Shortest job next
Encyclopedia
Shortest job next also known as Shortest Job First (SJF) or Shortest Process Next (SPN), is a scheduling policy that selects the waiting process
with the smallest execution time to execute next. SJN is a non-preemptive
algorithm. Shortest remaining time
is a preemptive variant of SJN.
Shortest job next is advantageous because of its simplicity and because it maximizes process throughput (in terms of the number of processes run to completion in a given amount of time). It also minimizes the average amount of time each process has to wait until its execution is complete. However, it has the potential for process starvation for processes which will require a long time to complete if short processes are continually added. Highest response ratio next
is similar but provides a solution to this problem.
Shortest job next can be effectively used with interactive processes which generally follow a pattern of alternating between waiting for a command and executing it. If the execution of a command is regarded as a separate "process", past behaviour can indicate which process to run next, based on an estimate of its running time.
Shortest job next scheduling is rarely used outside of specialized environments because it requires accurate estimations of the runtime of all processes that are waiting to execute. Estimating the running time of queued processes is sometimes done using a technique called aging
.
Process (computing)
In computing, a process is an instance of a computer program that is being executed. It contains the program code and its current activity. Depending on the operating system , a process may be made up of multiple threads of execution that execute instructions concurrently.A computer program is a...
with the smallest execution time to execute next. SJN is a non-preemptive
Preemption (computing)
In computing, preemption is the act of temporarily interrupting a task being carried out by a computer system, without requiring its cooperation, and with the intention of resuming the task at a later time. Such a change is known as a context switch...
algorithm. Shortest remaining time
Shortest remaining time
Shortest remaining time is a scheduling method that is a preemptive version of shortest job next scheduling. In this scheduling algorithm, the process with the smallest amount of time remaining until completion is selected to execute...
is a preemptive variant of SJN.
Shortest job next is advantageous because of its simplicity and because it maximizes process throughput (in terms of the number of processes run to completion in a given amount of time). It also minimizes the average amount of time each process has to wait until its execution is complete. However, it has the potential for process starvation for processes which will require a long time to complete if short processes are continually added. Highest response ratio next
Highest response ratio next
Highest Response Ratio Next scheduling is a non-preemptive discipline, similar to Shortest Job Next , in which the priority of each job is dependent on its estimated run time, and also the amount of time it has spent waiting. Jobs gain higher priority the longer they wait, which prevents...
is similar but provides a solution to this problem.
Shortest job next can be effectively used with interactive processes which generally follow a pattern of alternating between waiting for a command and executing it. If the execution of a command is regarded as a separate "process", past behaviour can indicate which process to run next, based on an estimate of its running time.
Shortest job next scheduling is rarely used outside of specialized environments because it requires accurate estimations of the runtime of all processes that are waiting to execute. Estimating the running time of queued processes is sometimes done using a technique called aging
Aging (scheduling)
Aging is the process of gradually increasing the priority of a task, based on its waiting time. The aging technique estimates the time a process will run based on a weighted average of previous estimates and measured values. Aging can be used to reduce starvation of low priority tasks...
.