If you really don't know until runtime what heap a pointer is in, then write barriers will have some kind of cost, but it should be low (definitely not atomic).

If you really don't know until runtime what heap a pointer is in, then write barriers will have some kind of cost, but it should be low (definitely not atomic). However, if you don't know statically what kind of heap a pointer is in, you're going to have a poorly performing GC system no matter what.