Module 2 Terms
What is a solution to the problem of indefinite blockage of low-priority processes?
"Aging"
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.
What is the difficulty with SJF in short term scheduling?
Knowing the length of the next CPU request
An I/O bound program will typically have:
Many very short CPU bursts
In a multi-programming environment:
More than one process resides in memory
CPU scheduling is the basis of _________
Multi programming Operating Systems.
Processes are classified into different groups in what scheduling algorithm?
Multilevel queue scheduling algorithm.
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.
What is time quantum defined as?
The Round robin scheduling algorithm
What is a short-term scheduler?
It selects which process has to be executed next and allocates CPU
Why is scheduling done?
-Increase CPU utilization -Increase the throughput -Decrease Turnaround Time -Decrease the Waiting Time (idle time is costly)
A process can be terminated due to:
A normal exit, fatal error, or being killed by another process
In multilevel feedback scheduling algorithm:
A process can move to a different classified ready queue
In Operating systems, each process has its own:
Address Space and Global Variables, Open Files, Pending alarms/signals/signal handlers
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.
What is a medium-term scheduler?
It selects which process to remove from memory by swapping.
The two steps of a process execution are:
CPU and I/O burst
What is interprocess communication?
Communication between two processes
The context of a process in the PCB of a process does NOT contain: -the value of the CPU registers -the process state -memory management information -context switch time
Context switch time
In a priority scheduling algorithm, when a process arrives at the ready queue, its priority is compared with the priority of _______
Currently running processes
What module gives control of the CPU to the process selected by the short-term scheduler?
Dispatcher
A set of processes is deadlock if:
Each process is blocked and will remain so forever Think blind train drivers taking siestas
Orders are processed in the sequence they arrive if ______ rule sequences the jobs.
First Come, first served
Which scheduling algorithm allocates the CPU first to the process that requests the CPU first?
First come , First served scheduling
What system called is used in Unix to create a new process?
Fork
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 happens when the process issues an I/O request?
It is placed in an I/O queue
What happens when a process terminates?
It is removed from all queues
What is Dispatch Latency?
It is the time to stop one process and start running another one.
What is Turnaround Time?
It is the total time from the completion till the submission of a process.
What is a long-term scheduler?
It selects which process has to be brought into the ready queue
Which of the following do not belong for queues for processes? -Job Queue, PCB Queue, Device Queue, or Ready Queue
PCB Queue (Process Control Block: Data Structure in the OS containing info needed to manage the scheduling of particular processes)
A process stack does not contain:
PID of a child process
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
The switching of the CPU from one process or thread to another is called:
Process switch, task switch, or context switch (all are correct)
The address of the next instruction to be executed by the current process is provided by what?
Program counter
The processes that are residing in main memory and are ready and waiting to execute are kept on a list called _________
Ready Queue
In a time-sharing operating system, when the time slot given to a process is completed, the process goes from the running state to the:
Ready State
Suppose that a process is in "Blocked" state waiting for some I/O service. When the service is completed it goes to:
Ready State
What is the only state transition that is initiated by the user process itself?
Ready State
A process is selected from the ________ queue by the _________ scheduler, to be executed.
Ready, Short Term
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
What is the most optimal scheduling algorithm?
Shortest Job First (SJF)
Which scheduling algorithm gives minimum average waiting time?
Shortest Job First. (SJF)
What happens in a priority scheduling algorithm?
The CPU is allocated to the process with the highest priority.
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 is the primary distinction between the short term scheduler and long term scheduler?
The frequency of their execution
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.
What is waiting time?
The total time spent in the ready queue.
What is response time?
The total time taken from the submission time til the first response is produced.
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.
With multi-programming ______ is used productively.
Time
Which of the following need not necessarily be saved on a context switch between processes? General purpose registers Translation look-aside buffer Program counter All of the mentioned
Translation look-aside buffer
The interval from the time of submission of a process to the time of completion is termed as __________
Turnaround Time
The systems which allows only one process execution at a time are called:
Uniprocessing Systems systems that allow more than one process execute at a time are called multiprogramming systems. Uniprocessing means 1 processor.
Which one of the following cannot be scheduled by the kernel?
User Level thread User Level threads are managed by the thread library and the kernel is unaware of them.
What is the ready state of a process?
When a process is scheduled to run after some execution When a process is unable to run until some task is completed, the process is in blocked state, and if the process is using the CPU, it is in running state
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
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.
What is scheduling?
allowing a job to use the processor
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
In the following cases non - preemptive scheduling occurs : a)When a process switches from the running state to the ready state b) When a process goes from the running state to the waiting state c) When a process switches from the waiting state to the ready state d) All of the mentioned
b) When a process goes from the running state to the waiting state This means there are no other choices
If all processes are I/O bound, the ready queue will almost always be ______ and the short term scheduler will have a _______ to do.
empty, lot
Which of the following does not interrupt a running process? A device, timer, scheduler process, or power failure
scheduler process
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
Which system call returns the process identifier of a terminated child?
wait