The map task names are sent to the reduces
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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
Assigned to: Owen O'Malley
Fix For: 0.3
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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