● A real-time operating system (RTOS) is any operating system (OS) intended to serve real-time applications that process data as it comes in, typically without buffer delays.
● Processing time requirements (including any OS delay) are measured in tenths of seconds or shorter increments of time.
● A real time system is a time bound system which has well defined fixed time constraints. Processing must be done within the defined constraints or the system will fail. They either are event driven or time sharing.
● Event driven systems switch between tasks based on their priorities while time sharing systems switch the task based on clock interrupts. Most RTOS’s use a pre-emptive scheduling algorithm.

Total number of tickets =40
Total 4 processors:

→ Long term scheduler controls the degree of multiprogramming.
→ Ready queue of the processes resides in main memory.
→ A process can have multiple sub processes.
→ Multiple process of a single program cannot exist because each program can have only one process.

Algorithm: LRTF (Longest Remaining Time First)

Avg TAT = 12+13+14/3 = 39/3 = 13 units

Total time needed to complete the execution = 47
Idle time = 2+3 = 5
Percentage of Idle time = 5/47 × 100 =10.6%

Total no.of context switches is 2.

● Starvation is method in which process with high priorities continuously uses the resources preventing low priority process to acquire the resources
● In the Shortest job next scheduling shortest job process will run first and later higher burst time.Here the process with higher burst time need to wait for longer time.

Let's take an example:

Consider scheduler schedule processes priority after S times units so order of execution will be P1 P2 P3 P4 P1 P2 P3 P4 (S< execution time of any process) which is exactly same as round robin so answer – B.

Priority scheduling algorithm could result starvation. Starvation is nothing but indefinite blocking.

TRUE: Shortest remaining time (SRT) algorithm is the preemptive version of the shortest job next(SJN) CPU scheduling algorithm.
TRUE: A “context switch” is the mechanism to store and restore the state or context of a CPU in PCB.

1. New
A program which is going to be picked up by the OS into the main memory is called a new process.
2. Ready
Whenever a process is created, it directly enters in the ready state, in which, it waits for the CPU to be assigned. The OS picks the new processes from the secondary memory and put all of them in the main memory. The processes which are ready for the execution and reside in the main memory are called ready state processes.
3. Running
One of the processes from the ready state will be chosen by the OS depending upon the scheduling algorithm.
4. Block or wait
From the Running state, a process can make the transition to the block or wait state depending upon the scheduling algorithm or the intrinsic behavior of the process.

The above diagram represents queueing diagram. Various queues are
→ All processes, upon entering into the system, are stored in the Job Queue.
→ Processes in the Ready state are placed in the Ready Queue.
→ Processes waiting for a device to become available are placed in Device Queues. There are unique device queues available for each I/O device.

→ In shortest job first scheduling algorithm, the processor selects the process with the smallest execution time to execute next.
→ SJF scheduling algorithm will result in the maximum throughput

→ Ageing is a scheduling technique used to avoid starvation. Fixed priority scheduling is a scheduling discipline, in which tasks queued for utilizing a system resource are assigned a priority each.
→ A task with a high priority is allowed to access a specific system resource before a task with a lower priority is allowed to do the same.
→ A disadvantage of this approach is that tasks assigned with a lower priority may be starved when a large number of high priority tasks are queued. Aging is used to gradually increase the priority of a task, based on its waiting time in the ready queue.

→ Actually they are given single job, so when we are using round robin algorithm it won’t perform any context switch because we are executing same job.
→ But according to their intention will cause total 6 switches. 0-2, 2-4, 4-6, 6-8, 8-10, 10-12, 12-14. We are initially started with 0. So, It will take 6 connected switches
Note: We thought they made this question wrong.

The gantt chart of the above RR scheduling algorithm is

The completion of order of processes are P3,P1 and P2

→ Starvation is the name given to the indefinite postponement of a process because it requires some resource before it can run, but the resource, though available for allocation, is never allocated to this process.
1. Priority: A process ready to run for CPU can wait indefinitely because of low priority
2. Shortest process next : longest process possibility to get starvation
3. Shortest remaining time first: longest process possibility to get starvation
Solution: Aging
→ FCFS and Round Robin scheduling never happen indefinite blocking.

Disk scheduling→ SCAN,C-SCAN,LOOK,C-LOOK
Batch processing→ FIFO
Time sharing→ Round robin with the help of time quantum.
Interrupt processing→ LIFO

→ SJF scheduling algorithm is either preemptive and Nonpreemptive.
→ Round robin is purely Nonpreemptive.
→ Priority based scheduling also preemptive and Nonpreemptive.
→ Shortest-Job-next scheduling algorithm is either preemptive and Nonpreemptive.

Round robin method essentially uses the preemptive version of FIFO. Because of time quantum, it preempt every process in FIFO order. But it is not FIFO exactly because of preemption. In FIFO won’t support preemption.

Ready→ Running: The process is dispatched.
Blocked→ Ready: Request made by the process is satisfied or an event for which it was waiting occurs.
Running→ Blocked: Process wishes to wait for some action by another process.
Running→ Ready: The process is preempted.

Given data,

→ Preemptive scheduling is the strategy of temporarily suspending a gunning process before the CPU time slice expires.
→ Preemptive scheduling means the processes is always interrupted by higher priorities.

Process states
1. New→ The process is being created.
2. Running→ Instructions are being executed.
3. Waiting→ The process is waiting for some event to occur
4. Ready→ The process is waiting to be assigned to a processor
5. Terminated→ The process has finished execution

A virtual memory based memory management algorithm partially swaps out a process. This is an example of medium term scheduling.
Medium-term scheduling:The decision to add to the number of processes that are partially or fully in main memory.
Long-term scheduling:The decision to add to the pool of processes to be executed.
Short-term scheduling:The decision as to which available process will be executed by the processor.

Disk Scheduling→ SCAN
Batch Processing→ FIFO
Time Sharing→ Round-robin
Interrupt Processing→ LIFO

Round Robin(RR) is one of preemptive scheduling algorithm. Priority based scheduling is non preemptive.

Since arrival time is not given assume arrival time of each process as"0" Option (A):

→ Round robin and priority scheduling algorithms are preemptive based algorithms.
→ FCFS and SJF is non-preemptive based algorithms but in SJF are supporting another version is SRTF(Shortest remaining time first)

→ Round robin and priority scheduling algorithms are preemptive based algorithms.
→ FCFS and SJF is non-preemptive based algorithms but in SJF are supporting another version is SRTF(Shortest remaining time first)

Fair-share scheduling is a scheduling algorithm for computer operating systems in which the CPU usage is equally distributed among system users or groups, as opposed to equal distribution among processes.