Question

In class- We calculated 2^30 * (1nsec + 1nsec) = 2.14 sec

1. In class, we demonstrated how long it takes to complete compaction for a 1GB RAM with 2 nsec access time assuming the entire memory had to be compacted -- meaning there is a hole at the first location and no hole at the last location.

How often would compaction have to be completed? Why? (2)

Answer 1

ANSWER :

* The algorithmic difficulty in a compaction algorithm is about updating all pointers, so that they point to the new object location.

* Through strict typing, the .NET virtual machine can unambiguously decide whether each word in RAM is a pointer or not, but updating all pointers efficiently without using too much extra RAM can be tricky.

* Additional complexity comes from generational collectors which try, most of the time, to leave most of the objects untouched, working preferentially with young objects only.

* Here, this means compacting only the end of the heap; full compaction being applied only rarely.

* The point here is that a full compaction, although linear, could still induce a noticeable pause.

Generational GC tries to make such pauses rarer.