Program for Shortest Job First (SJF) scheduling | Set 2 (Preemptive)


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

Preemptive-SJF-Diagram

P1 waiting time: 4-2 = 2
P2 waiting time: 0
The 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

This article is contributed by Sahil Chhabra. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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