[jira] [Created] (SPARK-17331) Avoid allocating 0-length arrays

["Sean Owen (JIRA)" <ji...@apache.org>]

Sean Owen created SPARK-17331:
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             Summary: Avoid allocating 0-length arrays
                 Key: SPARK-17331
                 URL: https://issues.apache.org/jira/browse/SPARK-17331
             Project: Spark
          Issue Type: Improvement
          Components: MLlib, Spark Core
    Affects Versions: 2.0.0
            Reporter: Sean Owen
            Assignee: Sean Owen
            Priority: Trivial


I've noticed a number of places in the code that allocate 0-length arrays. Since all 0-length arrays of a type are equivalent, it's often possible to avoid these allocations.

Where it actually likely matters is {{UTF8String}}, which does it in a several places and which can even be replaced by {{UTF8String.EMPTY_UTF8}}, saving even more allocations. 

It _could_ be worth refactoring other occurrences, mostly of "new byte[0]", to simply use a reference to one fixed static instance of it. But I avoided that in the Java code on the grounds that it's a little clunky and can be added if it proves to be a hotspot.

Same in Scala, where {{Array[T]()}} can be replaced by {{Array.empty}}. This actually still allocates a 0-length array. However the former call actually allocates *two* empty arrays because of varargs. The latter is simpler and widely used in the code, so, seems worth touching up to save some garbage.



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