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

    https://github.com/apache/spark/pull/15342#discussion_r82653577
  
    --- Diff: 
mllib/src/main/scala/org/apache/spark/mllib/clustering/KMeans.scala ---
    @@ -258,149 +252,106 @@ class KMeans private (
             }
         }
         val initTimeInSeconds = (System.nanoTime() - initStartTime) / 1e9
    -    logInfo(s"Initialization with $initializationMode took " + 
"%.3f".format(initTimeInSeconds) +
    -      " seconds.")
    +    logInfo(f"Initialization with $initializationMode took 
$initTimeInSeconds%.3f seconds.")
     
    -    val active = Array.fill(numRuns)(true)
    -    val costs = Array.fill(numRuns)(0.0)
    -
    -    var activeRuns = new ArrayBuffer[Int] ++ (0 until numRuns)
    +    var converged = false
    +    var cost = 0.0
         var iteration = 0
     
         val iterationStartTime = System.nanoTime()
     
    -    instr.foreach(_.logNumFeatures(centers(0)(0).vector.size))
    +    instr.foreach(_.logNumFeatures(centers.head.vector.size))
     
    -    // Execute iterations of Lloyd's algorithm until all runs have 
converged
    -    while (iteration < maxIterations && !activeRuns.isEmpty) {
    -      type WeightedPoint = (Vector, Long)
    -      def mergeContribs(x: WeightedPoint, y: WeightedPoint): WeightedPoint 
= {
    -        axpy(1.0, x._1, y._1)
    -        (y._1, x._2 + y._2)
    -      }
    -
    -      val activeCenters = activeRuns.map(r => centers(r)).toArray
    -      val costAccums = activeRuns.map(_ => sc.doubleAccumulator)
    -
    -      val bcActiveCenters = sc.broadcast(activeCenters)
    +    // Execute iterations of Lloyd's algorithm until converged
    +    while (iteration < maxIterations && !converged) {
    +      val costAccum = sc.doubleAccumulator
    +      val bcCenters = sc.broadcast(centers)
     
           // Find the sum and count of points mapping to each center
           val totalContribs = data.mapPartitions { points =>
    -        val thisActiveCenters = bcActiveCenters.value
    -        val runs = thisActiveCenters.length
    -        val k = thisActiveCenters(0).length
    -        val dims = thisActiveCenters(0)(0).vector.size
    +        val thisCenters = bcCenters.value
    +        val dims = thisCenters.head.vector.size
     
    -        val sums = Array.fill(runs, k)(Vectors.zeros(dims))
    -        val counts = Array.fill(runs, k)(0L)
    +        val sums = Array.fill(thisCenters.length)(Vectors.zeros(dims))
    +        val counts = Array.fill(thisCenters.length)(0L)
     
             points.foreach { point =>
    -          (0 until runs).foreach { i =>
    -            val (bestCenter, cost) = 
KMeans.findClosest(thisActiveCenters(i), point)
    -            costAccums(i).add(cost)
    -            val sum = sums(i)(bestCenter)
    -            axpy(1.0, point.vector, sum)
    -            counts(i)(bestCenter) += 1
    -          }
    +          val (bestCenter, cost) = KMeans.findClosest(thisCenters, point)
    +          costAccum.add(cost)
    +          val sum = sums(bestCenter)
    +          axpy(1.0, point.vector, sum)
    +          counts(bestCenter) += 1
             }
     
    -        val contribs = for (i <- 0 until runs; j <- 0 until k) yield {
    -          ((i, j), (sums(i)(j), counts(i)(j)))
    -        }
    -        contribs.iterator
    -      }.reduceByKey(mergeContribs).collectAsMap()
    -
    -      bcActiveCenters.destroy(blocking = false)
    -
    -      // Update the cluster centers and costs for each active run
    -      for ((run, i) <- activeRuns.zipWithIndex) {
    -        var changed = false
    -        var j = 0
    -        while (j < k) {
    -          val (sum, count) = totalContribs((i, j))
    -          if (count != 0) {
    -            scal(1.0 / count, sum)
    -            val newCenter = new VectorWithNorm(sum)
    -            if (KMeans.fastSquaredDistance(newCenter, centers(run)(j)) > 
epsilon * epsilon) {
    -              changed = true
    -            }
    -            centers(run)(j) = newCenter
    -          }
    -          j += 1
    -        }
    -        if (!changed) {
    -          active(run) = false
    -          logInfo("Run " + run + " finished in " + (iteration + 1) + " 
iterations")
    +        counts.indices.filter(counts(_) > 0).map(j => (j, (sums(j), 
counts(j)))).iterator
    +      }.reduceByKey { case ((sum1, count1), (sum2, count2)) =>
    +        axpy(1.0, sum2, sum1)
    +        (sum1, count1 + count2)
    +      }.collectAsMap()
    +
    +      bcCenters.destroy(blocking = false)
    +
    +      // Update the cluster centers and costs
    +      converged = true
    --- End diff --
    
    The logic is the same, yes, but it seems really strange to set something to 
false, then each iteration set it to true and then set it back false if some 
condition. Why not leave it false and change to true if convergence criteria is 
met? This is basically a trivial detail, so only change it if you want. I'm 
fine either way.


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