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https://issues.apache.org/jira/browse/SPARK-29856?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Sean R. Owen resolved SPARK-29856.
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Resolution: Duplicate
> Conditional unnecessary persist on RDDs in ML algorithms
> --------------------------------------------------------
>
> Key: SPARK-29856
> URL: https://issues.apache.org/jira/browse/SPARK-29856
> Project: Spark
> Issue Type: Improvement
> Components: ML, MLlib
> Affects Versions: 3.0.0
> Reporter: Dong Wang
> Priority: Major
>
> When I run example.ml.GradientBoostedTreeRegressorExample, I find that RDD
> _{color:#DE350B}baggedInput{color}_ in _ml.tree.impl.RandomForest.run()_ is
> persisted, but it only used once. So this persist operation is unnecessary.
> {code:scala}
> val baggedInput = BaggedPoint
> .convertToBaggedRDD(treeInput, strategy.subsamplingRate, numTrees,
> withReplacement,
> (tp: TreePoint) => tp.weight, seed = seed)
> .persist(StorageLevel.MEMORY_AND_DISK)
> ...
> while (nodeStack.nonEmpty) {
> ...
> timer.start("findBestSplits")
> RandomForest.findBestSplits(baggedInput, metadata, topNodesForGroup,
> nodesForGroup,
> treeToNodeToIndexInfo, splits, nodeStack, timer, nodeIdCache)
> timer.stop("findBestSplits")
> }
> baggedInput.unpersist()
> {code}
> However, the action on {color:#DE350B}_baggedInput_{color} is in a while
> loop.
> In GradientBoostedTreeRegressorExample, this loop only executes once, so only
> one action uses {color:#DE350B}_baggedInput_{color}.
> In most of ML applications, the loop will executes for many times, which
> means {color:#DE350B}_baggedInput_{color} will be used in many actions. So
> the persist is necessary now.
> That's the point why the persist operation is "conditional" unnecessary.
> Same situations exist in many other ML algorithms, e.g., RDD
> {color:#DE350B}_instances_{color} in ml.clustering.KMeans.fit(), RDD
> {color:#DE350B}_indices_{color} in mllib.clustering.BisectingKMeans.run().
> This issue is reported by our tool CacheCheck, which is used to dynamically
> detecting persist()/unpersist() api misuses.
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