By default, the user is given no indication that the garbage collector is operating. If no program ever runs out of space and no program using a lot of heap space requires an inordinate amount of processing time, then such information is unlikely to be needed.
However, if a program thought to be using much heap space runs out of
space or runs inordinately slowly, then the user may want to determine
whether more or less frequent garbage collections are necessary.
Information obtained from the garbage collector by turning on the
gc_trace Prolog flag can be helpful in this