Improve GC pressure dealing with cascading Tuples.

[johnynek]

Look here:

https://github.com/twitter/scalding/blob/develop/src/main/scala/com/twitter/scalding/TupleBase.scala#L55

We should add a method to set into an existing Tuple and not necessarily reallocate.

The reason is cascading will not keep a reference to your tuple after you put it into the collector, so it is safe to reuse just one Tuple inside the operations, which should reduce GC pressure.

Calling TupleSetter.set(t : T, ctup: CTuple)

is probably what we want, which will reset ctup to contain exactly t, and then just reuse one instance of ctup.

[azymnis]

So this should be done for all TupleSetters and TupleGetters, right?

[johnynek]

Getters don't allocate. Converters have to because scala tuples are immutable. But setters are creating cascading tuples, and they don't need to allocate. So, I think it is just the setters we need to address.

[johnynek]

Relevant thread on the mailing list:
https://groups.google.com/forum/#!topic/cascading-user/vbloF-5RjKo