Long Term Scheduling

Long term scheduling obviously controls the degree of multiprogramming in multitasking systems, following certain policies to decide whether the system can honour a new job submission or, if more than one job is submitted, which of them should be selected. The need for some form of compromise between degree of multiprogramming and throughput seems evident, especially when one considers interactive systems. The higher the number of processes, in fact, the smaller the time each of them may control CPU for, if a fair share of responsiveness is to be given to all processes. 

Moreover, we have already seen that a too high number of processes causes waste of CPU time for system housekeeping chores (trashing in virtual memory systems is a particularly nasty example of this). However, the number of active processes should be high enough to keep the CPU busy servicing the payload (i.e. the user processes) as much as possible, by ensuring that - on average - there always be a sufficient number of processes not waiting for I/O.

Simple policies for long term scheduling are

• Simple First Come First Served (FCFS): it's essentially a FIFO scheme. All job requests (e.g. a submission of a batch program, or an user trying to log in in a time shared system) are honoured up to a fixed system load limit, further requests being refused tout court, or enqueued for later processing.

• Priority schemes. Note that in the context of long term scheduling ``priority'' has a different meaning than in dispatching: here it affects the choice of a program to be entered the system as a process, there the choice of which ready process process should be executed.

Disediakan oleh: Che Nurlailatul Ashikin