Shortest Remaining Time First (Preemptive SJF) Scheduling Algorithm

In previous post, we have discussed Set 1 of SJF i.e. non-pre-emptive. In this post we will discuss the pre-emptive 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 Pre-emptive 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. 

Process	Burst Time	Arrival Time
P1	6 ms	2 ms
P2	2 ms	5 ms
P3	8 ms	1 ms
P4	3 ms	0 ms
P5	4 ms	4 ms

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

Time Instance	Process	Arrival Time	Waiting Table	Execution Time	Initial Burst Time	Remaining Burst  Time
0-1ms	P4	0ms		1ms	3ms	2ms

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.

Time Instance	Process	Arrival Time	Waiting Table	Execution Time	Initial Burst Time	Remaining Burst  Time
1-2ms	P4	0ms	P3	1ms	2ms	1ms
	P3	1ms		0ms	8ms	8ms

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.

Time Instance	Process	Arrival Time	Waiting Table	Execution Time	Initial Burst Time	Remaining Burst  Time
2-3ms	P4	0ms	P3, P1	1ms	1ms	0ms
	P3	1ms		0ms	8ms	8ms
	P1	2ms		0ms	6ms	6ms

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.

Time Instance	Process	Arrival Time	Waiting Table	Execution Time	Initial Burst Time	Remaining Burst  Time
3-4ms	P3	1ms	P3	0ms	8ms	8ms
	P1	2ms		1ms	6ms	5ms

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.

Time Instance	Process	Arrival Time	Waiting Table	Execution Time	Initial Burst Time	Remaining Burst  Time
4-5ms	P3	1ms	P3, P1	0ms	8ms	8ms
	P1	2ms		0ms	5ms	5ms
	P5	4ms		1ms	4ms	3ms

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.

Time Instance	Process	Arrival Time	Waiting Table	Execution Time	Initial Burst Time	Remaining Burst  Time
5-6ms	P3	1ms	P3, P5, P1	0ms	8ms	8ms
	P1	2ms		0ms	5ms	5ms
	P5	4ms		0ms	3ms	3ms
	P2	5ms		1ms	2ms	1ms

At time = 6, 

• Process P2 will keep executing.
• It will execute till time = 7 as the burst time of P2 is 2ms

Time Instance	Process	Arrival Time	Waiting Table	Execution Time	Initial Burst Time	Remaining Burst  Time
6-7ms	P3	1ms	P3, P5, P1	0ms	8ms	8ms
	P1	2ms		0ms	5ms	5ms
	P5	4ms		0ms	3ms	3ms
	P2	5ms		1ms	1ms	0ms

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.

Time Instance	Process	Arrival Time	Waiting Table	Execution Time	Initial Burst Time	Remaining Burst  Time
7-10ms	P3	1ms	P3, P1	0ms	8ms	8ms
	P1	2ms		0ms	5ms	5ms
	P5	4ms		3ms	3ms	0ms

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

Time Instance	Process	Arrival Time	Waiting Table	Execution Time	Initial Burst Time	Remaining Burst  Time
10-15ms	P3	1ms	P3	0ms	8ms	8ms
	P1	4ms		4ms	5ms	0ms

At time = 15,

• The process P1 finishes its execution and P3 is the only process left. 
• P3 will start executing.

Time Instance	Process	Arrival Time	Waiting Table	Execution Time	Initial Burst Time	Remaining Burst  Time
15-23ms	P3	1ms		8ms	8ms	0ms

At time = 23, 

• Process P3 will finish its execution.
• The overall execution of the processes will be as shown below:

Time Instance	Process	Arrival Time	Waiting Table	Execution Time	Initial Burst Time	Remaining Burst  Time
0-1ms	P4	0ms		1ms	3ms	2ms
1-2ms	P4	0ms	P3	1ms	2ms	1ms
	P3	1ms		0ms	8ms	8ms
2-3ms	P4	0ms	P3, P1	1ms	1ms	0ms
	P3	1ms		0ms	8ms	8ms
	P1	2ms		0ms	6ms	6ms
3-4ms	P3	1ms	P3	0ms	8ms	8ms
	P1	2ms		1ms	6ms	5ms
4-5ms	P3	1ms	P3, P1	0ms	8ms	8ms
	P1	2ms		0ms	5ms	5ms
	P5	4ms		1ms	4ms	3ms
5-6ms	P3	1ms	P3, P5, P1	0ms	8ms	8ms
	P1	2ms		0ms	5ms	5ms
	P5	4ms		0ms	3ms	3ms
	P2	5ms		1ms	2ms	1ms
6-7ms	P3	1ms	P3, P5, P1	0ms	8ms	8ms
	P1	2ms		0ms	5ms	5ms
	P5	4ms		0ms	3ms	3ms
	P2	5ms		1ms	1ms	0ms
7-10ms	P3	1ms	P3, P1	0ms	8ms	8ms
	P1	2ms		0ms	5ms	5ms
	P5	4ms		3ms	3ms	0ms
10-15ms	P3	1ms	P3	0ms	8ms	8ms
	P1	4ms		4ms	5ms	0ms
15-23ms	P3	1ms		8ms	8ms	0ms

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

Process	Completion Time	Turn Around Time	Waiting Time
P1	15	15-2 = 13	13-6 = 7
P2	7	7-5 = 2	2-2 = 0
P3	23	23-1 = 22	22-8 = 14
P4	3	3-0 = 3	3-3 = 0
P5	10	10-4 = 6	6-4 = 2

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

Some of the key characteristics of SRTF 

• Preemptive: SRTF is a preemptive algorithm, which means that the currently running process can be interrupted if a new process arrives with a shorter burst time. This helps in ensuring that the processes with the shortest burst times are executed first.
• Dynamic: SRTF is a dynamic algorithm, which means that it can adapt to changes in the arrival time and burst time of processes. It constantly re-evaluates the remaining burst time of each process and schedules the process with the shortest remaining time.
• Low waiting time: SRTF is known for its low waiting time. By selecting the process with the shortest remaining burst time, it ensures that the processes with the shortest burst times are executed first, which reduces the average waiting time of processes.
• SRTF has a higher complexity than other scheduling algorithms like FCFS (First Come First Serve) and RR (Round Robin), because it requires frequent context switches and preemptions.

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_time
  • Increment time lap by one.
• Find turnaround time (waiting_time + burst_time).

 Program to implement Shortest Remaining Time First:

C++

// 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
// 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++;
            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 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 << " 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 code
int 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;
}

C

// C program to implement Shortest Remaining Time First
// Shortest Remaining Time First (SRTF)

#include <stdio.h>
#include <limits.h>

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(struct 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;
    int check = 0; // changed boolean to integer

    // 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 = 1; // changed boolean to integer
            }
        }

        if (check == 0) {
            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 = 0; // changed boolean to integer

            // 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(struct 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(struct 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
    printf(" 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];
        printf(" %d\t\t"
            "%d\t\t %d"
            "\t\t %d\n", proc[i].pid,
            proc[i].bt, wt[i], tat[i]);
    }

    printf("\nAverage waiting time = "
        "%f", (float)total_wt / (float)n);
    printf("\nAverage turn around time = "
        "%f", (float)total_tat / (float)n);
}

// Driver code
int main() {
    struct 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

// 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

# 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#

// 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

Javascript

<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
    // processes
function 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 time
function 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 time
function 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 + 
            "&emsp;&emsp;&emsp;&emsp;&emsp;&emsp;"
            + proc[i].bt + "&emsp;&emsp;&emsp;&emsp;" + wt[i]
            + "&emsp;&emsp;&emsp;&emsp;" + tat[i]+"<br>");
        }
       
        document.write("Average waiting time = " +
                          total_wt / n+"<br>");
        document.write("Average turn around time = " +
                           total_tat / n+"<br>");
}

// Driver Method
let 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>

Output

 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(N)

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. 

Source:Wiki
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