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https://issues.apache.org/jira/browse/SPARK-21425?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16090017#comment-16090017
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Ryan Williams commented on SPARK-21425:
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Yea, static accumulators; I was able to reproduce it in cluster mode where
multiple tasks on the same executor are writing to that executor's single
instance of the same static accumulator.
I'm now thinking of this mostly as a surprise (to me!) about interactions
between closure-serialization and accumulators… I assumed (incorrectly) that
references to accumulators in task closures were treated specially in some way,
but I see now that they're not.
How unexpected/undesirable this behavior is debatable; I inadvertently fell
into a situation where I was making static accumulators while working around a
scalatest bug, as mentioned above, but in retrospect it feels pretty contrived
to have ended up in that situation.
Docs update, precautionary threadsafe-ing of \{Long,Double\}Accumulator, and
taking no action all seem reasonable to me.
> 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
> Priority: Minor
>
> [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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