Github user dbtsai commented on a diff in the pull request:
https://github.com/apache/spark/pull/3288#discussion_r20615000
--- Diff: mllib/src/main/scala/org/apache/spark/mllib/linalg/Vectors.scala
---
@@ -76,6 +76,22 @@ sealed trait Vector extends Serializable {
def copy: Vector = {
throw new NotImplementedError(s"copy is not implemented for
${this.getClass}.")
}
+
+ /**
+ * It will return the iterator for the active elements of dense and
sparse vector as
+ * (index, value) pair. Note that foreach method can be overridden for
better performance
+ * in different vector implementation.
+ *
+ * @param skippingZeros Skipping zero elements explicitly if true. It
will be useful when we
+ * iterator through dense vector having lots of
zero elements which
+ * we want to skip. Default is false.
+ * @return Iterator[(Int, Double)] where the first element in the tuple
is the index,
+ * and the second element is the corresponding value.
+ */
+ private[spark] def activeIterator(skippingZeros: Boolean):
Iterator[(Int, Double)]
--- End diff --
You are right; the `Tuple2[Int, Double]` is specialized, and I mistakenly
interpreted the bytecode.
For the flowing scala code,
```scala
def foreach[@specialized(Unit) U](f: ((Int, Double)) => U) {
var i = 0
val localValuesSize = values.size
val localValues = values
while (i < localValuesSize) {
f(i, localValues(i))
i += 1
}
}
```
the generated bytecode will be
```
public foreach(Lscala/Function1;)V
L0
LINENUMBER 296 L0
ICONST_0
ISTORE 2
L1
LINENUMBER 297 L1
GETSTATIC scala/Predef$.MODULE$ : Lscala/Predef$;
ALOAD 0
INVOKEVIRTUAL org/apache/spark/mllib/linalg/DenseVector.values ()[D
INVOKEVIRTUAL scala/Predef$.doubleArrayOps
([D)Lscala/collection/mutable/ArrayOps;
INVOKEINTERFACE scala/collection/mutable/ArrayOps.size ()I
ISTORE 3
L2
LINENUMBER 298 L2
ALOAD 0
INVOKEVIRTUAL org/apache/spark/mllib/linalg/DenseVector.values ()[D
ASTORE 4
L3
LINENUMBER 299 L3
FRAME APPEND [I I [D]
ILOAD 2
ILOAD 3
IF_ICMPGE L4
L5
LINENUMBER 300 L5
ALOAD 1
NEW scala/Tuple2$mcID$sp
DUP
ILOAD 2
ALOAD 4
ILOAD 2
DALOAD
INVOKESPECIAL scala/Tuple2$mcID$sp.<init> (ID)V
INVOKEINTERFACE scala/Function1.apply
(Ljava/lang/Object;)Ljava/lang/Object;
POP
L6
LINENUMBER 301 L6
ILOAD 2
ICONST_1
IADD
ISTORE 2
GOTO L3
```
However,
```
INVOKESPECIAL scala/Tuple2$mcID$sp.<init> (ID)V
INVOKEINTERFACE scala/Function1.apply
(Ljava/lang/Object;)Ljava/lang/Object;
```
is expensive, so that's why checking zero in the anonymous function will
slow down the whole thing.
I agree with you, the iterator is slow by nature, and we are only
interested in foreach implementation. I'll remove the iterator, and just have
foreach method in vector.
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