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https://issues.apache.org/jira/browse/SPARK-39931?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17573655#comment-17573655
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Enrico Minack commented on SPARK-39931:
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[~hyukjin.kwon] [~zero323] [~ruifengz] [~itholic]
> Improve performance of applyInPandas for very small groups
> ----------------------------------------------------------
>
> Key: SPARK-39931
> URL: https://issues.apache.org/jira/browse/SPARK-39931
> Project: Spark
> Issue Type: New Feature
> Components: PySpark
> Affects Versions: 3.4.0
> Reporter: Enrico Minack
> Priority: Major
>
> Calling `DataFrame.groupby(...).applyInPandas(...)` for very small groups in
> PySpark is very slow. The reason is that for each group, PySpark creates a
> Pandas DataFrame and calls into the Python code. For very small groups, the
> overhead is huge, for large groups, it is smaller.
> Here is a benchmarks (sendonds to groupBy(...).applyInPandas(...) 10m rows):
> ||groupSize||Scala||pyspark.sql||pyspark.pandas||
> |1024|8.9|20.9|7.8|
> |512|9.4|31.8|9.8|
> |256|9.3|47.0|20.2|
> |128|9.5|83.3|48.8|
> |64|9.5|137.8|91.9|
> |32|9.6|263.6|207.3|
> |16|9.6|525.9|261.5|
> |8|9.5|1,043|663.0|
> |4|9.8|2,073|1,168|
> |2|10.4|4,132|2,456|
> |1|11.3|8,162|4,642|
> *Idea to overcome this* is to call into Python side with a Pandas DataFrame
> that contains potentially multiple groups, then perform a Pandas
> DataFrame.groupBy(...).apply(...). With large groups, that Panadas DataFrame
> has all rows of single group, with small groups it contains many groups. This
> should improve efficiency.
> I have prepared a PoC to benchmark that idea but am struggling to massage the
> internal Rows before sending them to Python. The idea is to turn the
> {{Dataset[T]}} into {{Dataset[(K, T)]}}, create a bespoke {{GroupedIterator}}
> that splits the Dataset into chunks that contain complete groups, sends them
> to Python, which the groups by key and apply the udf on the values. The first
> bit seems to be the hard bit:
> {{sql/core/src/main/scala/org/apache/spark/sql/execution/python/FlatMapGroupsInPandasExec.scala}}:
> {code:scala}
> val inputRDD = ProjectExec(Seq(
> Alias(CreateNamedStruct(groupingAttributes.flatMap(a =>
> Literal(a.name) :: a :: Nil)), "key")(),
> Alias(CreateNamedStruct(dedupAttributes.flatMap(a => Literal(a.name)
> :: a :: Nil)), "val")()
> ), child).execute()
> {code}
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