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https://issues.apache.org/jira/browse/SPARK-18608?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15893596#comment-15893596
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zhengruifeng commented on SPARK-18608:
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[~mlnick] [~yuhaoyan] [~srowen] I think if we use {{train(dataset: Dataset[_],
handlePersistence: Boolean)}} instead of {{train(dataset: Dataset[_])}} may
result in extra problems for external implementers, because the existing
external algorithms overriding {{Predictor.train}} will not work.
I think we can do it in another way:
{code}
abstract class Predictor[
FeaturesType,
Learner <: Predictor[FeaturesType, Learner, M],
M <: PredictionModel[FeaturesType, M]]
extends Estimator[M] with PredictorParams {
protected var storageLevel = StorageLevel.NONE //
override def fit(dataset: Dataset[_]): M = {
storageLevel = dataset.storageLevel
...
}
protected def train(dataset: Dataset[_]): M
{code}
so in algorithm implementations we can use the orignial storageLevel of the
input dataset.
> Spark ML algorithms that check RDD cache level for internal caching
> double-cache data
> -------------------------------------------------------------------------------------
>
> Key: SPARK-18608
> URL: https://issues.apache.org/jira/browse/SPARK-18608
> Project: Spark
> Issue Type: Bug
> Components: ML
> Reporter: Nick Pentreath
>
> Some algorithms in Spark ML (e.g. {{LogisticRegression}},
> {{LinearRegression}}, and I believe now {{KMeans}}) handle persistence
> internally. They check whether the input dataset is cached, and if not they
> cache it for performance.
> However, the check is done using {{dataset.rdd.getStorageLevel == NONE}}.
> This will actually always be true, since even if the dataset itself is
> cached, the RDD returned by {{dataset.rdd}} will not be cached.
> Hence if the input dataset is cached, the data will end up being cached
> twice, which is wasteful.
> To see this:
> {code}
> scala> import org.apache.spark.storage.StorageLevel
> import org.apache.spark.storage.StorageLevel
> scala> val df = spark.range(10).toDF("num")
> df: org.apache.spark.sql.DataFrame = [num: bigint]
> scala> df.storageLevel == StorageLevel.NONE
> res0: Boolean = true
> scala> df.persist
> res1: df.type = [num: bigint]
> scala> df.storageLevel == StorageLevel.MEMORY_AND_DISK
> res2: Boolean = true
> scala> df.rdd.getStorageLevel == StorageLevel.MEMORY_AND_DISK
> res3: Boolean = false
> scala> df.rdd.getStorageLevel == StorageLevel.NONE
> res4: Boolean = true
> {code}
> Before SPARK-16063, there was no way to check the storage level of the input
> {{DataSet}}, but now we can, so the checks should be migrated to use
> {{dataset.storageLevel}}.
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