David Morávek created BEAM-9824:
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Summary: Multiple reshuffles are ignored in some cases on Flink
batch runner.
Key: BEAM-9824
URL: https://issues.apache.org/jira/browse/BEAM-9824
Project: Beam
Issue Type: Bug
Components: runner-flink
Affects Versions: 2.19.0, 2.20.0
Reporter: David Morávek
Assignee: David Morávek
Fix For: 2.21.0
Multiple reshuffles are ignored in some cases on Flink batch runner. This may
lead to huge performace penalty in IO connectors (when reshuffling splits).
In flink optimizer, when we `.rebalance()` dataset, is output channel is marked
as `FORCED_REBALANCED`. When we chain this with another `.rebalance()`, the
latter is ignored because it's source is already `FORCED_REBALANCED`, thus
requested property is met. This is correct beaviour because rebalance is
idempotent.
When we include `flatMap` in between rebalances ->
`.rebalance().flatMap(...).rebalance()`, we need to reshuffle again, because
dataset distribution may have changed (eg. you can possibli emit unbouded
stream from a single element). Unfortunatelly `flatMap` output is still
incorrectly marked as `FORCED_REBALANCED` and the second reshuffle gets ignored.
This especially affects IO connectors -> `FileIO.match()` returns reshuffled
list of matched files -> we split each file into ranges -> **reshuffle** ->
read. Ignoring the second reshuffle leads to huge perf. degradation (5m -> 2h
in one of our production pipelines)
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