Page 54 - DCAP103_Principle of operating system

Unit 2: Process Management-I

• The process could issue an I/O request, and then be placed in an I/O queue. Notes
• The process could create a new sub process and wait for its termination.
• The process could be removed forcibly from the CPU, as a result of an interrupt, and be
put back in the ready queue.

Figure 2.5: Queuing-diagram Representation of Process Scheduling

Ready queue CPU

I/O I/O queue I/O request

Time slice
expired

Child Tark a
executos child

Interrput Wait for an
occurs interrupt

In the first two cases, the process eventually switches from the waiting state to the ready state,
and is then put back in the ready queue. A process continues this cycle until it terminates, at
which time it is removed from all queues and has its PCB and resources deallocated.
2.3.2 Schedulers

A process migrates between the various scheduling queues throughout its lifetime. The operating
system must select, for scheduling purposes, processes from these queues in some fashion. The
selection process is carried out by the appropriate scheduler.
In a batch system, often more processes are submitted than can be executed immediately.
These processes are spooled to a mass-storage device (typically a disk), where they are kept
for later execution. The long-term scheduler, or job scheduler, selects processes from this pool
and loads them into memory for execution. The short-term scheduler, or CPU scheduler, selects
from among the processes that are ready to execute, and allocates the CPU to one of them.
The primary distinction between these two schedulers is the frequency of their execution. The
short-term scheduler must select a new process for the CPU frequently. A process may execute
for only a few milliseconds before waiting for an I/O request. Often, the short-term scheduler
executes at least once every 100 milliseconds. Because of the brief time between executions, the
short-term scheduler must be fast. If it takes 10 milliseconds to decide to execute a process for
100 milliseconds, then 10/(100 + 10) = 9 percent of the CPU is being used (or wasted) simply
for scheduling the work.
The long-term scheduler, on the other hand, executes much less frequently. There may be minutes
between the creation of new processes in the system. The long-term scheduler controls the degree
of multiprogramming—the number of processes in memory. If the degree of multiprogramming
is stable, then the average rate of process creation must be equal to the average departure rate

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