极客笔记FCFS调度的C程序
给定n个进程，即P1，P2，P3，…，Pn以及它们对应的执行时间。任务是使用FCFS CPU调度算法来计算平均等待时间和平均周转时间。
等待时间和周转时间是什么？
- 周转时间是进程提交和完成之间的时间间隔。
周转时间 = 进程完成时间 – 进程提交时间
- 等待时间是周转时间与执行时间的差。
等待时间 = 周转时间 – 执行时间
什么是FCFS调度？
先到先服务（FCFS）也被称为先进先出（FIFO），是一种CPU调度算法，根据进程在就绪队列中排队的顺序分配CPU。
FCFS采用非抢占调度，意味着一旦CPU被分配给一个进程，它就不会离开CPU，直到该进程终止或由于某些I/O中断而停止。
示例
假设有四个进程按P2，P3，P1的顺序到达，其对应的执行时间如下表所示。同时，将它们的到达时间设为0。
| Process | Order of arrival | Execution time in msec |
|---|---|---|
| P1 | 3 | 15 |
| P2 | 1 | 3 |
| P3 | 2 | 3 |
显示系统中进程P1、P2和P3的等待时间的甘特图
如上所示，
进程P2的等待时间为0
进程P3的等待时间为3
进程P1的等待时间为6
平均时间=（0 + 3 + 6）/ 3 = 3毫秒。
由于我们将到达时间设为0，因此周转时间和完成时间将相同。
示例
Input-: processes = P1, P2, P3
Burst time = 5, 8, 12
Output-:
Processes Burst Waiting Turn around
1 5 0 5
2 8 5 13
3 12 13 25
Average Waiting time = 6.000000
Average turn around time = 14.333333
算法
Start Step 1-> In function int waitingtime(int proc[], int n, int burst_time[], int wait_time[])
Set wait_time[0] = 0
Loop For i = 1 and i < n and i++
Set wait_time[i] = burst_time[i-1] + wait_time[i-1]
End For
Step 2-> In function int turnaroundtime( int proc[], int n, int burst_time[], int wait_time[], int tat[])
Loop For i = 0 and i < n and i++
Set tat[i] = burst_time[i] + wait_time[i]
End For
Step 3-> In function int avgtime( int proc[], int n, int burst_time[])
Declare and initialize wait_time[n], tat[n], total_wt = 0, total_tat = 0;
Call waitingtime(proc, n, burst_time, wait_time)
Call turnaroundtime(proc, n, burst_time, wait_time, tat)
Loop For i=0 and i<n and i++
Set total_wt = total_wt + wait_time[i]
Set total_tat = total_tat + tat[i]
Print process number, burstime wait time and turnaround time
End For
Print "Average waiting time =i.e. total_wt / n
Print "Average turn around time = i.e. total_tat / n
Step 4-> In int main()
Declare the input int proc[] = { 1, 2, 3}
Declare and initialize n = sizeof proc / sizeof proc[0]
Declare and initialize burst_time[] = {10, 5, 8}
Call avgtime(proc, n, burst_time)
Stop
示例
#include <stdio.h>
// Function to find the waiting time for all processes
int waitingtime(int proc[], int n,
int burst_time[], int wait_time[]) {
// waiting time for first process is 0
wait_time[0] = 0;
// calculating waiting time
for (int i = 1; i < n ; i++ )
wait_time[i] = burst_time[i-1] + wait_time[i-1] ;
return 0;
}
// Function to calculate turn around time
int turnaroundtime( int proc[], int n,
int burst_time[], int wait_time[], int tat[]) {
// calculating turnaround time by adding
// burst_time[i] + wait_time[i]
int i;
for ( i = 0; i < n ; i++)
tat[i] = burst_time[i] + wait_time[i];
return 0;
}
//Function to calculate average time
int avgtime( int proc[], int n, int burst_time[]) {
int wait_time[n], tat[n], total_wt = 0, total_tat = 0;
int i;
//Function to find waiting time of all processes
waitingtime(proc, n, burst_time, wait_time);
//Function to find turn around time for all processes
turnaroundtime(proc, n, burst_time, wait_time, tat);
//Display processes along with all details
printf("Processes Burst Waiting Turn around
");
// Calculate total waiting time and total turn
// around time
for ( i=0; i<n; i++) {
total_wt = total_wt + wait_time[i];
total_tat = total_tat + tat[i];
printf(" %d\t %d\t\t %d \t%d
", i+1, burst_time[i], wait_time[i], tat[i]);
}
printf("Average waiting time = %f
", (float)total_wt / (float)n);
printf("Average turn around time = %f
", (float)total_tat / (float)n);
return 0;
}
// main function
int main() {
//process id's
int proc[] = { 1, 2, 3};
int n = sizeof proc / sizeof proc[0];
//Burst time of all processes
int burst_time[] = {5, 8, 12};
avgtime(proc, n, burst_time);
return 0;
}
输出
Processes Burst Waiting Turn around
1 5 0 5
2 8 5 13
3 12 13 25
Average Waiting time = 6.000000
Average turn around time = 14.333333