Local Replacement Algorithm - Overview

Overview

For example, consider an allocation scheme where high-priority processes are allowed to select frames from low-priority processes for replacement. A process can select a replacement from among its own frames or the frames of any low-priority process. This approach allows a high-priority process to increase its frame allocation at the expense of a low-priority process.

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).

One problem with global replacement is that a process cannot control its own page-fault rate. The set of pages in memory for a process depends not only on the paging behavior of that process but also on the paging behavior of other processes. Therefore,the same process may perform quite differently (for example, taking 0.5 seconds for one execution and 10.3 seconds for the next execution) because of totally external circumstances.

Under local replacement, the set of pages in memory for a process is affected by the paging behavior of only that process. Local replacement might hinder a process, however, by not making available to it other, less used pages of memory. Thus global replacement generally results in greater system throughput and is therefore the more common method.