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https://issues.apache.org/jira/browse/FLINK-14607?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16968042#comment-16968042
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Zhu Zhu commented on FLINK-14607:
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Thanks for the explanation. The slot sharing can be simplified a lot in this
way.
I have marked this issue to minor.
> SharedSlot cannot fulfill pending slot requests before it's completely
> released
> -------------------------------------------------------------------------------
>
> Key: FLINK-14607
> URL: https://issues.apache.org/jira/browse/FLINK-14607
> Project: Flink
> Issue Type: Bug
> Components: Runtime / Coordination
> Affects Versions: 1.10.0, 1.9.1
> Reporter: Zhu Zhu
> Priority: Minor
>
> Currently a pending request can only be fulfilled when a physical
> slot({{AllocatedSlot}}) becomes available in {{SlotPool}}.
> A shared slot however, cannot be used to fulfill pending requests even if it
> becomes qualified. This may lead to resource deadlocks in certain cases.
> For example, running job A(parallelism=2) --(pipelined)--> B(parallelism=2)
> with 1 slot only, all vertices are in the same slot sharing group, here's
> what may happen:
> 1. Schedule A1 and A2. A1 acquires the only slot, A2's slot request is
> pending because a slot cannot host 2 instances of the same JobVertex at the
> same time. Shared slot status: \{A1\}
> 2. A1 produces data and triggers the scheduling of B1. Shared slot status:
> \{A1, B1\}
> 3. A1 finishes. Shared slot status: \{B1\}
> 4. B1 cannot finish since A2 has not finished, while A2 cannot get launched
> due to no physical slot becomes available, even though the shred slot is
> qualified to host it now. A resource deadlock happens.
> Maybe we should improve {{SlotSharingManager}}. One a task slot is released,
> its root {{MultiTaskSlot}} should be used to try fulfilling existing pending
> task slots from other pending root slots({{unresolvedRootSlots}}) in this
> {{SlotSharingManager}}(means in the same slot sharing group).
> We need to be careful to not cause any failures, and do not violate
> colocation constraints.
> cc [~trohrmann]
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