Page 83 - DCAP103_Principle of operating system

Principles of Operating Systems

Notes 3. Fork is
(a) the dispatching of a task (b) the creation of a new job
(c) the creation of a new process (d) increasing the priority of a task
4. The components that process data are located in the:
(a) input devices (b) output devices
(c) system unit (d) storage component
5. System software is the set of programs that enables your computer’s hardware devices
and ....................... software to work together.
(a) management (b) processing
(c) utility (d) application

3.4 Scheduling Criteria

Different CPU scheduling algorithms have different properties, and the choice of a particular
algorithm may favor one class of processes over another. In choosing which algorithm to use in
a particular situation, we must consider the properties of the various algorithms. Many criteria
have been suggested for comparing CPU scheduling algorithms. Which characteristics are used
for comparison can make a substantial difference in which algorithm is judged to be best. The
criteria include the following:
• CPU Utilization: We want to keep the CPU as busy as possible.
• Throughput: If the CPU is busy executing processes, then work is being done. One measure
of work is the number of processes that are completed per time unit, called throughput.
For long processes, this rate may be one process per hour; for short transactions, it may
be 10 processes per second.
• Turnaround Time: From the point of view of a particular process, the important criterion
is how long it takes to execute that process. The interval from the time of submission of
a process to the time of completion is the turnaround time. Turnaround time is the sum
of the periods spent waiting to get into memory, waiting in the ready queue, executing
on the CPU, and doing I/O.
• Waiting Time: The CPU scheduling algorithm does not affect the amount of the time
during which a process executes or does I/O; it affects only the amount of time that a
process spends waiting in the ready queue. Waiting time is the sums of periods spend
waiting in the ready queue.
• Response Time: In an interactive system, turnaround time may not be the best criterion.
Often, a process can produce some output fairly early and can continue computing
new results while previous results are being output to the user. Thus, another measure
is the time from the submission of a request until the first response is produced. This
measure, called response time, is the time it takes to start responding, not the time it
takes to output the response. The turnaround time is generally limited by the speed
of the output device.
It is desirable to maximize CPU utilization and throughput and to minimize turnaround time,
waiting time, and response time. In most cases, we optimize the average measure. However,
under some circumstances, it is desirable to optimize the minimum or maximum values rather
than the average. For example, to guarantee that all users get good service, we may want to
minimize the maximum response time. Investigators have suggested that, for interactive systems,
it is more important to minimize the variance in the response time than to minimize the average
response time. A system with reasonable and predictable response time may be considered more
desirable than a system that is faster on the average but is highly variable. However, little work
has been done on CPU-scheduling algorithms that minimize varianc.

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