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https://issues.apache.org/jira/browse/FLINK-2590?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14721935#comment-14721935
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ASF GitHub Bot commented on FLINK-2590:
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Github user HuangWHWHW commented on the pull request:
https://github.com/apache/flink/pull/1075#issuecomment-136245681
@rmetzger +1. I think add a test is helpful.
Otherwise can you give us a infomation that prove the 'id = (counter <<
shifter) + taskId; ' will never generate the same id in different task?
And a minor thing in you issue description:
Is log2(8)=3 not 4?
> DataSetUtils.zipWithUniqueID creates duplicate IDs
> --------------------------------------------------
>
> Key: FLINK-2590
> URL: https://issues.apache.org/jira/browse/FLINK-2590
> Project: Flink
> Issue Type: Bug
> Components: Java API, Scala API
> Affects Versions: 0.10, master
> Reporter: Martin Junghanns
> Assignee: Martin Junghanns
> Priority: Minor
>
> The function creates IDs using the following code:
> {code:java}
> shifter = log2(numberOfParallelSubtasks)
> id = counter << shifter + taskId;
> {code}
> As the binary function + is executed before the bitshift <<, this results in
> cases where different tasks create the same ID. It essentially calculates
> {code}
> counter*2^(shifter+taskId)
> {code}
> which is 0 for counter = 0 and all values of shifter and taskID.
> Consider the following example.
> numberOfParallelSubtaks = 8
> shifter = log2(8) = 4 (maybe rename the function?)
> produces:
> {code}
> start: 1, shifter: 4 taskId: 4 label: 256
> start: 2, shifter: 4 taskId: 3 label: 256
> start: 4, shifter: 4 taskId: 2 label: 256
> {code}
> I would suggest the following:
> {code}
> counter*2^(shifter)+taskId
> {code}
> which in code is equivalent to
> {code}
> shifter = log2(numberOfParallelSubtasks);
> id = (counter << shifter) + taskId;
> {code}
> and for our example produces:
> {code}
> start: 1, shifter: 4 taskId: 4 label: 20
> start: 2, shifter: 4 taskId: 3 label: 35
> start: 4, shifter: 4 taskId: 2 label: 66
> {code}
> So we move the counter to the left and add the task id. As there is space for
> 2^shifter numbers, this prevents collisions.
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