[I] Native shuffle fails with a 2GB task serialization OOM on jobs with very many partitions [datafusion-comet]

["parthchandra (via GitHub)" <gi...@apache.org>]

parthchandra opened a new issue, #5391:
URL: https://github.com/apache/datafusion-comet/issues/5391

   ### Describe the bug
   
   A job with a huge number of scan partitions fails at stage submission:
   
     ```
     Task serialization failed: java.lang.OutOfMemoryError: Required array length 2147483639 + 794 is too large
         at java.io.ByteArrayOutputStream.ensureCapacity(...)
         at org.apache.spark.serializer.JavaSerializerInstance.serialize(...)
         at org.apache.spark.scheduler.DAGScheduler.submitMissingTasks(DAGScheduler.scala:1528)
     ```
   
     A single `byte[]` can't exceed ~2GB, and the driver is serializing one object bigger than that. The failing job had ~38.7M partitions (multi-petabyte dataset), native scan into a native shuffle. The same job runs fine on plain Spark, so it's something Comet does differently, not just scale.
   
   
     ### Analysis
   
     `DAGScheduler.submitMissingTasks` serializes the `(RDD, ShuffleDependency)` pair once into a single broadcast byte array, which must fit in ~2GB. On the native shuffle path, `CometShuffleDependency.nativeShuffleSpec` is a non-transient field holding a `NativeExecContext` whose `perPartitionByKey: Map[String, Array[Array[Byte]]]` carries one
     serialized scan plan-data blob (the partition's file list) per map partition. With ~38.7M partitions that's ~38.7M protobufs baked into the broadcast task binary, so it exceeds 2GB — even though each write task only ever reads its own partition's slice.
   
     Spark's `FileScanRDD` keeps per-partition file lists `@transient`; each partition's files ride in its own `Partition` object and are serialized separately per task, so the full list  is never included in the shared broadcast blob.
   
   ### Steps to reproduce
   
   _No response_
   
   ### Expected behavior
   
   _No response_
   
   ### Additional context
   
   _No response_


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Re: [I] Native shuffle fails with a 2GB task serialization OOM on jobs with very many partitions [datafusion-comet]

["parthchandra (via GitHub)" <gi...@apache.org>]

parthchandra closed issue #5391: Native shuffle fails with a 2GB task serialization OOM on jobs with very many partitions
URL: https://github.com/apache/datafusion-comet/issues/5391


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