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From Koert Kuipers <ko...@tresata.com>
Subject dataset algos slow because of too many shuffles
Date Fri, 03 Feb 2017 05:49:14 GMT
we noticed that some algos we ported from rdd to dataset are significantly
slower, and the main reason seems to be more shuffles that we successfully
avoid for rdds by careful partitioning. this seems to be dataset specific
as it works ok for dataframe.

see also here:
http://blog.hydronitrogen.com/2016/05/13/shuffle-free-joins-in-spark-sql/

it kind of boils down to this... if i partition and sort dataframes that
get used for joins repeatedly i can avoid shuffles:

System.setProperty("spark.sql.autoBroadcastJoinThreshold", "-1")

val df1 = Seq((0, 0), (1, 1)).toDF("key", "value")

.repartition(col("key")).sortWithinPartitions(col("key")).persist(StorageLevel.DISK_ONLY)
val df2 = Seq((0, 0), (1, 1)).toDF("key2", "value2")

.repartition(col("key2")).sortWithinPartitions(col("key2")).persist(StorageLevel.DISK_ONLY)

val joined = df1.join(df2, col("key") === col("key2"))
joined.explain

== Physical Plan ==
*SortMergeJoin [key#5], [key2#27], Inner
:- InMemoryTableScan [key#5, value#6]
:     +- InMemoryRelation [key#5, value#6], true, 10000, StorageLevel(disk,
1 replicas)
:           +- *Sort [key#5 ASC NULLS FIRST], false, 0
:              +- Exchange hashpartitioning(key#5, 4)
:                 +- LocalTableScan [key#5, value#6]
+- InMemoryTableScan [key2#27, value2#28]
      +- InMemoryRelation [key2#27, value2#28], true, 10000,
StorageLevel(disk, 1 replicas)
            +- *Sort [key2#27 ASC NULLS FIRST], false, 0
               +- Exchange hashpartitioning(key2#27, 4)
                  +- LocalTableScan [key2#27, value2#28]

notice how the persisted dataframes are not shuffled or sorted anymore
before being used in the join. however if i try to do the same with dataset
i have no luck:

val ds1 = Seq((0, 0), (1, 1)).toDS

.repartition(col("_1")).sortWithinPartitions(col("_1")).persist(StorageLevel.DISK_ONLY)
val ds2 = Seq((0, 0), (1, 1)).toDS

.repartition(col("_1")).sortWithinPartitions(col("_1")).persist(StorageLevel.DISK_ONLY)

val joined1 = ds1.joinWith(ds2, ds1("_1") === ds2("_1"))
joined1.explain

== Physical Plan ==
*SortMergeJoin [_1#105._1], [_2#106._1], Inner
:- *Sort [_1#105._1 ASC NULLS FIRST], false, 0
:  +- Exchange hashpartitioning(_1#105._1, 4)
:     +- *Project [named_struct(_1, _1#83, _2, _2#84) AS _1#105]
:        +- InMemoryTableScan [_1#83, _2#84]
:              +- InMemoryRelation [_1#83, _2#84], true, 10000,
StorageLevel(disk, 1 replicas)
:                    +- *Sort [_1#83 ASC NULLS FIRST], false, 0
:                       +- Exchange hashpartitioning(_1#83, 4)
:                          +- LocalTableScan [_1#83, _2#84]
+- *Sort [_2#106._1 ASC NULLS FIRST], false, 0
   +- Exchange hashpartitioning(_2#106._1, 4)
      +- *Project [named_struct(_1, _1#100, _2, _2#101) AS _2#106]
         +- InMemoryTableScan [_1#100, _2#101]
               +- InMemoryRelation [_1#100, _2#101], true, 10000,
StorageLevel(disk, 1 replicas)
                     +- *Sort [_1#83 ASC NULLS FIRST], false, 0
                        +- Exchange hashpartitioning(_1#83, 4)
                           +- LocalTableScan [_1#83, _2#84]

notice how my persisted Datasets are shuffled and sorted again. part of the
issue seems to be in joinWith, which does some preprocessing that seems to
confuse the planner. if i change the joinWith to join (which returns a
dataframe) it looks a little better in that only one side gets shuffled
again, but still not optimal:

val ds1 = Seq((0, 0), (1, 1)).toDS

.repartition(col("_1")).sortWithinPartitions(col("_1")).persist(StorageLevel.DISK_ONLY)
val ds2 = Seq((0, 0), (1, 1)).toDS

.repartition(col("_1")).sortWithinPartitions(col("_1")).persist(StorageLevel.DISK_ONLY)

val joined1 = ds1.join(ds2, ds1("_1") === ds2("_1"))
joined1.explain

== Physical Plan ==
*SortMergeJoin [_1#83], [_1#100], Inner
:- InMemoryTableScan [_1#83, _2#84]
:     +- InMemoryRelation [_1#83, _2#84], true, 10000, StorageLevel(disk, 1
replicas)
:           +- *Sort [_1#83 ASC NULLS FIRST], false, 0
:              +- Exchange hashpartitioning(_1#83, 4)
:                 +- LocalTableScan [_1#83, _2#84]
+- *Sort [_1#100 ASC NULLS FIRST], false, 0
   +- Exchange hashpartitioning(_1#100, 4)
      +- InMemoryTableScan [_1#100, _2#101]
            +- InMemoryRelation [_1#100, _2#101], true, 10000,
StorageLevel(disk, 1 replicas)
                  +- *Sort [_1#83 ASC NULLS FIRST], false, 0
                     +- Exchange hashpartitioning(_1#83, 4)
                        +- LocalTableScan [_1#83, _2#84]

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