Clairvoyant Shortest Job first (SJF)

In this article, we discuss about Clairvoyant SJF. It is a theoretical concept in which the algorithm looks in the future and waits for the shortest process to arrive, this results in the least average waiting time.

Difference between Clairvoyant SJF and Shortest Job First :
Both algorithms work on the same principle of allocating CPU time to the shorter process. The difference lies in the fact that Clairvoyant can look into the future and wait for the shortest process and allocate the resource accordingly, whereas SJF has to allocate the resources to the process which have already arrived (i.e., are waiting the ready queue).
Examples:

Input: The processes are,



Process id    Arrival time    Burst time    
    p1            0             5        
    p2            1             2        
    p3            3             1        
    p4            4             3    

Output: Process scheduling according to Clairvoyant SJF is,

Process id    Arrival time    Burst time    
    p3           3              1        
    p2           1              2        
    p4           4              3        
    p1           0              5    

The average waiting time is : 2.5
Output: Process scheduling according to Shortest Job First is,

Process id    Arrival time    Burst time    
    p1           0              5        
    p3           3              1        
    p2           1              2        
    p4           4              3    

The average waiting time is : 2.75

Note:
Clairvoyant SJF and SJF will give the same result if all the processes arrive at the same time.

Input: The processes are,

Process id      Arrival time    Burst time      
    p1               0               4
    p2               0               9
    p3               0               5
    p4               0               3    

Output: Process scheduling according to Clairvoyant SJF is,

Process id    Arrival time    Burst time    
    p4              0               3
    p1              0               4
    p3              0               5
    p2              0               9    

The average waiting time is : 5.5
Output: Process scheduling according to Shortest Job First is,

Process id    Arrival time    Burst time    
    p4              0               3
    p1              0               4
    p3              0               5
    p2              0               9    

The average waiting time is : 5.5

Code :

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#include <bits/stdc++.h>
#define SIZE 4
using namespace std;
  
// Structure to store the information about the process
typedef struct proinfo {
    string pname; // Process name
    int atime; // Arrival time
    int btime; // Burst time
} proinfo;
  
// This function schedules the process 
// according to the Clairvoyant SJF scheduling algorithm.
void clairvoyantSjf(proinfo* arr)
{
    // To sort the processes according to the burst time
    int index = 0;
    for (int i = 0; i < SIZE - 1; i++) {
        index = i;
        for (int j = i + 1; j < SIZE; j++) {
            if (arr[j].btime < arr[index].btime) {
  
                index = j;
            }
        }
        swap(arr[i], arr[index]);
    }
}
  
void display(proinfo* arr)
{
    cout << endl;
    cout << "Process id"
         << "\t";
    cout << "Arrival time"
         << "\t";
    cout << "Burst time"
         << "\t";
    cout << endl;
    for (int i = 0; i < SIZE; i++) {
        cout << arr[i].pname << "\t\t";
        cout << arr[i].atime << "\t\t";
        cout << arr[i].btime << "\t\t";
        cout << endl;
    }
}
  
// To calculate the average waiting time
void avgWait(proinfo* arr)
{
    int ctime = 0;
    int twait = 0;
    for (int i = 0; i < SIZE; i++) {
        twait += abs(arr[i].atime - ctime);
        ctime += arr[i].btime;
    }
    cout << "The average waiting time is: " << (float)twait / SIZE << endl;
}
  
int main()
{
    // Array of process info structures.
    proinfo arr[SIZE];
    arr[0] = { "p1", 0, 5 };
    arr[1] = { "p2", 1, 2 };
    arr[2] = { "p3", 3, 1 };
    arr[3] = { "p4", 4, 3 };
  
    cout << "Process scheduling according to Clairvoyant SJF is: " << endl;
  
    clairvoyantSjf(arr);
    // To display the schedule
    display(arr);
    // to calculate the Average waiting time
    avgWait(arr);
}

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Output:

Process scheduling according to Clairvoyant SJF is: 

Process id    Arrival time    Burst time    
p3        3        1        
p2        1        2        
p4        4        3        
p1        0        5        
The average waiting time is: 2.5


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