RDD
Parallelism
Serialization and GC
References
Don’t collect large RDDs
When a collect operation is issued on a RDD, the dataset is copied to the driver, i.e. the master node. A memory exception will be thrown if the dataset is too large to fit in memory;
takeor takeSamplecan be used to retrieve only a capped number of elements instead.1
val parallel = sc.parallelize(1 to 9)
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val parts = parallel.partitions
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and then create a smaller rdd filtering out everything but a single partition. Collect the data from smaller rdd and iterate over values of a single partition:
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for(p <- parts){
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val idx = p.index
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val partRDD = parallel.mapPartitionsWithIndex((index: Int, it: Iterator[Int]) => if(index == idx) it else Iterator(), true)
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val data = partRDD.collect
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// Data contains all values from a single partition in the form of array.
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// Now you can do with the data whatever you want: iterate, save to a file, etc.
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}
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// You can use also the foreachPartition operation
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parallel.foreachPartition(partition => {
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partition.toArray
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// Your code
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})
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Of cause, it will work only if the partitions are small enough. If they aren't, you can always increase the number of partitions with
rdd.coalesce(numParts, true).Copy link