Github user mengxr commented on the pull request:
https://github.com/apache/spark/pull/2942#issuecomment-60873448
Had an offline discussion with @freeman-lab . We decided to introduce the
concept of `timeUnit` to describe decay. A `timeUnit` (like a second) could be
either a `batch` or a `point` (later we might add real "second"). Suppose that
we have some points at time `t`. The decay factor `alpha` is the discount we
take when we calculate the contribution from those points at time `t+1`. The
half life `h` is defined such that at time `t + h` the discount becomes 0.5. So
`alpha^h = 0.5` regardless of which `timeUnit` we use.
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