[jira] [Created] (HIVE-12310) Update memory estimation login in TopNHash

["Thejas M Nair (JIRA)" <ji...@apache.org>]

Thejas M Nair created HIVE-12310:
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             Summary: Update memory estimation login in TopNHash
                 Key: HIVE-12310
                 URL: https://issues.apache.org/jira/browse/HIVE-12310
             Project: Hive
          Issue Type: Bug
          Components: Query Processor
            Reporter: Thejas M Nair


HIVE-12084 changes TopNHash to use Runtime.getRuntime().freeMemory() for finding available memory.
However, it does not give the all the memory it could use, it ignores unallocated memory. This is because the heap size of jvm grows up to max heap size (-Xmx) as per it needs. totalMemory() gives total heap space it has allocated, and freeMemory() is the free memory within that.
See http://i.stack.imgur.com/GjuwM.png and http://stackoverflow.com/questions/3571203/what-is-the-exact-meaning-of-runtime-getruntime-totalmemory-and-freememory .
So instead of using Runtime.getRuntime().freeMemory() , I think it should use maxMemory() - totalMemory() + freeMemory()



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