OnlineLDAOptimizer uses RDD.reduce in two places where it could use treeAggregate. This can cause scalability issues. This should be an easy fix.
This is also a bug since it modifies the first argument to reduce, so we should use aggregate or treeAggregate.
See this line: https://github.com/apache/spark/blob/f12f11e578169b47e3f8b18b299948c0670ba585/mllib/src/main/scala/org/apache/spark/mllib/clustering/LDAOptimizer.scala#L452
and a few lines below it.