Mark-Sweep-Compact algorithms solve the shortcomings of Mark and Sweep by moving all marked – and thus alive – objects to the beginning of the memory region. The downside of this approach is an increased GC pause duration as we need to copy all objects to a new place and to update all references to such objects. The benefits to Mark and Sweep are also visible – after such a compacting operation new object allocation is again extremely cheap via pointer bumping. Using such approach the location of the free space is always known and no fragmentation issues are triggered either.

Java GC mark sweep compacting