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https://issues.apache.org/jira/browse/SPARK-40048?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17578369#comment-17578369
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sam commented on SPARK-40048:
-----------------------------
Also confirmed no eviction seems to be happening with
https://stackoverflow.com/questions/69596441/is-there-any-stable-method-on-the-sparksession-sparkcontext-rdd-that-we-can-call
Though hard to tell, since we only run this code at the _end_ of the job
> Partitions are traversed multiple times invalidating Accumulator consistency
> ----------------------------------------------------------------------------
>
> Key: SPARK-40048
> URL: https://issues.apache.org/jira/browse/SPARK-40048
> Project: Spark
> Issue Type: Bug
> Components: Spark Core
> Affects Versions: 2.4.4
> Reporter: sam
> Priority: Major
>
> We are trying to use Accumulators to count RDDs without having to force
> `.count()` on them for efficiency reasons. We are aware tasks can fail and
> re-run, which will invalidate the value of the accumulator, so we count the
> number of times a partition has been traversed, so we can detect this.
> The problem is that partitions are being traversed multiple times even though
> - We cache the RDD in memory
> - No tasks are failing, no executors are dying.
> - There is plenty of memory (no RDD eviction)
> The code we use:
> ```
> val count: LongAccumulator
> val partitionTraverseCounts: List[LongAccumulator]
> def incrementTimesCalled(partitionIndex: Int): Unit =
> partitionTraverseCounts(partitionIndex).add(1)
> def incrementForPartition[T](index: Int, it: Iterator[T]): Iterator[T] = {
> incrementTimesCalled(index)
> it.map { x =>
> increment()
> x
> }
> }
> ```
> How we use the above:
> ```
> rdd.mapPartitionsWithIndex(safeCounter.incrementForPartition)
> ```
> We have a 50 partition RDD, and we frequently see odd traverse counts:
> ```
> traverseCounts: List(2, 1, 2, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
> 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 2,
> 2, 2, 2, 1, 2)
> ```
> As you can see, some partitions are traversed twice, while others are
> traversed only once.
> To confirm no task failures:
> ```
> cat job.log | grep -i task | grep -i fail
> ```
> To confirm no memory issues:
> ```
> cat job.log | grep -i memory
> ```
> We see every log line has multiple GB memory free.
> We also don't see any errors or exceptions.
> Question:
> 1. Why is spark traversing a cached RDD multiple times?
> 2. Is there any way to disable this?
> Many thanks,
> Sam
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