Shortest Remaining Time First (Preemptive SJF) Scheduling Algorithm

10 May, 2022
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 the Shortest Remaining Time First (SRTF) 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.
Examples to show working of Preemptive Shortest Job First CPU Scheduling Algorithm:
Example-1: Consider the following table of arrival time and burst time for five processes P1, P2, P3, P4 and P5. 
The Shortest Job First CPU Scheduling Algorithm will work on the basis of steps as mentioned below:
At time = 0,
Process P4 arrives and starts executing
At time= 1, 
Process P3 arrives. 
But, as P4 has a shorter burst time. It will continue execution.
Thus, P3 will wait till P4 gets executed.
At time =2, 
Process P1 arrives with burst time = 6
As the burst time of P1 is more than that of P4 
Thus, P4 will continue its execution.
At time = 3, 
Process P4 will finish its execution. 
Then, the burst time of P3 and P1 is compared. 
Process P1 is executed because its burst time is less as compared to P3.
    
          P3
At time = 4, 
Process P5 arrives.
Then the burst time of P3, P5, and P1 is compared. 
Process P5 gets executed first among them because its burst time is lowest, and process P1 is preempted.
At time = 5, 
Process P2 arrives.
The burst time of all processes are compared, 
Process P2 gets executed as its burst time is lowest among all. 
Process P5 is preempted.
At time = 6, 
Process P2 will keep executing.
It will execute till time = 8 as the burst time of P2 is 2ms
P3, P5, P1
At time=7, 
The process P2 finishes its execution.
Then again the burst time of all remaining processes is compared. 
The Process P5 gets executed because its burst time is lesser than the others.
At time = 10, 
The process P5 will finish its execution. 
Then the burst time of the remaining processes P1 and P3 is compared. 
Thus, process P1 is executed as its burst time is less than P3
At time = 15,
The process P1 finishes its execution and P3 is the only process left. 
P3 will start executing.
At time = 23, 
Process P3 will finish its execution.
The overall execution of the processes will be as shown below:
    
        P3
Gantt chart for above execution:
Gantt chart for SRTF
Now, lets calculate average waiting time and turn around time:
As we know,
Turn Around time = Completion time – arrival time
Waiting Time = Turn around time – burst time
Now, 
Average Turn around time = (13 + 2 + 22 + 3 + 6)/5 = 9.2
Average waiting time = (7 + 0 + 14 + 0 + 2)/5 = 23/5 = 4.6
Implementation of SRTF Algorithm: 
Approach:
Traverse until all process gets completely executed.Find process with minimum remaining time at every single time lap.Reduce its time by 1.Check if its remaining time becomes 0 Increment the counter of process completion.Completion time of current process = current_time + 1;Calculate waiting time for each completed process. wt[i]= Completion time – arrival_time-burst_timeIncrement time lap by one.
Find process with minimum remaining time at every single time lap.
Reduce its time by 1.
Check if its remaining time becomes 0 
Increment the counter of process completion.
Completion time of current process = current_time + 1;
Calculate waiting time for each completed process. wt[i]= Completion time – arrival_time-burst_time
 wt[i]= Completion time – arrival_time-burst_time
Increment time lap by one.
Find turnaround time (waiting_time + burst_time).
 Program to implement Shortest Remaining Time First:
C++
Java
Python3
C#
Javascript
// C++ program to implement Shortest Remaining Time First// Shortest Remaining Time First (SRTF)  #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// processesvoid 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++;            check = false;              // 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 timevoid 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 timevoid 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 << " P\t\t"        << "BT\t\t"        << "WT\t\t"        << "TAT\t\t\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 codeint main(){    Process proc[] = { { 1, 6, 2 }, { 2, 2, 5 },                    { 3, 8, 1 }, { 4, 3, 0}, {5, 4, 4} };    int n = sizeof(proc) / sizeof(proc[0]);      findavgTime(proc, n);    return 0;}
// Java program to implement Shortest Remaining Time First// Shortest Remaining Time First (SRTF)  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++;                check = false;                       // 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);    }}
# Python3 program to implement Shortest Remaining Time First# Shortest Remaining Time First (SRTF)  # Function to find the waiting time # for all processes def findWaitingTime(processes, n, wt):     rt = [0] * n      # Copy the burst time into rt[]     for i in range(n):         rt[i] = processes[i][1]    complete = 0    t = 0    minm = 999999999    short = 0    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 j in range(n):            if ((processes[j][2] <= t) and                 (rt[j] < minm) and rt[j] > 0):                minm = rt[j]                short = j                check = True        if (check == False):            t += 1            continue                      # Reduce remaining time by one         rt[short] -= 1          # Update minimum         minm = rt[short]         if (minm == 0):             minm = 999999999          # If a process gets completely         # executed         if (rt[short] == 0):               # Increment complete             complete += 1            check = False              # Find finish time of current             # process             fint = t + 1              # Calculate waiting time             wt[short] = (fint - proc[short][1] -                                    proc[short][2])              if (wt[short] < 0):                wt[short] = 0                  # Increment time         t += 1  # Function to calculate turn around time def findTurnAroundTime(processes, n, wt, tat):           # Calculating turnaround time     for i in range(n):        tat[i] = processes[i][1] + wt[i]   # Function to calculate average waiting # and turn-around times. def findavgTime(processes, n):     wt = [0] * n    tat = [0] * n       # Function to find waiting time     # of all processes     findWaitingTime(processes, n, wt)       # Function to find turn around time    # for all processes     findTurnAroundTime(processes, n, wt, tat)       # Display processes along with all details     print("Processes    Burst Time     Waiting",                     "Time     Turn-Around Time")    total_wt = 0    total_tat = 0    for i in range(n):          total_wt = total_wt + wt[i]         total_tat = total_tat + tat[i]         print(" ", processes[i][0], "\t\t",                    processes[i][1], "\t\t",                    wt[i], "\t\t", tat[i])      print("\nAverage waiting time = %.5f "%(total_wt /n) )    print("Average turn around time = ", total_tat / n)       # Driver code if __name__ =="__main__":          # Process id's     proc = [[1, 6, 1], [2, 8, 1],            [3, 7, 2], [4, 3, 3]]    n = 4    findavgTime(proc, n)      # This code is contributed# Shubham Singh(SHUBHAMSINGH10)
// C# program to implement Shortest Remaining Time First// Shortest Remaining Time First (SRTF)  using System;  public class Process{    public int pid; // Process ID    public int bt; // Burst Time    public 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 = int.MaxValue;        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.MaxValue;                  // If a process gets completely            // executed            if (rt[shortest] == 0)             {                      // Increment complete                complete++;                check = false;                      // 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];int []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        Console.WriteLine("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];            Console.WriteLine(" " + proc[i].pid + "\t\t"                            + proc[i].bt + "\t\t " + wt[i]                            + "\t\t" + tat[i]);        }              Console.WriteLine("Average waiting time = " +                        (float)total_wt / (float)n);        Console.WriteLine("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);    }}  // This code has been contributed by 29AjayKumar
<script>// Javascript program to implement // Shortest Remaining Time First// Shortest Remaining Time First (SRTF)  class Process{    constructor(pid,bt,art)    {        this.pid = pid;    // Process ID        this.bt = bt;    // Burst Time        this.art = art;    // Arrival Time    }}  // Method to find the waiting time for all    // processesfunction findWaitingTime( proc,n,wt){    let rt = new Array(n);                 // Copy the burst time into rt[]        for (let i = 0; i < n; i++)            rt[i] = proc[i].bt;                 let complete = 0, t = 0, minm = Number.MAX_VALUE;        let shortest = 0, finish_time;        let 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 (let 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 = Number.MAX_VALUE;                     // If a process gets completely            // executed            if (rt[shortest] == 0) {                         // Increment complete                complete++;                check = false;                         // 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 timefunction findTurnAroundTime(proc,n,wt,tat){     // calculating turnaround time by adding        // bt[i] + wt[i]        for (let i = 0; i < n; i++)            tat[i] = proc[i].bt + wt[i];}  // Method to calculate average timefunction findavgTime(proc,n){    let wt = new Array(n), tat = new Array(n);        let  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        document.write("Processes " +                           " Burst time " +                           " Waiting time " +                           " Turn around time<br>");                 // Calculate total waiting time and        // total turnaround time        for (let i = 0; i < n; i++) {            total_wt = total_wt + wt[i];            total_tat = total_tat + tat[i];            document.write(" " + proc[i].pid +             "      "            + proc[i].bt + "    " + wt[i]            + "    " + tat[i]+"<br>");        }                 document.write("Average waiting time = " +                          total_wt / n+"<br>");        document.write("Average turn around time = " +                           total_tat / n+"<br>");}  // Driver Methodlet 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);    // This code is contributed by rag2127  </script>
 P        BT        WT        TAT        
 1        6         7         13
 2        2         0         2
 3        8         14         22
 4        3         0         3
 5        4         2         6

Average waiting time = 4.6
Average turn around time = 9.2
Time Complexity: O(N)Auxiliary Space: O(1)
Advantages: 
Short processes are handled very quickly. 
The system also requires very little overhead since it only makes a decision when a process completes or a new process is added. 
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.
Disadvantages: 
Like shortest job first, it has the potential for process starvation. 
Long processes may be held off indefinitely if short processes are continually added. 
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