At the moment compacting GC takes pains to reuse the existing arenas so that it can work when we are out of memory. However if we are not totally out of memory, it can be beneficial to compact into a new set of areans, for two reasons: - objects allocated sequentially are more likely to stay together in memory - we protect reclaimed arenas to catch bugs where we don't update pointers correctly, and this would allow us to protect all arenas we compacted not just those that were emptied