When should I repartition an RDD?

7
votes

I know that I can repartition an RDD to increase its partitions and use coalesce to decrease its partitions. I have two questions regarding this that I cannot completely understand after reading different resources.

Spark will use a sensible default (1 partition per block which is 64MB in first versions and now 128MB) when generating an RDD. But I also read that it is recommended to use 2 or 3 times the number of cores running the jobs. So here comes the question:

  1. How many partitions should I use for a given file? For example, suppose I have a 10GB .parquet file, 3 executors with 2 cores and 3gb memory each. Should I repartition? How many partitions should I use? What is the better way to make that choice?

  2. Are all data types (ie .txt, .parquet, etc..) repartitioned by default if no partitioning is provided?

1
apache-sparkrddpartitioning
As a rule of thumb, num of Partitions = num of Cores but you might want to oversubscribe. You'll find a lot of articles explaining why. Also, you can decrease the number of partitions with repartition as well. - philantrovert
If you are decreasing the number of partitions in this RDD, consider using coalesce, which can avoid performing a shuffle. - Gakuo

1 Answers

7
votes

Spark can run a single concurrent task for every partition of an RDD, up to the total number of cores in the cluster.

For example : 

val rdd= sc.textFile ("file.txt", 5)

The above line of code will create an RDD named textFile with 5 partitions.

Suppose that you have a cluster with 4 cores and assume that each partition needs to process for 5 minutes. In case of the above RDD with 5 partitions, 4 partition processes will run in parallel as there are 4 cores and the 5th partition process will process after 5 minutes when one of the 4 cores, is free.

The entire processing will be completed in 10 minutes and during the 5th partition process, the resources (remaining 3 cores) will remain idle.

The best way to decide on the number of partitions in a RDD is to make the number of partitions equal to the number of cores in the cluster so that all the partitions will process in parallel and the resources will be utilized in an optimal way.

Question : Are all data types (ie .txt, .parquet, etc..) repartitioned by default if no partitioning is provided?

There will be default no of partitions for every rdd. to check you can use rdd.partitions.length right after rdd created.

to use existing cluster resources in optimal way and to speed up, we have to consider re-partitioning to ensure that all cores are utilized and all partitions have enough number of records which are uniformly distributed.

For better understanding, also have a look at https://jaceklaskowski.gitbooks.io/mastering-apache-spark/spark-rdd-partitions.html

Note : There is no fixed formula for this. general convention most of them follow is

(numOf executors * no of cores) * replicationfactor (which may be 2 or 3 times more )