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https://issues.apache.org/jira/browse/TEZ-1750?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14203068#comment-14203068
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Siddharth Seth commented on TEZ-1750:
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Missed this.
bq. Shouldnt this just break. If one output vertex is scheduled then all would
be scheduled, right?
The 0 task case may get in the way. Otherwise, I think it would be safe to
break here.
> Add a DAGScheduler which schedules tasks only when sources have been scheduled
> ------------------------------------------------------------------------------
>
> Key: TEZ-1750
> URL: https://issues.apache.org/jira/browse/TEZ-1750
> Project: Apache Tez
> Issue Type: Improvement
> Reporter: Siddharth Seth
> Assignee: Siddharth Seth
> Priority: Critical
> Attachments: TEZ-1750.1.txt, TEZ-1750.2.txt
>
>
> Splitting out the patch on TEZ-1522 into a separate jira.
> There's several scenarios in which we end up scheduling downstream tasks
> before their sources have been scheduled - and then get into a situation
> where the sources are starved. Currently, anywhere a ShuffleVertexManager is
> used can cause such behaviour - since it starts scheduling it's tasks after a
> certain number of sources are complete, but subsequen non-shuffle
> VertexManagers will scheduled immediately.
> Disabling slow-start is one option to achieve this (or setting slow start on
> all vertices), but it doesn't work for the situation where dynamic reducer
> parallelism kicks in - since it has to wait for source tasks to complete.
> The intent here is to add a DAGScheduler, which affectively negates the slow
> start, and in case of dynamic parallelism determination, waits for upstream
> tasks to be scheduled before scheduling downstream tasks.
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