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

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

    [ https://issues.apache.org/jira/browse/SPARK-21425?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16088842#comment-16088842 ] 

holdenk commented on SPARK-21425:
---------------------------------

Thanks for finding this. I've given some thought in the past to accumulators and I'd be happy to take a look at this if @rxin and such are busy.

> 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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