[jira] [Created] (SPARK-21425) LongAccumulator, DoubleAccumulator not threadsafe

["Ryan Williams (JIRA)" <ji...@apache.org>]

Ryan Williams created SPARK-21425:
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             Summary: LongAccumulator, DoubleAccumulator not threadsafe
                 Key: SPARK-21425
                 URL: https://issues.apache.org/jira/browse/SPARK-21425
             Project: Spark
          Issue Type: Bug
          Components: Spark Core
    Affects Versions: 2.2.0
            Reporter: Ryan Williams


[AccumulatorV2 docs|https://github.com/apache/spark/blob/v2.2.0/core/src/main/scala/org/apache/spark/util/AccumulatorV2.scala#L42-L43] acknowledge that accumulators must be concurrent-read-safe, but afaict they must also be concurrent-write-safe.

The same docs imply that {{Int}} and {{Long}} meet either/both of these criteria, when afaict they do not.

Relatedly, the provided [LongAccumulator|https://github.com/apache/spark/blob/v2.2.0/core/src/main/scala/org/apache/spark/util/AccumulatorV2.scala#L291] and [DoubleAccumulator|https://github.com/apache/spark/blob/v2.2.0/core/src/main/scala/org/apache/spark/util/AccumulatorV2.scala#L370] are not thread-safe, and should be expected to behave undefinedly when multiple concurrent tasks on the same executor write to them.

[Here is a repro repo|https://github.com/ryan-williams/spark-bugs/tree/accum] with some simple applications that demonstrate incorrect results from {{LongAccumulator}}'s.



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