Github user viirya commented on the pull request:
https://github.com/apache/spark/pull/3899#issuecomment-68837277
Hi @mengxr, Thanks for comment. I may be wrong, but I think that we should
branch based on `sign(label)` instead of `sign(label * margin)`?
Because according to the definition of logistic loss function, `log(1 +
exp(-yp))`, where y = +1 for positive label and y = -1 for negative label.
When we branch on `sign(label * margin)`, we can not make sure what the
sign of label is, right? Because when `sign(label * margin) == 1`, it could be
`(sign(label), sign(margin)) == (1, 1)` or `(sign(label), sign(margin)) == (-1,
-1)`.
Besides,
[https://github.com/apache/spark/blob/master/mllib/src/main/scala/org/apache/spark/mllib/tree/loss/LogLoss.scala#L67](https://github.com/apache/spark/blob/master/mllib/src/main/scala/org/apache/spark/mllib/tree/loss/LogLoss.scala#L67)
and
[https://github.com/apache/spark/blob/master/mllib/src/main/scala/org/apache/spark/mllib/tree/loss/LogLoss.scala#L69](https://github.com/apache/spark/blob/master/mllib/src/main/scala/org/apache/spark/mllib/tree/loss/LogLoss.scala#L69)
are mathematically the same equation. I think it should be a bug. As the
function note said, it is not used by the gradient boosting algorithm but only
for debugging, so it is not found before.
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