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https://issues.apache.org/jira/browse/SPARK-58004?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Dongjoon Hyun resolved SPARK-58004.
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Fix Version/s: 4.3.0
Resolution: Fixed
Issue resolved by pull request 57087
[https://github.com/apache/spark/pull/57087]
> Add a hard reducer-partition fan-in bound to AQE shuffle coalescing
> -------------------------------------------------------------------
>
> Key: SPARK-58004
> URL: https://issues.apache.org/jira/browse/SPARK-58004
> Project: Spark
> Issue Type: Improvement
> Components: SQL
> Affects Versions: 4.3.0
> Reporter: Chao Sun
> Assignee: Chao Sun
> Priority: Major
> Labels: pull-request-available
> Fix For: 4.3.0
>
>
> AQE's CoalesceShufflePartitions rule currently packs contiguous reducer
> partitions using post-shuffle byte size as its primary bound. For a shuffle
> with a very large number of tiny or empty reducer partitions, one
> CoalescedPartitionSpec can therefore span many thousands of original reducer
> partitions while remaining below the advisory target size. This can create
> excessive per-task shuffle fan-in and may force users to disable AQE
> coalescing entirely.
> For example, an AQE-enabled aggregation with 100,000 initial shuffle
> partitions and sparse output can produce coalesced tasks whose byte sizes are
> reasonable but whose reducer-partition ranges are extremely wide.
> Proposed behavior:
> * Add spark.sql.adaptive.coalescePartitions.maxReducerPartitionsPerTask as a
> positive integer configuration with an effectively unbounded default.
> * Enforce both the advisory byte target and the maximum reducer-partition
> span while coalescing.
> * Count empty reducer partitions toward the span limit.
> * Prevent the small-tail backward merge from violating the limit.
> * Preserve skew-split PartialReducerPartitionSpec semantics and apply the
> same ranges to all shuffle inputs in a coalesce group.
> * Preserve existing behavior when the configuration is not set.
> A limit based on remote shuffle blocks is intentionally out of scope because
> reliable block-count information is not available in this AQE planning path.
> That can be considered separately if such information becomes available.
> Related: SPARK-50257 discusses another case of overly aggressive AQE
> partition coalescing, but does not provide a general reducer-partition fan-in
> bound.
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