Till Rohrmann created FLINK-7851:
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Summary: Improve scheduling balance in case of fewer sub tasks
than input operator
Key: FLINK-7851
URL: https://issues.apache.org/jira/browse/FLINK-7851
Project: Flink
Issue Type: Improvement
Components: Distributed Coordination
Affects Versions: 1.3.2, 1.4.0
Reporter: Till Rohrmann
Fix For: 1.4.0
When having a job where we have a mapper {{m1}} running with dop {{n}} followed
by a key by and a mapper {{m2}} (all-to-all communication) which runs with dop
{{m}} and {{n > m}}, it happens that the sub tasks of {{m2}} are not uniformly
spread out across all currently used {{TaskManagers}}.
For example: {{n = 4}}, {{m = 2}} and we have 2 TaskManagers with 2 slots each.
The deployment would look the following:
TM1:
Slot 1: {{m1_1}} -> {{m_2_1}}
Slot 2: {{m1_3}} -> {{m_2_2}}
TM2:
Slot 1: {{m1_2}}
Slot 2: {{m1_4}}
The problem for this behaviour is that when there are too many preferred
locations (currently 8) due to an all-to-all communication pattern, then we
will simply poll the next slot from the MultiMap in
{{SlotSharingGroupAssignment}}. The polling algorithm first drains all
available slots for a single machine before it polls slots from another
machine.
I think it would be better to poll slots in a round robin fashion wrt to the
machines. That way we would get a better resource utilisation by spreading the
tasks more evenly.
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