Ok, point taken, however their cost analysis is based on "simulated cycles", which is extremely simplified. With modern CPUs doing caching, prefetching, out-of-order execution I seriously doubt its accurate. malloc/free have typically a tendency to scatter objects around the whole heap, while compacting GCs allocate from a contiguous region - so a properly designed research experiment would take that into account. Hans Boehm did experiments on real programs and found that using compacting GCs actually speeded up some programs because of better cache friendliness.

As for heap fragmentation - it does not exist in some GC systems, like G1 or C4. And fragmentation is also extremely workload dependent - it might be "very small" for most cases and for some might be as much as 5x (Firefox struggled a lot from this).