CPU Scheduling Algorithms

What is a solution to the problem of indefinite blockage of low-priority processes?

"Aging"

Which scheduling algorithm gives minimum average waiting time?

Shortest Job First. (SJF)

Orders are processed in the sequence they arrive if ______ rule sequences the jobs.

First Come, first served

What is 'Aging'?

Increasing the priority of jobs to ensure termination in a finite time

What is one of the disadvantages of the priority scheduling algorithm?

It can lead to some low priority process waiting indefinitely for the CPU

What does the FIFO algorithm do?

It first executes the job that came in first in the queue.

What is the difficulty with SJF in short term scheduling?

Knowing the length of the next CPU request

The FCFS algorithm is particularly troublesome for:

Multiprogramming systems In a time sharing system, each user needs to get a share of the CPU at regular intervals.

Round Robin Scheduling falls under the category of:

Preemptive scheduling

The strategy of making processes that are logically runnable to be temporarily suspended is called:

Preemptive scheduling

What is the most optimal scheduling algorithm?

Shortest Job First (SJF)

Preemptive Shortest Job First scheduling is sometimes called:

SRTN scheduling - Shortest Remaining Time Next

Under multiprogramming, turnaround time for short jobs is usually ________ and that for long jobs is slightly ___________

Shortened ; Lengthened

An SJF algorithm is simply a priority algorithm where the priority is:

The predicted next CPU burst The larger the CPU burst, the lower the priority.

Consider the following set of processes, the length of the CPU burst time given in milliseconds : Process Burst time P1 6 P2 8 P3 7 P4 3 Assuming the above process being scheduled with the SJF scheduling algorithm, what is the wait time for P1?

The waiting time for process P1 is 3ms. P1 has to wait for P4, which has a Burst time for 3ms.

What is scheduling?

allowing a job to use the processor

Complex scheduling algorithms: a) are very appropriate for very large computers b) use minimal resources c) use many resources d) all of the mentioned

a) are very appropriate for very large computers Large computers are overloaded with greater number of processes

The portion of the process schedule in an operating system that dispatches processes is concerned with:

assigning ready processes to the CPU

Which of the following algorithms tends to minimize the process flow time ? a) First come First served b) Shortest Job First c) Earliest Deadline First d) Longest Job First

b) Shortest Job First

There are 10 different processes running on a workstation. Idle processes are waiting for an input event in the input queue. Busy processes are scheduled with the Round-Robin timesharing method. Which out of the following quantum times is the best value for small response times, if the processes have a short runtime, e.g. less than 10ms ? a) tQ = 15ms b) tQ = 40ms c) tQ = 45ms d) tQ = 50ms

tQ = 15ms

What is Preemptive scheduling?

The OS moves a process from running to ready without the process requesting it. Without preemption, the system implements "run to completion" (or yield or block)

What are the objectives of a good scheduling policy?

-Fairness. -Efficiency. -Low response time (important for interactive jobs). -Low turnaround time (important for batch jobs). High throughput [the above are from Tanenbaum]. -Repeatability. Dartmouth (DTSS) "wasted cycles'' and limited logins for repeatability. -Fair across projects. -Cheating'' in unix by using multiple processes. - TOPS-10. - Fair share research project. Degrade gracefully under load.

Which of the following statements are true ? I. Shortest remaining time first scheduling may cause starvation II. Preemptive scheduling may cause starvation III. Round robin is better than FCFS in terms of response time

All of the Above I) Shortest remaining time first scheduling is a preemptive version of shortest job scheduling. It may cause starvation as shorter processes may keep coming and a long CPU burst process never gets CPU. II) Preemption may cause starvation. If priority based scheduling with preemption is used, then a low priority process may never get CPU. III) Round Robin Scheduling improves response time as all processes get CPU after a specified time.

With round robin scheduling algorithm in a time shared system: a) using very large time slices converts it into First come First served scheduling algorithm b) using very small time slices converts it into First come First served scheduling algorithm c) using extremely small time slices increases performance d) using very small time slices converts it into Shortest Job First algorithm

a) using very large time slices converts it into First come First served scheduling algorithm All the Processes will be able to get completed.