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From "Pradeep Kamath (JIRA)" <j...@apache.org>
Subject [jira] Updated: (PIG-636) PERFORMANCE: Use lightweight bag implementations which do not register with SpillableMemoryManager with Combiner
Date Tue, 27 Jan 2009 01:18:59 GMT

     [ https://issues.apache.org/jira/browse/PIG-636?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

Pradeep Kamath updated PIG-636:

    Status: Patch Available  (was: Open)

Attached patch with implementations for SingleTupleBag and NonSpillableDataBag. SingleTupleBag
is a simplistic implementation of the DataBag interface which has just a single Tuple member
representing the contents of the bag. The iterator() simply returns this member Tuple object.
NonSpillableDataBag is more generic and can hold any number of tuples. It essentially has
all functionality of a DefaultDataBag without the ability to spill to disk. Both these classes
are to be used by directly calling their constructors without going through the BagFactory
and are meant only for use in internal operators to transfer data in bags.

POPreCombinerLocalRearrange has been changed to use SingleTupleBag and POCombinerPackage has
been changed to use NonSpillableDataBag. The CombinerOptimizer has been changed to include
a new visitor which visits the POProjects in the map plan with result type BAG. Such Projects
are annotated with a flag to use SingleTupleBags. This is necessary since only changing POPrecombinerLocalRearrange
to use SingleTupleBag will still result in the problem reported in the description for algebraic
aggregates which work on projects of bags like the following:
a = load...
b = group a by all;
c = foreach b generate SUM(a.$0), GRUNT(a.$1);

POProject has been changed to check the above flag and use a SingleTupleBag in getNext(DataBag).

> PERFORMANCE: Use lightweight bag implementations which do not register with SpillableMemoryManager
with Combiner
> ----------------------------------------------------------------------------------------------------------------
>                 Key: PIG-636
>                 URL: https://issues.apache.org/jira/browse/PIG-636
>             Project: Pig
>          Issue Type: Improvement
>    Affects Versions: types_branch
>            Reporter: Pradeep Kamath
>            Assignee: Pradeep Kamath
>             Fix For: types_branch
> Currently whenever Combiner is used in pig, in the map, the POPrecombinerLocalRearrange
operator puts the single "value" tuple corresponding to a key into a DataBag and passes this
to the foreach which is being combined. This will generate as many bags as there are input
records. These bags all will have a single tuple and hence are small and should not need to
be spilt to disk. However since the bags are created through the BagFactory mechanism, each
bag creation is registered with the SpillableMemoryManager and a weak reference to the bag
is stored in a linked list. This linked list grows really big over time causing unnecessary
Garbage collection runs. This can be avoided by having a simple lightweight implementation
of the DataBag interface to store the single tuple in a bag. Also these SingleTupleBags should
be created without registering with the spillableMemoryManager. Likewise the bags created
in POCombinePackage are supposed to fit in Memory and not spill. Again a NonSpillableDataBag
implementation of DataBag interface which does not register with the SpillableMemoryManager
would help.

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