Github user sethah commented on a diff in the pull request: https://github.com/apache/spark/pull/16037#discussion_r90773915 --- Diff: mllib/src/main/scala/org/apache/spark/mllib/optimization/LBFGS.scala --- @@ -241,16 +241,27 @@ object LBFGS extends Logging { val bcW = data.context.broadcast(w) val localGradient = gradient - val (gradientSum, lossSum) = data.treeAggregate((Vectors.zeros(n), 0.0))( - seqOp = (c, v) => (c, v) match { case ((grad, loss), (label, features)) => - val l = localGradient.compute( - features, label, bcW.value, grad) - (grad, loss + l) - }, - combOp = (c1, c2) => (c1, c2) match { case ((grad1, loss1), (grad2, loss2)) => - axpy(1.0, grad2, grad1) - (grad1, loss1 + loss2) - }) + // Given (current accumulated gradient, current loss) and (label, features) + // tuples, updates the current gradient and current loss + val seqOp = (c: (Vector, Double), v: (Double, Vector)) => + (c, v) match { + case ((grad, loss), (label, features)) => + val denseGrad = grad.toDense + val l = localGradient.compute(features, label, bcW.value, denseGrad) + (denseGrad, loss + l) + } + + // Adds two (gradient, loss) tuples + val combOp = (c1: (Vector, Double), c2: (Vector, Double)) => + (c1, c2) match { case ((grad1, loss1), (grad2, loss2)) => + val denseGrad1 = grad1.toDense --- End diff -- This is necessary because we may have empty partitions, right? Might be nice to add a test explicitly for this (it seems it failed in pyspark without it, but that was just a coincidence?) so someone doesn't remove these lines in the future.
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