sri hari kali charan Tummala sri hari kali charan Tummala - 3 months ago 13
Scala Question

difference between rdd.collect().toMap to rdd.collectAsMap()?

Is there any performance impact when I use collectAsMap on my RDD instead of rdd.collect().toMap ?

I have a key value rdd and I want to convert to HashMap as far I know collect() is not efficient on large data sets as it runs on driver can I use collectAsMap instead is there any performance impact ?

Original:

val QuoteHashMap=QuoteRDD.collect().toMap
val QuoteRDDData=QuoteHashMap.values.toSeq
val QuoteRDDSet=sc.parallelize(QuoteRDDData.map(x => x.toString.replace("(","").replace(")","")))
QuoteRDDSet.saveAsTextFile(Quotepath)


Change:

val QuoteHashMap=QuoteRDD.collectAsMap()
val QuoteRDDData=QuoteHashMap.values.toSeq
val QuoteRDDSet=sc.parallelize(QuoteRDDData.map(x => x.toString.replace("(","").replace(")","")))
QuoteRDDSet.saveAsTextFile(Quotepath)

Answer

The implementation of collectAsMap is the following

def collectAsMap(): Map[K, V] = self.withScope {
    val data = self.collect()
    val map = new mutable.HashMap[K, V]
    map.sizeHint(data.length)
    data.foreach { pair => map.put(pair._1, pair._2) }
    map
  }

Thus, there is no performance difference between collect and collectAsMap, because collectAsMap calls under the hood also collect.