What are real-life applications of Shortest job First scheduling?
Shortest job first (SJF) or shortest job next, is a scheduling policy that selects the waiting process with the smallest execution time to execute next. SJN is a non-preemptive algorithm.Shortest Job first has the advantage of having minimum average waiting time among all scheduling algorithms.It is a Greedy Algorithm.It may cause starvation if shorter processes keep coming. This problem can be solved using the concept of aging.It is practically infeasible as Operating System may not know burst time and therefore may not sort them. While it is not possible to predict execution time, several methods can be used to estimate the execution time for a job, such as a weighted average of previous execution times. SJF can be used in specialized environments where accurate estimates of running time are available.Program for Shortest Job First (or SJF) scheduling | Set 1 (Non- preemptive) - GeeksforGeeks
How do I calculate the waiting time for a non-preemptive, shortest job first (SJF) algorithm in an operating system?
Other name of this algorithm is Shortest-Process-Next (SPN). Like FCFS, SJF is non preemptive therefore, it is not useful in time-sharing environment in which reasonable response time must be guaranteed.It is a Greedy Algorithm.Here is the program linkC PROGRAM FOR NON PREEMPTIVE SHORTEST JOB FIRST SCHEDULING
What cases would make Thrashing better or worst, or even simply do nothing?
Terminating processes first. The trashing happens when multiple processes are fighting for space in RAM and routinely fetch pages from disk. Thrashing (computer science) - WikipediaThen augmenting RAM will prevent scarcity of space. There is an anomaly where adding RAM could increase page faults and then trashing, but it is rare. Bélády's anomaly - WikipediaRewriting code to use less resources would be a possible solution, but this is possible if is your application to cause trashing. It is not viable to rewrite other programs.Adding more disk doesn't help, as the problem here is RAM management, not disk.Other options which could help:Faster access to hard diskLess use of virtual memoryRunning smaller programsA scheduler with shortest job first preemptive with agingNot helping:Faster CPU -> more trashing!A memory debugger program. Causes many more switches aggravating the problem.
What are the scheduling algorithms in an OS?
Scheduling usually refers to selecting which process to run next - but can also refer to which input/output (or disk) operation to do next. (Usually, if no modifier is attached, scheduling refers to process scheduling.)This sounds like such a simple thing - but when you start considering it, it becomes more and more complicated. In operating system design, process scheduling is a matter of tremendous and sometimes heated debates.For a long time, computers could literally do only one thing at a time. The illusion of doing many things was made possible by doing things in rapid rotation, so fast that the human being behind the screen (as it were) perceived everything happening at once. Today, with multi-core machines, the CPU can - again literally - do multiple things at once - but each of these cores are performing the same fast process switching process, and the collection of cores add considerations to process scheduling.Here are some of the things one has to consider in a scheduling algorithm:Is the process "real-time"? That is, it cannot wait for processing.Is the process long-running?Is the process interactive? (such as a shell or windowing environment)Is the process fast or slow?Is the process swapped out?Is the process waiting for a disk response?Is the process waiting for an interactive response?Is the process the most recent started process?Is the process the oldest started process?Which processor core is the most heavily used? (for multi-core machines)Which processor core is the least heavily used? (for multi-core machines)When you consider I/O Scheduling, even more constraints come into play - and it gets even more interesting if you wish to consider a networked-cluster environment, where the CPU could be across the network.Each one of these - Process Scheduling and I/O Scheduling - can be a very complex topic. They could be an entire college course each - and probably are.
What is a commonly used scheduling algo in Linux?
I want to add some more details to Narayan Menariya answer, he only explained process scheduler.Process Scheduler: O(n) scheduler, O(1) scheduler, CFS (default) (Completely Fair Scheduler), BFS (Brain Fuck Scheduler).I/O Scheduler: Noop, deadline, CFQ(default), FSCAN, BFQ.
How do I draw the following scheduling in a Gantt diagram for FIFO algorithm?
In FIFO algorithm (also known as FCFS (first come first serve)), CPU executes the process which came earliest until it is complete. So,Process P1 starts execution at time 0 and it takes 4 CPU (0-1, 1-2, 2-3, 3-4) burst to be executed. and while this process is in execution all other process will have to wait.As soon as P1 finishes execution, CPU starts executing P2. So, P2 starts execution at time 4 and it takes 1 CPU (4-5) burst to be executed.Similarly, other processes are executedNow coming to wait time:P1 arrived at time 0 and CPU started its execution at time 0 , so wait-time for P1 = 0 - 0 = 0P2 arrived at time 1 and CPU started its execution at time 4 , sowait-time for P2 = 4 - 1 = 3P3 arrived at time 2 and CPU started its execution at time 5 , sowait-time for P3 = 5 - 2 = 3........ SO ONsee the image