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https://issues.apache.org/jira/browse/FLINK-10727?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16671728#comment-16671728
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ASF GitHub Bot commented on FLINK-10727:
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tillrohrmann commented on issue #6974: [FLINK-10727][network] remove
unnecessary synchronization in SingleInputGate#requestPartitions()
URL: https://github.com/apache/flink/pull/6974#issuecomment-435073195
This change breaks Flink's iteration mechanism and potentially even more
because `SingleInputGate#requestedPartitionsFlag` is read in
`SingleInputGate#updateInputChannel` which is not called from the `Task`
thread. The result can be that `updateInputChannel` does not request the sub
partition of newly registered partitions. Due to that it can happen that a job
gets stuck because it never consumes the input from a producer.
You can easily reproduce the problem by adding a `Thread.sleep(10L)` before
setting `requestedPartitionsFlag = true`.
I'm wondering how much improvement these kind of changes actually bring. I'm
a bit concerned that changes to such a critical component like the network
stack get merged into master just before feature freeze. If at all, something
like this should be merged at the beginning of the release cycle to give it
more exposure. Moreover, Travis never passed and actually failed with an IT
case running in exactly this problem. And also IntelliJ warns about the fact
that `requestedPartitionsFlag` is accessed both in synchronized and
unsynchronized context which should be red flag in most cases. I think we
should be more careful in the future!
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> Remove unnecessary synchronization in SingleInputGate#requestPartitions()
> -------------------------------------------------------------------------
>
> Key: FLINK-10727
> URL: https://issues.apache.org/jira/browse/FLINK-10727
> Project: Flink
> Issue Type: Improvement
> Components: Network
> Affects Versions: 1.5.5, 1.6.2, 1.7.0
> Reporter: Nico Kruber
> Assignee: Nico Kruber
> Priority: Major
> Labels: pull-request-available
> Fix For: 1.7.0
>
>
> For every {{SingleInputGate#getNextBufferOrEvent()}},
> {{SingleInputGate#requestPartitions()}} is called and this always
> synchronizes on the {{requestLock}} before checking the
> {{requestedPartitionsFlag}}. Since {{SingleInputGate#requestPartitions()}} is
> only called from the same thread (the task thread getting the record), it is
> enough to check the {{requestedPartitionsFlag}} first before synchronizing
> for the actual requests (if needed). {{UnionInputGate}} already goes the same
> way in its {{requestPartitions()}} implementation.
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