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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. -- This message was sent by Atlassian JIRA (v6.4.14#64029) --------------------------------------------------------------------- To unsubscribe, e-mail: issues-unsubscr...@spark.apache.org For additional commands, e-mail: issues-h...@spark.apache.org