use specialized axpy in RowMatrix

Author: Li Pu

mllib/src/main/scala/org/apache/spark/mllib/linalg/distributed/RowMatrix.scala

@@ -214,7 +214,13 @@ class RowMatrix(
       seqOp = (U, r) => {
         val rBrz = r.toBreeze
         val a = rBrz.dot(vbr.value)
-        brzAxpy(a, rBrz, U.asInstanceOf[BV[Double]])
+        rBrz match {
+          // use specialized axpy for better performance
+          case _: BDV[_] => brzAxpy(a, rBrz.asInstanceOf[BDV[Double]], U)
+          case _: BSV[_] => brzAxpy(a, rBrz.asInstanceOf[BSV[Double]], U)
+          case _ => throw new UnsupportedOperationException(
+            s"Do not support vector operation from type ${rBrz.getClass.getName}.")
+        }
         U
       },
       combOp = (U1, U2) => U1 += U2