[ http://issues.apache.org/jira/browse/HADOOP-200?page=all ]
Owen O'Malley updated HADOOP-200:
---------------------------------
Attachment: map-id.patch
Here is the patch. It does dramatically lower the slugishness when the reduces
are launching.
> The map task names are sent to the reduces
> ------------------------------------------
>
> Key: HADOOP-200
> URL: http://issues.apache.org/jira/browse/HADOOP-200
> Project: Hadoop
> Type: Bug
> Components: mapred
> Versions: 0.2
> Reporter: Owen O'Malley
> Assignee: Owen O'Malley
> Fix For: 0.3
> Attachments: map-id.patch
>
> As each reduce is created, it is given the entire set of potential map names.
> For my large sort jobs with 64k maps, this means that each reduce task is
> given a two dimensional array that is 5 tasks/map * 64k maps = 320k strings.
> Since the reduce task is passed from the job tracker to the task tracker and
> down to the task runner, passing the entire list is very expensive. I suspect
> that this is the cause of the slow downs that I see in the task trackers
> heart beats when the reduce tasks are being launched.
> I propose that the ReduceTask be changed to just get the count of maps, with
> ids from 0 .. maps -1.
> public ReduceTask(String jobFile, String taskId, int maps, int partition);
> Then we need to change the protocol for finding map outputs:
> MapOutputLocation[] locateMapOutputs(String jobId, int[] mapIds, int
> partition);
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