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Unit 5: Scheduling

the time each of them may control CPU for, if a fair share of responsiveness is to be given to all Notes
processes. Moreover you 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:
1. 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.
2. 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.

Long term scheduling is performed when a new process is created. It is shown in the fi gure
below. If the number of ready processes in the ready queue becomes very high, then there is a
overhead on the operating system (i.e., processor) for maintaining long lists, context switching
and dispatching increases. Therefore, allow only limited number of processes in to the ready
queue. The “long-term scheduler” managers this. Long-term scheduler determines which
programs are admitted into the system for processing. Once when admit a process or job, it
becomes process and is added to the queue for the short-term scheduler. In some systems, a
newly created process begins in a swapped-out condition, in which case it is added to a queue
for the medium-term scheduler scheduling manage queues to minimize queueing delay and to
optimize performance.
Figure 5.8: Long-term Scheduling

New
Long-term Long-term
scheduling scheduling
Short-term
Ready scheduling
suspend Ready Running
Exit
Medium-term
scheduling
Event wait Terminated
Waiting
suspend Waiting

The long-term scheduler limits the number of processes to allow for processing by taking the
decision to add one or more new jobs, based on FCFS (First-Come, fi rst-serve) basis or priority
or execution time or Input/Output requirements. Long-term scheduler executes relatively
infrequently.

5.5.2 Medium Term Scheduling

Medium term scheduling is essentially concerned with memory management, hence it’s very
often designed as a part of the memory management subsystem of an OS. Its effi cient interaction
with the short term scheduler is essential for system performances, especially in virtual memory

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