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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