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From Nathan McCarthy <>
Subject Re: SparkSQL schemaRDD & MapPartitions calls - performance issues - columnar formats?
Date Fri, 09 Jan 2015 07:37:46 GMT
Any ideas? :)

From: Nathan <<>>
Date: Wednesday, 7 January 2015 2:53 pm
To: "<>" <<>>
Subject: SparkSQL schemaRDD & MapPartitions calls - performance issues - columnar formats?


I’m trying to use a combination of SparkSQL and ‘normal' Spark/Scala via rdd.mapPartitions(…).
Using the latest release 1.2.0.

Simple example; load up some sample data from parquet on HDFS (about 380m rows, 10 columns)
on a 7 node cluster.

  val t = sqlC.parquetFile("/user/n/sales-tran12m.parquet”)

Now lets do some operations on it; I want the total sales & quantities sold for each hour
in the day so I choose 3 out of the 10 possible columns...

  sqlC.sql("select Hour, sum(ItemQty), sum(Sales) from test1 group by Hour").collect().foreach(println)

After the table has been 100% cached in memory, this takes around 11 seconds.

Lets do the same thing but via a MapPartitions call (this isn’t production ready code but
gets the job done).

  val try2 = sqlC.sql("select Hour, ItemQty, Sales from test1”)
  rddPC.mapPartitions { case hrs =>
    val qtySum = new Array[Double](24)
    val salesSum = new Array[Double](24)

    for(r <- hrs) {
      val hr = r.getInt(0)
      qtySum(hr) += r.getDouble(1)
      salesSum(hr) += r.getDouble(2)
    (salesSum zip qtySum)
  }.reduceByKey((a,b) => (a._1 + b._1, a._2 + b._2)).collect().foreach(println)

Now this takes around ~49 seconds… Even though test1 table is 100% cached. The number of
partitions remains the same…

Now if I create a simple RDD of a case class HourSum(hour: Int, qty: Double, sales: Double)

Convert the SchemaRDD;
val rdd = sqlC.sql("select * from test1").map{ r => HourSum(r.getInt(1), r.getDouble(7),
r.getDouble(8)) }.cache()
//cache all the data

Then run basically the same MapPartitions query;

rdd.mapPartitions { case hrs =>
  val qtySum = new Array[Double](24)
  val salesSum = new Array[Double](24)

  for(r <- hrs) {
    val hr = r.hour
    qtySum(hr) += r.qty
    salesSum(hr) += r.sales
  (salesSum zip qtySum)
}.reduceByKey((a,b) => (a._1 + b._1, a._2 + b._2)).collect().foreach(println)

This takes around 1.5 seconds! Albeit the memory footprint is much larger.

My thinking is that because SparkSQL does store things in a columnar format, there is some
unwrapping to be done out of the column array buffers which takes time and for some reason
this just takes longer when I switch out to map partitions (maybe its unwrapping the entire
row, even though I’m using just a subset of columns, or maybe there is some object creation/autoboxing
going on when calling getInt or getDouble)…

I’ve tried simpler cases too, like just summing sales. Running sum via SQL is fast (4.7
seconds), running a mapPartition sum on a double RDD is even faster (2.6 seconds). But MapPartitions
on the SchemaRDD;

sqlC.sql("select SalesInclGST from test1").mapPartitions(iter => Iterator(iter.foldLeft(0.0)((t,r)
=> t+r.getDouble(0)))).sum

 takes a long time (33 seconds). In all these examples everything is fully cached in memory.
And yes for these kinds of operations I can use SQL, but for more complex queries I’d much
rather be using a combo of SparkSQL to select the data (so I get nice things like Parquet
pushdowns etc.) & functional Scala!

I think I’m doing something dumb… Is there something I should be doing to get faster performance
on MapPartitions on SchemaRDDs? Is there some unwrapping going on in the background that catalyst
does in a smart way that I’m missing?


Nathan McCarthy
Level 25, 8 Chifley, 8-12 Chifley Square
Sydney NSW 2000

T: +61 2 8224 8922
F: +61 2 9292 6444



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