Github user rxin commented on a diff in the pull request:
https://github.com/apache/spark/pull/1083#discussion_r13779775
--- 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
--- End diff --
This paragraph doesn't make a lot of sense to me. In general it is just
unlikely for two threads to work on the same rdd partition. However, if we ever
pass the first if (where it returns None), it already means we are in the
"unlikely" case.
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