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From "dhruba borthakur (JIRA)" <j...@apache.org>
Subject [jira] Commented: (MAPREDUCE-1797) The JobTracker lock can be a reader/writer lock
Date Tue, 18 May 2010 22:12:43 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-1797?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12868865#action_12868865
] 

dhruba borthakur commented on MAPREDUCE-1797:
---------------------------------------------

We are seeing a scalability bottleneck in our 1600 node map-reduce cluster. 

A job is submitted to the JT. The JT spawns a thread A to read in the splits from hdfs. The
JT keeps the JobInProgress lock while reading in the splits from the splits file. while this
is happening, another client issues a getCounter() call to the JT. The getCounter RPC is handled
by a thread B, it first acquires the JT lock and then blocks while trying to acquire the JobInProgress
lock. This causes the entire JT to block until thread A release the JobInProgress lock.

One suggestion is to change the JobTracker lock into a reader/writer lock.

> The JobTracker lock can be a reader/writer lock
> -----------------------------------------------
>
>                 Key: MAPREDUCE-1797
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-1797
>             Project: Hadoop Map/Reduce
>          Issue Type: Improvement
>          Components: jobtracker
>            Reporter: dhruba borthakur
>
> The Jobtracker has a global lock and a per-job JobInProgress lock. The aim for the JobInprogress
lock is to support the ability to lock a single job's metadata without  blocking out the entire
JobTracker. However, many code paths acquire the JobTracker lock and then acquire the JobInProgress
lock while keeping the JobTracker lock. This somewhat defeats the benefit of having a per-job
lock.

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