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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