A process can be swapped temporarily out of memory to a backing store, and then brought back into memory for continued execution
Backing store – fast disk large enough to accommodate copies of all memory images for all users
Roll out, roll in – swapping variant used for priority-based scheduling algorithms
lower-priority process is swapped out so higher-priority process can be loaded and executed
System maintains a ready queue of ready-to-run processes which have memory images on disk
Major part of swap time is transfer time
total transfer time is directly proportional to the amount of memory swapped