Why does partition parameter of SparkContext.textFile not take effect?

scala> val p=sc.textFile("file:///c:/_home/so-posts.xml", 8) //i've 8 cores
p: org.apache.spark.rdd.RDD[String] = MapPartitionsRDD[56] at textFile at <console>:21

scala> p.partitions.size
res33: Int = 729

I was expecting 8 to be printed and I see 729 tasks in Spark UI

EDIT:

After calling repartition() as suggested by @zero323

scala> p1 = p.repartition(8)
scala> p1.partitions.size
res60: Int = 8
scala> p1.count

I still see 729 tasks in the Spark UI even though the spark-shell prints 8.

Solution 1:

If you take a look at the signature

textFile(path: String, minPartitions: Int = defaultMinPartitions): RDD[String] 

you'll see that the argument you use is called minPartitions and this pretty much describes its function. In some cases even that is ignored but it is a different matter. Input format which is used behind the scenes still decides how to compute splits.

In this particular case you could probably use mapred.min.split.size to increase split size (this will work during load) or simply repartition after loading (this will take effect after data is loaded) but in general there should be no need for that.

Solution 2:

@zero323 nailed it, but I thought I'd add a bit more (low-level) background on how this minPartitions input parameter influences the number of partitions.

tl;dr The partition parameter does have an effect on SparkContext.textFile as the minimum (not the exact!) number of partitions.

In this particular case of using SparkContext.textFile, the number of partitions are calculated directly by org.apache.hadoop.mapred.TextInputFormat.getSplits(jobConf, minPartitions) that is used by textFile. TextInputFormat only knows how to partition (aka split) the distributed data with Spark only following the advice.

From Hadoop's FileInputFormat's javadoc:

FileInputFormat is the base class for all file-based InputFormats. This provides a generic implementation of getSplits(JobConf, int). Subclasses of FileInputFormat can also override the isSplitable(FileSystem, Path) method to ensure input-files are not split-up and are processed as a whole by Mappers.

It is a very good example how Spark leverages Hadoop API.

BTW, You may find the sources enlightening ;-)