catch {
+ case e: Exception =>
+ logWarning(s"Exception while waiting for another thread to
load $id", e)
+ }
+ }
+ logInfo(s"Finished waiting for $id")
+ /* See whether someone else has successfully loaded it. The main
way this would fail
+ * is for the RDD-level cache eviction policy if someone else has
loaded the same RDD
+ * partition but we didn't want to make space for it. However,
that case is unlikely
+ * because it's unlikely that two threads would work on the same
RDD partition. One
+ * downside of the current code is that threads wait serially if
this does happen. */
+ val values = blockManager.get(id)
+ if (!values.isDefined) {
+ logInfo(s"Whoever was loading $id failed; we'll try it
ourselves")
+ loading.add(id)
+ }
+ values
+ }
+ }
+ }
+
+ /**
+ * Cache the values of a partition, keeping track of any updates in the
storage statuses
+ * of other blocks along the way.
+ */
+ private def cacheValues[T](
+ key: BlockId,
+ value: Iterator[T],
+ storageLevel: StorageLevel,
+ updatedBlocks: ArrayBuffer[(BlockId, BlockStatus)]): Iterator[T] = {
+
+ if (!storageLevel.useMemory) {
+ /* This RDD is not to be cached in memory, so we can just pass the
computed values
+ * as an iterator directly to the BlockManager, rather than first
fully unrolling
+ * it in memory. The latter option potentially uses much more memory
and risks OOM
+ * exceptions that can be avoided. */
+ assume(storageLevel.useDisk || storageLevel.useOffHeap, s"Empty
storage level for $key!")
--- End diff --
Might make sense to remove this assume; in case we add a new storage level
in the future, this won't hold any more and because this code is so far away
from the storage level code, we will likely forget to update this location.
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