In previous post, we have discussed Set 1 of SJF i.e. non-preemptive. In this post we will discuss the preemptive version of SJF known as Shortest Remaining Time First (SRTF).

In this scheduling algorithm, the process with the smallest amount of time remaining until completion is selected to execute. Since the currently executing process is the one with the shortest amount of time remaining by definition, and since that time should only reduce as execution progresses, processes will always run until they complete or a new process is added that requires a smaller amount of time.

### Preemptive SJF: Example

Process | Duration | Order | Arrival Time |
---|---|---|---|

P1 | 9 | 1 | 0 |

P2 | 2 | 2 | 2 |

**P1 waiting time: 4-2 = 2P2 waiting time: 0The average waiting time(AWT): (0 + 2) / 2 = 1**

**Advantage:**

1- Short processes are handled very quickly.

2- The system also requires very little overhead since it only makes a decision when a process completes or a new process is added.

3- When a new process is added the algorithm only needs to compare the currently executing process with the new process, ignoring all other processes currently waiting to execute.

**Disadvantage:**

1- Like shortest job first, it has the potential for process starvation.

2- Long processes may be held off indefinitely if short processes are continually added.

Source:Wiki

Implementation:1- Traverse until all process gets completely executed. a) Find process with minimum remaining time at every single time lap. b) Reduce its time by 1. c) Check if its remaining time becomes 0 d) Increment the counter of process completion. e) Completion time of current process = current_time +1; e) Calculate waiting time for each completed process. wt[i]= Completion time - arrival_time-burst_time f)Increment time lap by one. 2- Find turnaround time (waiting_time+burst_time).

## C/C++

// C++ program to implement Shortest Remaining // Time First #include <bits/stdc++.h> using namespace std; struct Process { int pid; // Process ID int bt; // Burst Time int art; // Arrival Time }; // Function to find the waiting time for all // processes void findWaitingTime(Process proc[], int n, int wt[]) { int rt[n]; // Copy the burst time into rt[] for (int i = 0; i < n; i++) rt[i] = proc[i].bt; int complete = 0, t = 0, minm = INT_MAX; int shortest = 0, finish_time; bool check = false; // Process until all processes gets // completed while (complete != n) { // Find process with minimum // remaining time among the // processes that arrives till the // current time` for (int j = 0; j < n; j++) { if ((proc[j].art <= t) && (rt[j] < minm) && rt[j] > 0) { minm = rt[j]; shortest = j; check = true; } } if (check == false) { t++; continue; } // Reduce remaining time by one rt[shortest]--; // Update minimum minm = rt[shortest]; if (minm == 0) minm = INT_MAX; // If a process gets completely // executed if (rt[shortest] == 0) { // Increment complete complete++; // Find finish time of current // process finish_time = t + 1; // Calculate waiting time wt[shortest] = finish_time - proc[shortest].bt - proc[shortest].art; if (wt[shortest] < 0) wt[shortest] = 0; } // Increment time t++; } } // Function to calculate turn around time void findTurnAroundTime(Process proc[], int n, int wt[], int tat[]) { // calculating turnaround time by adding // bt[i] + wt[i] for (int i = 0; i < n; i++) tat[i] = proc[i].bt + wt[i]; } // Function to calculate average time void findavgTime(Process proc[], int n) { int wt[n], tat[n], total_wt = 0, total_tat = 0; // Function to find waiting time of all // processes findWaitingTime(proc, n, wt); // Function to find turn around time for // all processes findTurnAroundTime(proc, n, wt, tat); // Display processes along with all // details cout << "Processes " << " Burst time " << " Waiting time " << " Turn around time\n"; // Calculate total waiting time and // total turnaround time for (int i = 0; i < n; i++) { total_wt = total_wt + wt[i]; total_tat = total_tat + tat[i]; cout << " " << proc[i].pid << "\t\t" << proc[i].bt << "\t\t " << wt[i] << "\t\t " << tat[i] << endl; } cout << "\nAverage waiting time = " << (float)total_wt / (float)n; cout << "\nAverage turn around time = " << (float)total_tat / (float)n; } // Driver code int main() { Process proc[] = { { 1, 6, 1 }, { 2, 8, 1 }, { 3, 7, 2 }, { 4, 3, 3 } }; int n = sizeof(proc) / sizeof(proc[0]); findavgTime(proc, n); return 0; }

## Java

// Java program to implement Shortest Remaining // Time First class Process { int pid; // Process ID int bt; // Burst Time int art; // Arrival Time public Process(int pid, int bt, int art) { this.pid = pid; this.bt = bt; this.art = art; } } public class GFG { // Method to find the waiting time for all // processes static void findWaitingTime(Process proc[], int n, int wt[]) { int rt[] = new int[n]; // Copy the burst time into rt[] for (int i = 0; i < n; i++) rt[i] = proc[i].bt; int complete = 0, t = 0, minm = Integer.MAX_VALUE; int shortest = 0, finish_time; boolean check = false; // Process until all processes gets // completed while (complete != n) { // Find process with minimum // remaining time among the // processes that arrives till the // current time` for (int j = 0; j < n; j++) { if ((proc[j].art <= t) && (rt[j] < minm) && rt[j] > 0) { minm = rt[j]; shortest = j; check = true; } } if (check == false) { t++; continue; } // Reduce remaining time by one rt[shortest]--; // Update minimum minm = rt[shortest]; if (minm == 0) minm = Integer.MAX_VALUE; // If a process gets completely // executed if (rt[shortest] == 0) { // Increment complete complete++; // Find finish time of current // process finish_time = t + 1; // Calculate waiting time wt[shortest] = finish_time - proc[shortest].bt - proc[shortest].art; if (wt[shortest] < 0) wt[shortest] = 0; } // Increment time t++; } } // Method to calculate turn around time static void findTurnAroundTime(Process proc[], int n, int wt[], int tat[]) { // calculating turnaround time by adding // bt[i] + wt[i] for (int i = 0; i < n; i++) tat[i] = proc[i].bt + wt[i]; } // Method to calculate average time static void findavgTime(Process proc[], int n) { int wt[] = new int[n], tat[] = new int[n]; int total_wt = 0, total_tat = 0; // Function to find waiting time of all // processes findWaitingTime(proc, n, wt); // Function to find turn around time for // all processes findTurnAroundTime(proc, n, wt, tat); // Display processes along with all // details System.out.println("Processes " + " Burst time " + " Waiting time " + " Turn around time"); // Calculate total waiting time and // total turnaround time for (int i = 0; i < n; i++) { total_wt = total_wt + wt[i]; total_tat = total_tat + tat[i]; System.out.println(" " + proc[i].pid + "\t\t" + proc[i].bt + "\t\t " + wt[i] + "\t\t" + tat[i]); } System.out.println("Average waiting time = " + (float)total_wt / (float)n); System.out.println("Average turn around time = " + (float)total_tat / (float)n); } // Driver Method public static void main(String[] args) { Process proc[] = { new Process(1, 6, 1), new Process(2, 8, 1), new Process(3, 7, 2), new Process(4, 3, 3)}; findavgTime(proc, proc.length); } }

Output:

Processes Burst time Waiting time Turn around time 1 6 3 9 2 8 16 24 3 7 8 15 4 3 0 3 Average waiting time = 6.75 Average turn around time = 12.75

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