在简单的字符串拆分操作上，Spark Dataframe本机性能与Pyspark RDD映射

Question

I don't expect the following code to benefit from the Dataframe Catalyst query optimizer, but I do expect there to be a performance difference between the Scala/native performance of string split and the Python performance. However, my performance results are disappointing, as the native Dataframe API appears to be slower.

My test is as follows:

def get_df(spark):
    return spark.read.load('s3://BUCKET/test-data.csv',
                           format='com.databricks.spark.csv',
                           inferSchema='true', header='true')

def upsize_df(df, exponent=10):
    for i in range(exponent):
        df = df.unionAll(df)
    return df

def rdd_ver(df):
    df = df.rdd.map(lambda row: row + tuple(
                        row.order_id.split('-'))).toDF(
                            df.columns + ['psrid', 'eoid'])
    df.show()

def df_ver(df):
    split_col = pyspark.sql.functions.split(df['order_id'], '-')
    df = df.withColumn('psrid', split_col.getItem(0))
    df = df.withColumn('eoid', split_col.getItem(1))
    df.show()

Cluster/YARN details:

• Spark 2.0 on AWS
• 6 executors
• 2 cores per executor

Test procedure:

• Create new PySpark shell in IPython
• Get dataframe of toy-sized dataset (1000 rows)
• repartition Dataframe to 12 partitions
• upsize_df with unionAll , to get to 1 million rows
• run df.count() to force execution of repartition and upsize_df
• finally, run %time rdd_ver(df) or %time df_ver(df)

My results so far have been surprising and disappointing. Here is a sampling of the results I've received, in seconds:

rdd_ver : 14.5, 22.4, 13.1, 24.7, 17.8 --- mean: 18.5

df_ver : 30.5, 26.9, 32.0, 29.7, 39.8 --- mean: 31.8

I'd appreciate any thoughts, either on the test procedure itself (the operation itself is derived from some production code) or on the poor performance of the Dataframe version.

EDIT:

The Spark Web UI indicates that my jobs are not actually being scheduled/submitted very quickly. I am not sure how reliable the Web UI's information is, but the 'Submitted' time displayed on the active job in this screenshot is over a minute after I initially hit 'enter' in the active Pyspark session to kick off %time df_ver(df)

Furthermore, it seems that none of the 6 executors are doing anything. They've all apparently been killed by Spark since I wasn't actively doing anything in the Spark session for more than a few seconds. It seems like the entire job is being run by the driver node, but I can't confirm that since I don't know the Spark Web UI well enough.

Answer 1

Why do you think it should be faster in scala? Python string operations are very fast:

Python:

In [58]: %time "this is my string".split()
CPU times: user 5 µs, sys: 1 µs, total: 6 µs
Wall time: 7.87 µs

Scala:

bash-3.2$ echo '
object TimeSplit {
   def main(args: Array[String]): Unit = {
     val now = System.nanoTime
     val split = "this is my string".split(" ")
     val diff = System.nanoTime - now
     println("%d microseconds".format(diff/1000))
   }
 }' > timesplit.scala

bash-3.2$ scalac timesplit.scala
bash-3.2$ scala TimeSplit
21 microseconds