时间片轮转——进程调度

原创

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上一篇博客写了最高优先级算法——进程调度：http://www.cnblogs.com/chiweiming/p/9028002.html

此篇介绍时间片轮转调度，时间片轮转调度比最高优先级调度更为简单，每次都从PCB（进程存在的唯一标识）队列中将

首进程调入CPU，增加其已用CPU时间，改变其进程状态；然后判断其已用CPU时间是否大于等于需要运行时间，大于将

其进程状态置为完成状态，否则将此PCB插入队列尾部，再次在队列中寻找优先级最高的PCB...两篇博客的代码大同小异。

/*
    时间片轮转算法 
*/
#include<stdio.h>
#include<stdlib.h>
#include<time.h>
#define N 3
#define Time_film 2    //时间片

int count = 0;

void print(struct PCB *head);

struct PCB{
    int process_name;    //进程名 
    int priority_number;    //优先数,全置0 
    int arrive_time;    //到达时间,为进程的创建时间
    int need_time;    //需要运行的时间,随机产生
    int used_time;    //已用CPU的时间,初始值为0
    int process_state;    //进程状态,1表示运行,0表示完成,-1表示就绪,初始值为-1 
    struct PCB *cre;    //前驱指针域
    struct PCB *next;    //后驱指针域
};

void Process_scheduling(struct PCB *head){
    /*
    扫描队列，将进程按到底顺序调入CPU运行；
    如果 use_CPU == need_time 撤销此PCB； 
    否则用完一个时间片后放回队列尾部，继续扫描；
    */
    //****************************
    struct PCB *Move=head;
    struct PCB *Max_Pri=head->next;
    struct PCB *Tail;    //尾指针 
    //****************************
    Move->next = Max_Pri->next;        //调出队列首进程
    if(Move->next != NULL){
        Move = Max_Pri->next;
        Move->cre = Max_Pri->cre;
    }
    //****************************
    printf("        进程 %d 被调度: \n",Max_Pri->process_name);
    Max_Pri->used_time += Time_film;    //增加CPU占用时间
    if(Max_Pri->used_time >= Max_Pri->need_time){
        Max_Pri->used_time = Max_Pri->need_time;    //进程状态改变
        Max_Pri->process_state = 0;
        count++;
    }
    else{
        Max_Pri->process_state = 1;
    }
    printf(" %d     %d     %d        %d         %d      %d \n\n",Max_Pri->process_name,Max_Pri->priority_number,Max_Pri->arrive_time,Max_Pri->need_time,Max_Pri->used_time,Max_Pri->process_state);
    if(count == N){    //所有进程执行完毕 
        printf("        所有进程执行完毕！");
        return;
    }
    printf("        就绪队列：\n");
    print(head);    //输出就绪队列
    printf("\n");
    //****************************
    if(Max_Pri->process_state !=0){
        Move = head;
        while( Move->next!=NULL ){    //当被调出进程未完成时将其插入就绪队列尾部 
            Move = Move->next; 
        }
        Tail = Move;
        Max_Pri->cre = Tail;
        Max_Pri->next = NULL;
        Tail->next = Max_Pri;
        Max_Pri->process_state = -1;
    }
    //****************************
    Process_scheduling(head);
}

void print(struct PCB *head){    //输出队列函数 
    if(head->next == NULL){
        printf("就绪队列已空\n");
        return;
    }
    printf("name priority arr_time need_time use_CPU pro_state\n");
    struct PCB *fry = head->next;
    while(fry != NULL){
        printf(" %d     ",fry->process_name);
        printf("%d     ",fry->priority_number);
        printf("%d        ",fry->arrive_time);
        printf("%d         ",fry->need_time);
        printf("%d      ",fry->used_time);
        printf("%d ",fry->process_state);
        printf("\n");
        fry = fry->next;    
    }
    printf("\n"); 
}

int main(){
    struct PCB *head;    //头指针
    struct PCB Pro[N+1];    //创建 N+1 个进程
    head = &Pro[0];
    srand(time(0));
    
    //****************************
    //设置进程参数
    Pro[0].process_name = 0;
    Pro[0].cre = NULL;
    Pro[0].next = &Pro[1];
    Pro[0].priority_number = 0;
    int i=0;
    for(i=1;i<=N;i++){
        Pro[i].process_name = i;
        Pro[i].priority_number = 0;
        Pro[i].arrive_time = i;
        Pro[i].need_time = rand()%7;
        while(Pro[i].need_time == 0){
            Pro[i].need_time = rand()%7;
        }
        Pro[i].used_time = 0;
        Pro[i].process_state = -1;
    }
    for(i=1;i<=N;i++){    //形成双向队列
        if( i == N ){
            Pro[i].cre = &Pro[i-1];
            Pro[i].next = NULL;
            break;
        }
        Pro[i].cre = &Pro[i-1];
        Pro[i].next = &Pro[i+1];
    }
    //****************************
    
    printf("        进程初始状态: \n");
    print(head);    //输出初始队列状态
    
    Process_scheduling(head);    //调用进程调度函数(时间片轮转)
    return 0;
} 

（运行结果部分截图）

15:52:07

2018-05-13