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Date: Thu, 1 Jul 2010 17:50:52 -0400 (EDT)
From: "Dick King (JIRA)" <jira@apache.org>
To: mapreduce-dev@hadoop.apache.org
Subject: [jira] Created: (MAPREDUCE-1909) TrackerDistributedCacheManager
 takes a blocking lock fo a loop that executes 10K times
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TrackerDistributedCacheManager takes a blocking lock fo a loop that executes 10K times
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                 Key: MAPREDUCE-1909
                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-1909
             Project: Hadoop Map/Reduce
          Issue Type: Improvement
            Reporter: Dick King
            Assignee: Dick King


In {{TrackerDistributedCachaManager.java}} , the portion where the cache is cleaned up, the lock is taken on the main hash table and then all the entries are scanned to see if they can be deleted.  That's a long lockage.  The table is likely to have 10K entries.

I would like to reduce the longest lock duration by maintaining the set of {{CacheStatus}} es to delete incrementally.

1: Let there be a new {{HashSet}}, {{deleteSet}}, that's protected under {{synchronized(cachedArchives)}}

2: When {{refcount}} is decreased to 0, move the {{CacheStatus}} from {{cachedArchives}} to {{deleteSet}}

3: When seeking an existing {{CacheStatus}}, look in {{deleteSet}} if it isn't in {{cachedArchives}}

4: When {{refcount}} is increased from 0 to 1 in a pre-existing {{CacheStatus}} [see 3:, above] move the {{CacheStatus}} from {{deleteSet}} to {{cachedArchives}}

5: When we clean the cache, under {{synchronized(cachedArchives)}} , move {{deleteSet}} to a local variable and create a new empty {{HashSet}}.  This is constant time.

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