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# Longest Remaining Time First (LRTF) CPU Scheduling Program

• Difficulty Level : Hard
• Last Updated : 13 Sep, 2021

Prerequisite – CPU Scheduling | Longest Remaining Time First (LRTF) algorithm
We have given some process with arrival time and Burst Time and we have to find the completion time (CT), Turn Around Time(TAT), Average Turn Around Time (Avg TAT), Waiting Time(WT), Average Waiting Time (AWT) for the given processes.

Example: Consider the following table of arrival time and burst time for four processes P1, P2, P3 and P4.

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```Process   Arrival time   Burst Time
P1            1 ms          2 ms
P2            2 ms          4 ms
P3            3 ms          6 ms
p4            4 ms          8 ms  ```

Gantt chart will be as following below, Since, completion time (CT) can be directly determined by Gantt chart, and

```Turn Around Time (TAT)
= (Complition Time) - (Arrival Time)

Also, Waiting Time (WT)
= (Turn Around Time) - (Burst Time) ```

Therefore,

Output:

```Total Turn Around Time = 68 ms
So, Average Turn Around Time = 68/4 = 17.00 ms

And, Total Waiting Time = 48 ms
So, Average Waiting Time = 12.00 ms ```

Algorithm –

• Step-1: Create a structure of process containing all necessary fields like AT (Arrival Time), BT(Burst Time), CT(Completion Time), TAT(Turn Around Time), WT(Waiting Time).
• Step-2: Sort according to the AT;
• Step-3: Find the process having Largest Burst Time and execute for each single unit. Increase the total time by 1 and reduce the Burst Time of that process with 1.
• Step-4: When any process have 0 BT left, then update the CT(Completion Time of that process CT will be Total Time at that time).
• Step-2: After calculating the CT for each process, find TAT and WT.
```(TAT = CT - AT)
(WT  = TAT - BT) ```

Implementation of Algorithm –

## C++

 `#include ` `using` `namespace` `std;` `// creating a structure of a process``struct` `process {``    ``int` `processno;``    ``int` `AT;``    ``int` `BT;` `    ``// for backup purpose to print in last``    ``int` `BTbackup;``    ``int` `WT;``    ``int` `TAT;``    ``int` `CT;``};` `// creating a structure of 4 processes``struct` `process p;` `// variable to find the total time``int` `totaltime = 0;``int` `prefinaltotal = 0;` `// comparator function for sort()``bool` `compare(process p1, process p2)``{``    ``// compare the Arrival time of two processes``    ``return` `p1.AT < p2.AT;``}` `// finding the largest Arrival Time among all the available``// process at that time``int` `findlargest(``int` `at)``{``    ``int` `max = 0, i;``    ``for` `(i = 0; i < 4; i++) {``        ``if` `(p[i].AT <= at) {``            ``if` `(p[i].BT > p[max].BT)``                ``max = i;``        ``}``    ``}` `    ``// returning the index of the process having the largest BT``    ``return` `max;``}` `// function to find the completion time of each process``int` `findCT()``{` `    ``int` `index;``    ``int` `flag = 0;``    ``int` `i = p.AT;``    ``while` `(1) {``        ``if` `(i <= 4) {``            ``index = findlargest(i);``        ``}` `        ``else``            ``index = findlargest(4);``        ``cout << ``"Process executing at time "` `<< totaltime``             ``<< ``" is: P"` `<< index + 1 << ``"\t"``;` `        ``p[index].BT -= 1;``        ``totaltime += 1;``        ``i++;` `        ``if` `(p[index].BT == 0) {``            ``p[index].CT = totaltime;``            ``cout << ``" Process P"` `<< p[index].processno``                 ``<< ``" is completed at "` `<< totaltime;``        ``}``        ``cout << endl;` `        ``// loop termination condition``        ``if` `(totaltime == prefinaltotal)``            ``break``;``    ``}``}` `int` `main()``{` `    ``int` `i;` `    ``// initializing the process number``    ``for` `(i = 0; i < 4; i++) {``        ``p[i].processno = i + 1;``    ``}` `    ``// cout<<"arrival time of 4 processes : ";``    ``for` `(i = 0; i < 4; i++) ``// taking AT``    ``{``        ``p[i].AT = i + 1;``    ``}` `    ``// cout<<" Burst time of 4 processes : ";``    ``for` `(i = 0; i < 4; i++) {` `        ``// assigning {2, 4, 6, 8} as Burst Time to the processes``        ``// backup for displaying the output in last``        ``// calculating total required time for terminating``        ``// the function().``        ``p[i].BT = 2 * (i + 1);``        ``p[i].BTbackup = p[i].BT;``        ``prefinaltotal += p[i].BT;``    ``}` `    ``// displaying the process before executing``    ``cout << ``"PNo\tAT\tBT\n"``;` `    ``for` `(i = 0; i < 4; i++) {``        ``cout << p[i].processno << ``"\t"``;``        ``cout << p[i].AT << ``"\t"``;``        ``cout << p[i].BT << ``"\t"``;``        ``cout << endl;``    ``}``    ``cout << endl;` `    ``// sorting process according to Arrival Time``    ``sort(p, p + 4, compare);` `    ``// calculating initial time when execution starts``    ``totaltime += p.AT;` `    ``// calculating to terminate loop``    ``prefinaltotal += p.AT;``    ``findCT();``    ``int` `totalWT = 0;``    ``int` `totalTAT = 0;``    ``for` `(i = 0; i < 4; i++) {``        ``// since, TAT = CT - AT``        ``p[i].TAT = p[i].CT - p[i].AT;``        ``p[i].WT = p[i].TAT - p[i].BTbackup;` `        ``// finding total waiting time``        ``totalWT += p[i].WT;` `        ``// finding total turn around time``        ``totalTAT += p[i].TAT;``    ``}` `    ``cout << ``"After execution of all processes ... \n"``;` `    ``// after all process executes``    ``cout << ``"PNo\tAT\tBT\tCT\tTAT\tWT\n"``;` `    ``for` `(i = 0; i < 4; i++) {``        ``cout << p[i].processno << ``"\t"``;``        ``cout << p[i].AT << ``"\t"``;``        ``cout << p[i].BTbackup << ``"\t"``;``        ``cout << p[i].CT << ``"\t"``;``        ``cout << p[i].TAT << ``"\t"``;``        ``cout << p[i].WT << ``"\t"``;``        ``cout << endl;``    ``}` `    ``cout << endl;``    ``cout << ``"Total TAT = "` `<< totalTAT << endl;``    ``cout << ``"Average TAT = "` `<< totalTAT / 4.0 << endl;``    ``cout << ``"Total WT = "` `<< totalWT << endl;``    ``cout << ``"Average WT = "` `<< totalWT / 4.0 << endl;``    ``return` `0;``}`

## Python3

 `# Python3 program to implement``# Longest Remaining Time First` `# creating a structure of 4 processes``p ``=` `[]``for` `i ``in` `range``(``4``):``    ``p.append([``0``, ``0``, ``0``, ``0``, ``0``, ``0``, ``0``])` `# variable to find the total time``totaltime ``=` `0``prefinaltotal ``=` `0` `# finding the largest Arrival Time``# among all the available process``# at that time``def` `findlargest(at):``    ``max` `=` `0``    ``for` `i ``in` `range``(``4``):``        ``if` `(p[i][``1``] <``=` `at):``            ``if` `(p[i][``2``] > p[``max``][``2``]) :``                ``max` `=` `i``    ` `    ``# returning the index of the``    ``# process having the largest BT``    ``return` `max` `# function to find the completion``# time of each process``def` `findCT(totaltime):``    ``index ``=` `0``    ``flag ``=` `0``    ``i ``=` `p[``0``][``1``]``    ``while` `(``1``):``        ``if` `(i <``=` `4``):``            ``index ``=` `findlargest(i)``        ``else``:``            ``index ``=` `findlargest(``4``)``        ``print``(``"Process execute at time "``,``                    ``totaltime, end ``=` `" "``)``        ``print``(``" is: P"``, index ``+` `1``,``                        ``sep ``=` `"``", end = "` `")``        ``p[index][``2``] ``-``=` `1``        ``totaltime ``+``=` `1``        ``i ``+``=` `1``        ``if` `(p[index][``2``] ``=``=` `0``):``                ``p[index][``6``] ``=` `totaltime``                ``print``(``"Process P"``, p[index][``0``],``                           ``sep ``=` `"``", end = "` `")``                ``print``(``" is completed at "``,``                     ``totaltime, end ``=` `" "``)``        ``print``()``        ` `        ``# loop termination condition``        ``if` `(totaltime ``=``=` `prefinaltotal):``            ``break` `# Driver code``if` `__name__ ``=``=``"__main__"``:``    ` `    ``# initializing the process number``    ``for` `i ``in` `range``(``4``):``        ``p[i][``0``] ``=` `i ``+` `1` `    ``for` `i ``in` `range``(``4``): ``# taking AT``        ``p[i][``1``] ``=` `i ``+` `1` `    ``for` `i ``in` `range``(``4``):` `        ``# assigning 2, 4, 6, 8 as Burst Time``        ``# to the processes backup for displaying``        ``# the output in last calculating total``        ``# required time for terminating the function().``        ``p[i][``2``] ``=` `2` `*` `(i ``+` `1``)``        ``p[i][``3``] ``=` `p[i][``2``]``        ``prefinaltotal ``+``=` `p[i][``2``]` `    ``# displaying the process before executing``    ``print``(``"PNo\tAT\tBT"``)` `    ``for` `i ``in` `range``(``4``):``        ``print``(p[i][``0``], ``"\t"``,``              ``p[i][``1``], ``"\t"``, p[i][``2``])``    ``print``()``    ` `    ``# sorting process according to Arrival Time``    ``p ``=` `sorted``(p, key ``=` `lambda` `p:p[``1``])` `    ``# calculating initial time when``    ``# execution starts``    ``totaltime ``+``=` `p[``0``][``1``]` `    ``# calculating to terminate loop``    ``prefinaltotal ``+``=` `p[``0``][``1``]``    ``findCT(totaltime)``    ``totalWT ``=` `0``    ``totalTAT ``=` `0``    ``for` `i ``in` `range``(``4``):``        ` `        ``# since, TAT = CT - AT``        ``p[i][``5``] ``=` `p[i][``6``]``-` `p[i][``1``]``        ``p[i][``4``] ``=` `p[i][``5``] ``-` `p[i][``3``]` `        ``# finding total waiting time``        ``totalWT ``+``=` `p[i][``4``]` `        ``# finding total turn around time``        ``totalTAT ``+``=` `p[i][``5``]` `    ``print``(``"\nAfter execution of all processes ... "``)` `    ``# after all process executes``    ``print``(``"PNo\tAT\tBT\tCT\tTAT\tWT"` `)` `    ``for` `i ``in` `range``(``4``):``        ``print``(p[i][``0``], ``"\t"``, p[i][``1``], ``"\t"``,``              ``p[i][``3``], ``"\t"``, end ``=` `" "``)``        ``print``(p[i][``6``], ``"\t"``,``              ``p[i][``5``], ``"\t"``, p[i][``4``])``    ``print``()``    ``print``(``"Total TAT = "``, totalTAT)``    ``print``(``"Average TAT = "``, totalTAT ``/` `4.0``)``    ``print``(``"Total WT = "``, totalWT)``    ``print``(``"Average WT = "``, totalWT ``/` `4.0``)` `# This code is contributed by``# Shubham Singh(SHUBHAMSINGH10)`

Output:

```PNo    AT    BT
1    1    2
2    2    4
3    3    6
4    4    8

Process executing at time 1 is: P1
Process executing at time 2 is: P2
Process executing at time 3 is: P3
Process executing at time 4 is: P4
Process executing at time 5 is: P4
Process executing at time 6 is: P4
Process executing at time 7 is: P3
Process executing at time 8 is: P4
Process executing at time 9 is: P3
Process executing at time 10 is: P4
Process executing at time 11 is: P2
Process executing at time 12 is: P3
Process executing at time 13 is: P4
Process executing at time 14 is: P2
Process executing at time 15 is: P3
Process executing at time 16 is: P4
Process executing at time 17 is: P1     Process P1 is completed at 18
Process executing at time 18 is: P2     Process P2 is completed at 19
Process executing at time 19 is: P3     Process P3 is completed at 20
Process executing at time 20 is: P4     Process P4 is completed at 21
After execution of all processes ...
PNo    AT    BT    CT    TAT    WT
1    1    2    18    17    15
2    2    4    19    17    13
3    3    6    20    17    11
4    4    8    21    17    9

Total TAT = 68
Average TAT = 17
Total WT = 48
Average WT = 12 ```

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