What is the relation between async threads and SMP

The async thread pool has nothing with SMP todo. The asynch threads are only
used by the file driver and by user written drivers that specifically uses the thread pool. The file driver uses this to avoid locking of the whole Erlang VM for a longertime period in case of a lengthy file operation. The asynch threads was introduced long before the SMP support in the VM and works for the non SMP VM as well. In fact the asynch threads are even more important for a non SMP system because without it a lengthy file operation will block the whole VM.