[jira] [Assigned] (FLINK-7851) Improve scheduling balance in case of fewer sub tasks than input operator

["Till Rohrmann (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/FLINK-7851?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Till Rohrmann reassigned FLINK-7851:
------------------------------------

    Assignee: Till Rohrmann

> 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: 1.4.0, 1.3.2
>            Reporter: Till Rohrmann
>            Assignee: 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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