[jira] [Created] (SPARK-14277) UnsafeSorterSpillReader should do buffered read from underlying compression stream

["Sital Kedia (JIRA)" <ji...@apache.org>]

Sital Kedia created SPARK-14277:
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             Summary: UnsafeSorterSpillReader should do buffered read from underlying compression stream
                 Key: SPARK-14277
                 URL: https://issues.apache.org/jira/browse/SPARK-14277
             Project: Spark
          Issue Type: Improvement
    Affects Versions: 1.6.1
            Reporter: Sital Kedia


While running a Spark job which is spilling a lot of data in reduce phase, we see that significant amount of CPU is being consumed in native Snappy ArrayCopy method (Please see the stack trace below). 

Stack trace - 
org.xerial.snappy.SnappyNative.$$YJP$$arrayCopy(Native Method)
org.xerial.snappy.SnappyNative.arrayCopy(SnappyNative.java)
org.xerial.snappy.Snappy.arrayCopy(Snappy.java:85)
org.xerial.snappy.SnappyInputStream.rawRead(SnappyInputStream.java:190)
org.xerial.snappy.SnappyInputStream.read(SnappyInputStream.java:163)
java.io.DataInputStream.readFully(DataInputStream.java:195)
java.io.DataInputStream.readLong(DataInputStream.java:416)
org.apache.spark.util.collection.unsafe.sort.UnsafeSorterSpillReader.loadNext(UnsafeSorterSpillReader.java:71)
org.apache.spark.util.collection.unsafe.sort.UnsafeSorterSpillMerger$2.loadNext(UnsafeSorterSpillMerger.java:79)
org.apache.spark.sql.execution.UnsafeExternalRowSorter$1.next(UnsafeExternalRowSorter.java:136)
org.apache.spark.sql.execution.UnsafeExternalRowSorter$1.next(UnsafeExternalRowSorter.java:123)

The reason for that is the SpillReader does a lot of small reads from the underlying snappy compressed stream and we pay a heavy cost of jni calls for these small reads. The SpillReader should instead do a buffered read from the underlying snappy compressed stream.



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