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https://issues.apache.org/jira/browse/SPARK-5140?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Patrick Wendell updated SPARK-5140:
-----------------------------------
Fix Version/s: (was: 1.2.1)
(was: 1.3.0)
> Two RDDs which are scheduled concurrently should be able to wait on parent in
> all cases
> ---------------------------------------------------------------------------------------
>
> Key: SPARK-5140
> URL: https://issues.apache.org/jira/browse/SPARK-5140
> Project: Spark
> Issue Type: New Feature
> Reporter: Corey J. Nolet
> Labels: features
>
> Not sure if this would change too much of the internals to be included in the
> 1.2.1 but it would be very helpful if it could be.
> This ticket is from a discussion between myself and [~ilikerps]. Here's the
> result of some testing that [~ilikerps] did:
> bq. I did some testing as well, and it turns out the "wait for other guy to
> finish caching" logic is on a per-task basis, and it only works on tasks that
> happen to be executing on the same machine.
> bq. Once a partition is cached, we will schedule tasks that touch that
> partition on that executor. The problem here, though, is that the cache is in
> progress, and so the tasks are still scheduled randomly (or with whatever
> locality the data source has), so tasks which end up on different machines
> will not see that the cache is already in progress.
> {code}
> Here was my test, by the way:
> import scala.concurrent.ExecutionContext.Implicits.global
> import scala.concurrent._
> import scala.concurrent.duration._
> val rdd = sc.parallelize(0 until 8).map(i => { Thread.sleep(10000); i
> }).cache()
> val futures = (0 until 4).map { _ => Future { rdd.count } }
> Await.result(Future.sequence(futures), 120.second)
> {code}
> bq. Note that I run the future 4 times in parallel. I found that the first
> run has all tasks take 10 seconds. The second has about 50% of its tasks take
> 10 seconds, and the rest just wait for the first stage to finish. The last
> two runs have no tasks that take 10 seconds; all wait for the first two
> stages to finish.
> What we want is the ability to fire off a job and have the DAG figure out
> that two RDDs depend on the same parent so that when the children are
> scheduled concurrently, the first one to start will activate the parent and
> both will wait on the parent. When the parent is done, they will both be able
> to finish their work concurrently. We are trying to use this pattern by
> having the parent cache results.
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