# 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).

### Shortest Remaining Time First (SRTF) scheduling

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.

### Shortest Remaining Time First (Preemptive SJF): Example

Process Duration Order Arrival Time
P1 9 1 0
P2 2 2 2 P1 waiting time: 4-2 = 2
P2 waiting time: 0
The average waiting time(AWT): (0 + 2) / 2 = 1

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.

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

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

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 ` `// Shortest Remaining Time First (SRTF) ` ` `  `#include ` `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 << ``"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); ` ` `  `    ``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 `

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.