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From "Ryan Williams (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (SPARK-21425) LongAccumulator, DoubleAccumulator not threadsafe
Date Sun, 16 Jul 2017 12:58:02 GMT

    [ https://issues.apache.org/jira/browse/SPARK-21425?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16088919#comment-16088919
] 

Ryan Williams commented on SPARK-21425:
---------------------------------------

Digging a bit more, it seems like:

- there's no special handling of accumulators w/r/t sending them to tasks; a separate copy
gets sent to each task just like would happen to any local, mutable variable on the driver.
- If a task-closure references a global variable, like a field of a singleton, all tasks on
a given executor/JVM will reference the same/global instance of that variable, which is why
I am seeing this "bug".

It seems like the lesson is to be a little careful having accumulators that are global or
otherwise get directly referenced by multiple tasks after closure-serialization, as opposed
to the normal pattern where they are declared as local variables scoped to some instance that
the closure-serializer makes a copy of for each task.

For context, I had my accumulator-definitions out in a singleton object in the first because
I needed to avoid local accumulators causing an enclosing Scalatest suite to be serialized,
cf. [scalatest#1013|https://github.com/scalatest/scalatest/issues/1013].

We can probably say "not a bug" here, though I was surprised that accumulator-references aren't
explicitly handled under serialization the way {{Broadcast}}'s are, but I guess that should
have been evident by a lack of explicit {{Broadcast.value}}-style API for referencing them.

> 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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