PERFORMANCE: Use lightweight bag implementations which do not register with
SpillableMemoryManager with Combiner
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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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