Github user srowen commented on a diff in the pull request:

    https://github.com/apache/spark/pull/14335#discussion_r72011821
  
    --- Diff: 
mllib/src/main/scala/org/apache/spark/mllib/clustering/LDAOptimizer.scala ---
    @@ -472,12 +473,13 @@ final class OnlineLDAOptimizer extends LDAOptimizer {
             gammaPart = gammad :: gammaPart
           }
           Iterator((stat, gammaPart))
    -    }
    +    }.persist(StorageLevel.MEMORY_AND_DISK)
         val statsSum: BDM[Double] = 
stats.map(_._1).treeAggregate(BDM.zeros[Double](k, vocabSize))(
           _ += _, _ += _)
    -    expElogbetaBc.unpersist()
         val gammat: BDM[Double] = breeze.linalg.DenseMatrix.vertcat(
           stats.map(_._2).flatMap(list => 
list).collect().map(_.toDenseMatrix): _*)
    --- End diff --
    
    Yes, I'm wondering why not do this in a distributed way; am I missing why 
it's only done serially on the driver? is it that the dense matrix class 
doesn't serialize or serialize well?


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