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https://issues.apache.org/jira/browse/HADOOP-2990?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12577261#action_12577261
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Holden Robbins commented on HADOOP-2990:
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Nm, found it under: src/java/org/apache/hadoop/mapred/lib/MultithreadedMapRunner
> Ability to thread task execution
> --------------------------------
>
> Key: HADOOP-2990
> URL: https://issues.apache.org/jira/browse/HADOOP-2990
> Project: Hadoop Core
> Issue Type: Improvement
> Components: mapred
> Environment: All
> Reporter: Holden Robbins
> Original Estimate: 48h
> Remaining Estimate: 48h
>
> Currently Hadoop spawns a single threaded JVM for each task. While good for
> many tasks, this does not maximize resource usage for slaves that have many
> cores (machines with more cores are getting more cost effective everyday)
> _and_ are running jobs that require many gigabytes of read-only in-memory
> resources to maximize throughput. Running in separate JVMs requires
> redundantly loading large amounts of data, reducing the possible number of
> parallel tasks that can run per a machine even though more cpus are available.
> Adding this ability will give hadoop users the flexibility to balance their
> need for maximizing memory usage & throughput and task segmentation.
> Note: This is a blocking bug in porting processes over to hadoop for my own
> organization. I am testing a patch for this now that leaves the existing
> behavior for single threaded operation in-tact. All synchronization is done
> through wrapper classes and helper methods and should not add any overhead to
> non-threaded processes.
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