Compute size of Spark dataframe - SizeEstimator gives unexpected results

问题

I am trying to find a reliable way to compute the size (in bytes) of a Spark dataframe programmatically.

The reason is that I would like to have a method to compute an "optimal" number of partitions ("optimal" could mean different things here: it could mean having an optimal partition size, or resulting in an optimal file size when writing to Parquet tables - but both can be assumed to be some linear function of the dataframe size). In other words, I would like to call coalesce(n) or repartition(n) on the dataframe, where n is not a fixed number but rather a function of the dataframe size.

Other topics on SO suggest using SizeEstimator.estimate from org.apache.spark.util to get the size in bytes of the dataframe, but the results I'm getting are inconsistent.

First of all, I'm persisting my dataframe to memory:

df.cache().count

The Spark UI shows a size of 4.8GB in the Storage tab. Then, I run the following command to get the size from SizeEstimator:

import org.apache.spark.util.SizeEstimator
SizeEstimator.estimate(df)

This gives a result of 115'715'808 bytes =~ 116MB. However, applying SizeEstimator to different objects leads to very different results. For instance, I try computing the size separately for each row in the dataframe and sum them:

df.map(row => SizeEstimator.estimate(row.asInstanceOf[ AnyRef ])).reduce(_+_)

This results in a size of 12'084'698'256 bytes =~ 12GB. Or, I can try to apply SizeEstimator to every partition:

df.mapPartitions(
    iterator => Seq(SizeEstimator.estimate(
        iterator.toList.map(row => row.asInstanceOf[ AnyRef ]))).toIterator
).reduce(_+_)

which results again in a different size of 10'792'965'376 bytes =~ 10.8GB.

I understand there are memory optimizations / memory overhead involved, but after performing these tests I don't see how SizeEstimator can be used to get a sufficiently good estimate of the dataframe size (and consequently of the partition size, or resulting Parquet file sizes).

What is the appropriate way (if any) to apply SizeEstimator in order to get a good estimate of a dataframe size or of its partitions? If there isn't any, what is the suggested approach here?

回答1:

Unfortunately, I was not able to get reliable estimates from SizeEstimator, but I could find another strategy - if the dataframe is cached, we can extract its size from queryExecution as follows:

df.cache.foreach(_=>_)
val catalyst_plan = df.queryExecution.logical
val df_size_in_bytes = spark.sessionState.executePlan(
    catalyst_plan).optimizedPlan.stats.sizeInBytes

For the example dataframe, this gives exactly 4.8GB (which also corresponds to the file size when writing to an uncompressed Parquet table).

This has the disadvantage that the dataframe needs to be cached, but it is not a problem in my case.

回答2:

SizeEstimator returns the number of bytes an object takes up on the JVM heap. This includes objects referenced by the object, the actual object size will almost always be much smaller.

The discrepancies in sizes you've observed are because when you create new objects on the JVM the references take up memory too, and this is being counted.

Check out the docs here 🤩
https://spark.apache.org/docs/2.2.0/api/scala/index.html#org.apache.spark.util.SizeEstimator$

回答3:

Apart from Size estimator, which you have already tried(good insight)..

below is another option

RDDInfo[] getRDDStorageInfo()

Return information about what RDDs are cached, if they are in mem or on both, how much space they take, etc.

actually spark storage tab uses this.Spark docs

Below is the implementation from spark

 /**
   * :: DeveloperApi ::
   * Return information about what RDDs are cached, if they are in mem or on disk, how much space
   * they take, etc.
   */
  @DeveloperApi
  def getRDDStorageInfo: Array[RDDInfo] = {
    getRDDStorageInfo(_ => true)
  }

  private[spark] def getRDDStorageInfo(filter: RDD[_] => Boolean): Array[RDDInfo] = {
    assertNotStopped()
    val rddInfos = persistentRdds.values.filter(filter).map(RDDInfo.fromRdd).toArray
    rddInfos.foreach { rddInfo =>
      val rddId = rddInfo.id
      val rddStorageInfo = statusStore.asOption(statusStore.rdd(rddId))
      rddInfo.numCachedPartitions = rddStorageInfo.map(_.numCachedPartitions).getOrElse(0)
      rddInfo.memSize = rddStorageInfo.map(_.memoryUsed).getOrElse(0L)
      rddInfo.diskSize = rddStorageInfo.map(_.diskUsed).getOrElse(0L)
    }
    rddInfos.filter(_.isCached)
  }

yourRDD.toDebugString from RDD also uses this. code here

General Note :

In my opinion, to get optimal number of records in each partition and check your repartition is correct and they are uniformly distributed, I would suggest to try like below... and adjust your re-partition number. and then measure the size of partition... would be more sensible. to address this kind of problems

yourdf.rdd.mapPartitionsWithIndex{case (index,rows) => Iterator((index,rows.size))}
  .toDF("PartitionNumber","NumberOfRecordsPerPartition")
  .show

or existing spark functions (based on spark version)

import org.apache.spark.sql.functions._ 

df.withColumn("partitionId", sparkPartitionId()).groupBy("partitionId").count.show

来源：https://stackoverflow.com/questions/49492463/compute-size-of-spark-dataframe-sizeestimator-gives-unexpected-results

标签

apache-spark

spark-dataframe