Github user wangmiao1981 commented on a diff in the pull request:

    https://github.com/apache/spark/pull/16464#discussion_r94984564
  
    --- Diff: mllib/src/main/scala/org/apache/spark/ml/r/LDAWrapper.scala ---
    @@ -172,6 +187,8 @@ private[r] object LDAWrapper extends 
MLReadable[LDAWrapper] {
           model,
           ldaModel.logLikelihood(preprocessedData),
           ldaModel.logPerplexity(preprocessedData),
    +      trainingLogLikelihood,
    +      logPrior,
    --- End diff --
    
    LogPrior is calculated based on the serialized topics etc, which are also 
used by the trainingLikelyhood. But the trainingLikelyhood is the same for both 
original and loaded model. Let me debug more. It looks like a bug. The original 
MLLIB implementation doesn't serialize the two parameters as they can be 
calculated from other saved values. In addition, there is no unit test for 
comparing the two values, which could be the reason of not catching this issue. 


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