Global versus Local Allocation

• Another important factor in the way frames are allocated to the various processes is page replacement.
• With multiple processes competing for frames, we can classify page-replacement algorithms into two broad categories:
  1. Global replacement. Global replacement allows a process to select a replacement frame from the set of all frames, even if that frame is currently allocated to some other process; that is, one process can take a frame from another.
  2. Local replacement. Local replacement requires that each process select from only its own set of allocated frames.
• With a local replacement strategy, the number of frames allocated to a process does not change.
• With global replacement, a process may happen to select only frames allocated to other processes, thus increasing the number of frames allocated to it (assuming that other processes do not choose its frames for replacement).
• Global replacement generally results in greater system throughput and is therefore the more common method.