I think it's pretty clever. The memory fragmentation caused by incessant consing leads to gc phase after gc phase. Their ORO trick prevents this, and as a bonus

I think it's pretty clever. The memory fragmentation caused by incessant consing leads to gc phase after gc phase. Their ORO trick prevents this, and as a bonus improves cache locality. They report they also have a true mark and sweep phase to take care of the corner cases. Does their ORO scheme have any significant downsides?