Compacting garbage collection with ambiguous roots

Many modern garbage collectors 4 recover space by copying. Using an initial "root" set of pointers which are stored in known locations, all accessible objects are copied into a "new space". Two of the attractive properties of such a collector are that it results in memory compaction and it can have a running time proportional to the amount of accessible storage 2. However, such schemes place a large burden on the underlying system as all pointers to the objects must be found and changed.