[jira] [Created] (BEAM-8191) Multiple Flatten.pCollections() transforms generate an overwhelming number of tasks

["Peter Backx (Jira)" <ji...@apache.org>]

Peter Backx created BEAM-8191:
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             Summary: Multiple Flatten.pCollections() transforms generate an overwhelming number of tasks
                 Key: BEAM-8191
                 URL: https://issues.apache.org/jira/browse/BEAM-8191
             Project: Beam
          Issue Type: Bug
          Components: runner-spark
    Affects Versions: 2.15.0, 2.14.0, 2.12.0
            Reporter: Peter Backx


The Flatten.pCollections() is translated into a Spark union operation. The resulting RDD will have the sum of the partitions in the originating RDDs.

If you flatten 2 PCollections with each 10 partitions, the result will have 20 partitions.

This is ok in small pipelins, but in our main pipeline, this means the number of tasks grows out of hand quite easily (over 500k tasks in one stage). This overloads the driver and crashes the process.

I have created a small repro case:

[https://github.com/pbackx/beam-flatmap-test]

 

A possible solution is to add a coalesce call after the union. We have been testing this and it seems to do exactly what we want, but I'm not sure if this fix is applicable for all cases. 

I will open a PR for this so that you can review my proposed change and discuss whether or not it's a good idea.



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