How does Round Robin partitioning in Spark work?

Question

I've trouble to understand Round Robin Partitioning in Spark. Consider the following exampl. I split a Seq of size 3 into 3 partitions:

val df = Seq(0,1,2).toDF().repartition(3)

df.explain

== Physical Plan ==
Exchange RoundRobinPartitioning(3)
+- LocalTableScan [value#42]

Now if I inspect the partitions, I get:

df
  .rdd
  .mapPartitionsWithIndex{case (i,rows) => Iterator((i,rows.size))}
  .toDF("partition_index","number_of_records")
  .show

+---------------+-----------------+
|partition_index|number_of_records|
+---------------+-----------------+
|              0|                0|
|              1|                2|
|              2|                1|
+---------------+-----------------+

If I do the same with Seq of size 8 and split it into 8 partitions, I get even worse skew:

(0 to 7).toDF().repartition(8)
  .rdd
  .mapPartitionsWithIndex{case (i,rows) => Iterator((i,rows.size))}
  .toDF("partition_index","number_of_records")
  .show

+---------------+-----------------+
|partition_index|number_of_records|
+---------------+-----------------+
|              0|                0|
|              1|                0|
|              2|                0|
|              3|                0|
|              4|                0|
|              5|                0|
|              6|                4|
|              7|                4|
+---------------+-----------------+

Can somebody explain this behavior. As far as I understand round robin partitioning, all partitions show be ~same size.

Answer 1

I can't explain why but somehow it is link to the local master.

if you explicit set :

• --master local => 1 row per partition (no parallelism)

• --master "local[2]" => 2 rows per partition (4 partitions empty)

• --master "local[4]" => 4 rows per partition (6 partitions empty)

• --master "local[8]" => 8 rows per partition (7 partitions empty)

Answer 2

(Checked for Spark version 2.1-2.4)

As far as I can see from ShuffleExchangeExec code, Spark tries to partition the rows directly from original partitions (via mapPartitions) without bringing anything to the driver.

The logic is to start with a randomly picked target partition and then assign partitions to the rows in a round-robin method. Note that "start" partition is picked for each source partition and there could be collisions.

The final distribution depends on many factors: a number of source/target partitions and the number of rows in your dataframe.