Not really though you may want to. The main CPU could compact memory if it felt fragmentation was high enough to warrant it. By its nature the traffic is asynch

Not really though you may want to. The main CPU could compact memory if it felt fragmentation was high enough to warrant it. By its nature the traffic is asynchronous so the CPU won't do a perfect compaction (it won't have processed all the inbound messages to mark garbage as free) but it would probably be close enough. The big win here is making it all async. No need to stop the world, issue write barriers to application activity, or otherwise synchronize for GC. You'd have a bigger highwater mark for used memory but otherwise the main CPU can pickup the "the memory from x to y is now free" messages on any thread and whenever it is convenient.