I think we have a version of mapPartitions that allows you to tell Spark the partitioning is preserved:
https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/rdd/RDD.scala#L639 We could also add a map function that does same. Or you can just write your map using an iterator. - Patrick On Thu, Mar 26, 2015 at 3:07 PM, Jonathan Coveney <jcove...@gmail.com> wrote: > This is just a deficiency of the api, imo. I agree: mapValues could > definitely be a function (K, V)=>V1. The option isn't set by the function, > it's on the RDD. So you could look at the code and do this. > https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/rdd/RDD.scala > > def mapValues[U](f: V => U): RDD[(K, U)] = { > val cleanF = self.context.clean(f) > new MapPartitionsRDD[(K, U), (K, V)](self, > (context, pid, iter) => iter.map { case (k, v) => (k, cleanF(v)) }, > preservesPartitioning = true) > } > > What you want: > > def mapValues[U](f: (K, V) => U): RDD[(K, U)] = { > val cleanF = self.context.clean(f) > new MapPartitionsRDD[(K, U), (K, V)](self, > (context, pid, iter) => iter.map { case t@(k, _) => (k, cleanF(t)) }, > preservesPartitioning = true) > } > > One of the nice things about spark is that making such new operators is very > easy :) > > 2015-03-26 17:54 GMT-04:00 Zhan Zhang <zzh...@hortonworks.com>: > >> Thanks Jonathan. You are right regarding rewrite the example. >> >> I mean providing such option to developer so that it is controllable. The >> example may seems silly, and I don't know the use cases. >> >> But for example, if I also want to operate both the key and value part to >> generate some new value with keeping key part untouched. Then mapValues may >> not be able to do this. >> >> Changing the code to allow this is trivial, but I don't know whether there >> is some special reason behind this. >> >> Thanks. >> >> Zhan Zhang >> >> >> >> >> On Mar 26, 2015, at 2:49 PM, Jonathan Coveney <jcove...@gmail.com> wrote: >> >> I believe if you do the following: >> >> >> sc.parallelize(List(1,2,3,4,5,5,6,6,7,8,9,10,2,4)).map((_,1)).reduceByKey(_+_).mapValues(_+1).reduceByKey(_+_).toDebugString >> >> (8) MapPartitionsRDD[34] at reduceByKey at <console>:23 [] >> | MapPartitionsRDD[33] at mapValues at <console>:23 [] >> | ShuffledRDD[32] at reduceByKey at <console>:23 [] >> +-(8) MapPartitionsRDD[31] at map at <console>:23 [] >> | ParallelCollectionRDD[30] at parallelize at <console>:23 [] >> >> The difference is that spark has no way to know that your map closure >> doesn't change the key. if you only use mapValues, it does. Pretty cool that >> they optimized that :) >> >> 2015-03-26 17:44 GMT-04:00 Zhan Zhang <zzh...@hortonworks.com>: >>> >>> Hi Folks, >>> >>> Does anybody know what is the reason not allowing preserverPartitioning >>> in RDD.map? Do I miss something here? >>> >>> Following example involves two shuffles. I think if preservePartitioning >>> is allowed, we can avoid the second one, right? >>> >>> val r1 = sc.parallelize(List(1,2,3,4,5,5,6,6,7,8,9,10,2,4)) >>> val r2 = r1.map((_, 1)) >>> val r3 = r2.reduceByKey(_+_) >>> val r4 = r3.map(x=>(x._1, x._2 + 1)) >>> val r5 = r4.reduceByKey(_+_) >>> r5.collect.foreach(println) >>> >>> scala> r5.toDebugString >>> res2: String = >>> (8) ShuffledRDD[4] at reduceByKey at <console>:29 [] >>> +-(8) MapPartitionsRDD[3] at map at <console>:27 [] >>> | ShuffledRDD[2] at reduceByKey at <console>:25 [] >>> +-(8) MapPartitionsRDD[1] at map at <console>:23 [] >>> | ParallelCollectionRDD[0] at parallelize at <console>:21 [] >>> >>> Thanks. >>> >>> Zhan Zhang >>> >>> --------------------------------------------------------------------- >>> To unsubscribe, e-mail: user-unsubscr...@spark.apache.org >>> For additional commands, e-mail: user-h...@spark.apache.org >>> >> >> > --------------------------------------------------------------------- To unsubscribe, e-mail: user-unsubscr...@spark.apache.org For additional commands, e-mail: user-h...@spark.apache.org
