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Posted to issues@spark.apache.org by "Ryan Williams (JIRA)" <ji...@apache.org> on 2015/10/17 00:59:05 UTC
[jira] [Commented] (SPARK-11161) Viewing the web UI for the first
time unpersists a cached RDD
[ https://issues.apache.org/jira/browse/SPARK-11161?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14961501#comment-14961501 ]
Ryan Williams commented on SPARK-11161:
---------------------------------------
Digging in to the code a bit, I determined that this likely has to do with the timing of driver GC cycles and is specific to RDDs which I am not retaining a reference to in my {{spark-shell}}.
Is it intended that cached RDDs be unpersisted when user code no longer retains a reference to them? That appears to be a necessary condition for what I'm observing.
> Viewing the web UI for the first time unpersists a cached RDD
> -------------------------------------------------------------
>
> Key: SPARK-11161
> URL: https://issues.apache.org/jira/browse/SPARK-11161
> Project: Spark
> Issue Type: Bug
> Components: Spark Core, Web UI
> Affects Versions: 1.5.1
> Reporter: Ryan Williams
> Priority: Minor
>
> This one is a real head-scratcher. [Here's a screencast|http://f.cl.ly/items/0P0N413t1V3j2B0A3V1a/Screen%20Recording%202015-10-16%20at%2005.43%20PM.gif]:
> !http://f.cl.ly/items/0P0N413t1V3j2B0A3V1a/Screen%20Recording%202015-10-16%20at%2005.43%20PM.gif!
> The three windows, left-to-right, are:
> * a {{spark-shell}} on YARN with dynamic allocation enabled, at rest with one executor. [Here's an example app's environment|https://gist.github.com/ryan-williams/6dd3502d5d0de2f030ac].
> * [Spree|https://github.com/hammerlab/spree], opened to the above app's "Storage" tab.
> * my YARN resource manager, showing a link to the web UI running on the driver.
> At the start, nothing has been run in the shell, and I've not visited the web UI.
> I run a simple job in the shell and cache a small RDD that it computes:
> {code}
> sc.parallelize(1 to 100000000, 100).map(_ % 100 -> 1).reduceByKey(_+_, 100).setName("foo").cache.count
> {code}
> As the second stage runs, you can see the partitions show up as cached in Spree.
> After the job finishes, a few requested executors continue to fill in, which you can see in the console at left or the nav bar of Spree in the middle.
> Once that has finished, everything is at rest with the RDD "foo" 100% cached.
> Then, I click the YARN RM's "ApplicationMaster" link which loads the web UI on the driver for the first time.
> Immediately, the console prints some activity, including that RDD 2 has been removed:
> {code}
> 15/10/16 21:43:12 INFO storage.BlockManagerInfo: Removed broadcast_1_piece0 on 172.29.46.15:33156 in memory (size: 1517.0 B, free: 7.2 GB)
> 15/10/16 21:43:12 INFO storage.BlockManagerInfo: Removed broadcast_1_piece0 on demeter-csmaz10-17.demeter.hpc.mssm.edu:56997 in memory (size: 1517.0 B, free: 12.2 GB)
> 15/10/16 21:43:13 INFO spark.ContextCleaner: Cleaned accumulator 2
> 15/10/16 21:43:13 INFO storage.BlockManagerInfo: Removed broadcast_0_piece0 on 172.29.46.15:33156 in memory (size: 1666.0 B, free: 7.2 GB)
> 15/10/16 21:43:13 INFO storage.BlockManagerInfo: Removed broadcast_0_piece0 on demeter-csmaz10-17.demeter.hpc.mssm.edu:56997 in memory (size: 1666.0 B, free: 12.2 GB)
> 15/10/16 21:43:13 INFO spark.ContextCleaner: Cleaned accumulator 1
> 15/10/16 21:43:13 INFO spark.ContextCleaner: Cleaned shuffle 0
> 15/10/16 21:43:13 INFO storage.BlockManager: Removing RDD 2
> 15/10/16 21:43:13 INFO spark.ContextCleaner: Cleaned RDD 2
> {code}
> Accordingly, Spree shows that the RDD has been unpersisted, and I can see in the event log (not pictured in the screencast) that an Unpersist event has made its way through the various SparkListeners:
> {code}
> {"Event":"SparkListenerUnpersistRDD","RDD ID":2}
> {code}
> Simply loading the web UI causes an RDD unpersist event to fire!
> I can't nail down exactly what's causing this, and I've seen evidence that there are other sequences of events that can also cause it:
> * I've repro'd the above steps ~20 times. The RDD always gets unpersisted when I've not visited the web UI until the RDD is cached, and when the app is dynamically allocating executors.
> * One time, I observed the unpersist to fire without my even visiting the web UI at all. Other times I wait a long time before visiting the web UI, so that it is clear that the loading of the web UI is causal, and it always is, but apparently there's another way for the unpersist to happen, seemingly rarely, without visiting the web UI.
> * I tried a couple of times without dynamic allocation and could not reproduce it.
> * I've tried a couple of times with dynamic allocation and starting with a higher minimum number of executors than 1 and have been unable to reproduce it.
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