Forget past outputs to free memory

Question

While tinkering with a notebook that loads a large chunk of data in memory, I noticed that if I run it multiple times, the past chunks reside in memory. I understand that the reason (or one of) for which this happens is that the past output mechanism (%) keeps a reference to them, so they cannot be garbage collected.

Is there a way to explicitly make Mathematica forget some data?

Answer 1

Use

 $HistoryLength = k

to keep the last k lines of input and output.