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From "Ashutosh Chauhan (JIRA)" <>
Subject [jira] [Commented] (HIVE-4838) Refactor MapJoin HashMap code to improve testability and readability
Date Wed, 07 Aug 2013 18:41:47 GMT


Ashutosh Chauhan commented on HIVE-4838:

Actually memory monitoring I was talking of was about local task which generates hashtable
which happens locally on client. To generate a hashtable (which is then ship to task nodes)
we launch local job on client in separate process. Logic of memory management for this local
task is convoluted (not of MR job which actually does the join in mapper). This local task
monitors its own memory, but seems like MapredLocalTask is catching OOM exception anyways.
One of this is not required. My thinking is there shouldn't be any memory monitoring and we
should just catch OOM exception when it fails. Anyways join is converted into mapjoin only
when size of small table is small (governed by config knob), so this OOM should be very very
rare. So, my suggestion is to remove MemoryHandler altogether.

ORC memory manger won't be a problem here, since ORC makes use of memory manager only while
writing data and here we are dumping hashtable in java serialized format, so that wont be
relevant. For similar reason (that this is local task) java.opts and io.sort.mb arent relevant
> Refactor MapJoin HashMap code to improve testability and readability
> --------------------------------------------------------------------
>                 Key: HIVE-4838
>                 URL:
>             Project: Hive
>          Issue Type: Bug
>            Reporter: Brock Noland
>            Assignee: Brock Noland
>         Attachments: HIVE-4838.patch, HIVE-4838.patch, HIVE-4838.patch, HIVE-4838.patch,
> MapJoin is an essential component for high performance joins in Hive and the current
code has done great service for many years. However, the code is showing it's age and currently
suffers  from the following issues:
> * Uses static state via the MapJoinMetaData class to pass serialization metadata to the
Key, Row classes.
> * The api of a logical "Table Container" is not defined and therefore it's unclear what
apis HashMapWrapper 
> needs to publicize. Additionally HashMapWrapper has many used public methods.
> * HashMapWrapper contains logic to serialize, test memory bounds, and implement the table
container. Ideally these logical units could be seperated
> * HashTableSinkObjectCtx has unused fields and unused methods
> * CommonJoinOperator and children use ArrayList on left hand side when only List is required
> * There are unused classes MRU, DCLLItemm and classes which duplicate functionality MapJoinSingleKey
and MapJoinDoubleKeys

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