Feichi Feng created HUDI-724:
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             Summary: Parallelize GetSmallFiles For Partitions
                 Key: HUDI-724
                 URL: https://issues.apache.org/jira/browse/HUDI-724
             Project: Apache Hudi (incubating)
          Issue Type: Improvement
          Components: Performance, Writer Core
            Reporter: Feichi Feng
         Attachments: gap.png, nogapAfterImprovement.png

When writing data, a gap was observed between spark stages. By tracking down 
where the time was spent on the spark driver, it's get-small-files operation 
for partitions.

When creating the UpsertPartitioner and trying to assign insert records, it 
uses a normal for-loop for get the list of small files for all partitions that 
the load is going to load data to, and the process is very slow when there are 
a lot of partitions to go through. While the operation is running on spark 
driver process, all other worker nodes are sitting idle waiting for tasks.

For all those partitions, they don't affect each other, so the get-small-files 
operations can be parallelized. The change I made is to pass the 
JavaSparkContext to the UpsertPartitioner, and create RDD for the partitions 
and eventually send the get small files operations to multiple tasks.



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