Github user jkbradley commented on the pull request:

    https://github.com/apache/spark/pull/4047#issuecomment-70146828
  
    @witgo  I agree that there are 2 different use regimes for LDA: 
interpretable topics and featurization.  The current implementation follows 
pretty much every other graph-based implementation I’ve seen:
    * 1 vertex per document + 1 vertex per term
    * Each vertex stores a vector of length # topics.
    * On each iteration, each doc vertex must communicate its vector to any 
connected term vertices (and likewise for term vertices), via map-reduce stages 
over triplets.
    
    I have not heard of methods which can avoid this amount of communication 
for LDA.  I’m sure the implementation can be optimized, so please make 
comments here or JIRAs afterwards about that.  For modified models, it might be 
possible to communicate less: sparsity-inducing priors, hierarchical models, 
etc.



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