There is an interesting bug that was found during the latest performance tuning we performed for Spark 2.2 (2.3 is also affected). It was a batch Spark job scheduled to be executed hourly and to process about 1Tb worth of data stored in parquet format. There were a number of things we tuned and this resulted in around 10% performance boost.
We were looking for known issues and found explode related issue SPARK-21657. Although explode was used in our job it didn’t cause any issues due to the low number of exploding rows. But this forced us to take a closer look into wholeStage codegen. Simple test with disabled
spark.sql.codegen.wholeStage resulted in 30-40% time savings.
We spent another day working on each piece of code and performed a test case with nothing but sum aggregations.
val cnt = 50 val rows = 5000000 val dummy = udf(() => 1) def addConstColumns(inputDF: DataFrame) = (0 until cnt).foldLeft(inputDF)((df, idx) => df.withColumn(s"col_$idx", dummy())) spark.range(rows).toDF() .withColumn("grp", lit(1)) .transform(addConstColumns) .groupBy("grp") .agg(sum("col_0"), (1 until cnt).map(idx => sum(s"col_$idx")): _*) .collect()
This code creates a dataframe with 50 columns that are sum aggregated. I did this test locally on a laptop (HotSpot JVM) with
WHOLESTAGE_CODEGEN_ENABLED=true/false and noticed that timing was about 15 sec vs 3 sec.
The result was it created a Spark Jira issue SPARK-23791.
Apparently I found one very similar SPARK-20184 and SPARK-20479 issues targeted Spark 2.4.
After a series of tests we also found that with a low number of cnt <= 13 and cnt>=100 there were no noticeable differences; with 14 <= cnt <= 83 it’s about a 4-7 time difference and finally with 84 <= cnt <= 99 code simply failed with nasty a error:
java.lang.ClassFormatError: Too many arguments in method signature in class file org/apache/spark/sql/catalyst/expressions/GeneratedClass$GeneratedIteratorForCodegenStage2 at java.lang.ClassLoader.defineClass1(Native Method)
If you have a performance critical Spark job that’s performing a lot of aggregations with Dataframe API be careful, there is no time savings that can be achieved with disabling wholeStage code generation.
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