轮询调度（Round-Robin Scheduling）
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轮询调度（Round Robin Scheduling）算法就是以轮询的方式依次将请求调度不同的服务器，即每次调度执行i = (i + 1) mod n，并选出第i台服务器。算法的优点是其简洁性，它无需记录当前所有连接的状态，所以它是一种无状态调度。
轮询调度算法的原理是每一次把来自用户的请求轮流分配给内部中的服务器，从1开始，直到N(内部服务器个数)，然后重新开始循环。
轮询调度算法流程
假设有一组服务器N台，S = {S1, S2, …, Sn}，一个指示变量i表示上一次选择的服务器ID。变量i被初始化为N-1。一个很经典的算法程序如下：
轮询调度算法假设所有服务器的处理性能都相同，不关心每台服务器的当前连接数和响应速度。当请求服务间隔时间变化比较大时，轮询调度算法容易导致服务器间的负载不平衡。
所以此种均衡算法适合于服务器组中的所有服务器都有相同的软硬件配置并且平均服务请求相对均衡的情况。
1、先来先服务算法（FCFS）First Come First Service
这是一种比较简单的磁盘调度算法。它根据进程请求访问磁盘的先后次序进行调度。此算法的优点是公平、简单，且每个进程的请求都能依次得到处理，不会出现某一进程的请求长期得不到满足的情况。此算法由于未对寻道进行优化，在对磁盘的访问请求比较多的情况下，此算法将降低设备服务的吞吐量，致使平均寻道时间可能较长，但各进程得到服务的响应时间的变化幅度较小。
先来先服务 （125）86.147.91.177.94.150.102.175.130
[java] view plain copy print?
没java的 发段源代码给你 有兴趣自己慢慢理解
#include time.h
#include dos.h
#include math.h
#include conio.h
#include stdio.h
#include stdlib.h
#include time.h
#include graphics.h
#define ESC 0x1b
#define ENTER 0x0d
#define TRUE 1
#define FALSE 0
/*每隔TIME秒就变换一次优先级*/
#define TIME 5
/*数据结构*/
/****************************************************************/
enum _Status/*进程状态枚举*/
{
READY =0,/*就绪*/
RUN,/*执行中*/
SUSPEND,/*挂起*/
};
typedef enum _Status Status;
/****************************************************************/
struct _Pcb/*进程结构*/
{
int PID;/*进程ID，ID为负数的进程为系统后备队列的作业*/
int Time;/*进程运行需要的时间*/
int Prior;/*进程的优先级，越大越优先*/
Status Sts;/*状态*/
struct _Pcb *Next;/*指向本进程队列中下个进程的PCB*/
};
typedef struct _Pcb PCB;
/****************************************************************/
struct _Batch/*多道处理中的道结构*/
{
PCB *pcb;/*该道当前正在处理的进程*/
struct _Batch *Next;/*下一道*/
};
typedef struct _Batch Batch;
/****************************************************************/
/*多道系统相关全局变量*/
PCB *ProcQueue = NULL;/*进程链表，按优先级从大到小排列*/
Batch *BatchQueue = NULL;/*系统多道链表*/
/****************************************************************/
/*动态优先权抢占式调度算法及相关函数声明*/
/****************************************************************/
int InitBatchs(int n);/*初始化多道系统，n为道数*/
int InsertProc(int prior, int time);/*向进程链表中按优先级大小插入一个新进程*/
int InsertIDLE();/*向进程队列中加入后备进程，当系统空闲时将被调入*/
int SortProcQueue();/*将进程链表按优先级从大到小排列*/
int AddBatch();/*增加系统道数*/
int DeleteBatch();/*减少系统道数*/
int UnSuspendProc(int id);/*解除ID为id的进程的挂起状态*/
int UpdateBatchs();/*多道系统根据进程链表进行更新，并将执行完毕的进程删除*/
int PassSeconds(int n);/*系统经过n秒后计算数据并进行优先级调度*/
/****************************************************************/
/*各函数的定义*/
/****************************************************************/
int InitBatchs(int n)
{
int i;
for (i=0; in; ++i)
{
AddBatch();
}
return (UpdateBatchs());
}
int InsertProc(int prior, int time)
{
static int sysid = 0;/*该系统已经加入过多少进程，此值将是新进程的ID*/
PCB *last,*now,*pcb;
pcb = (PCB*)malloc(sizeof(PCB));
if (pcb == NULL) return FALSE;
pcb-Prior = prior;
pcb-Time = time;
pcb-PID = (++sysid);
pcb-Sts = READY;
if (ProcQueue == NULL)/*如果进程队列为空*/
{
ProcQueue = pcb;
pcb-Next = NULL;
return TRUE;
}
last = ProcQueue;
now = last-Next;
if (pcb-Prior last-Prior)/*pcb将排在队头*/
{
pcb-Next = ProcQueue;
ProcQueue = pcb;
return TRUE;
}
while ((now != NULL) (pcb-Prior now-Prior))/*寻找插入位置*/
{
last = now;
now = last-Next;
}
last-Next = pcb;
pcb-Next = now;
return TRUE;
}
int InsertIDLE()
{
PCB *now = ProcQueue;
PCB *idle = (PCB*)malloc(sizeof(PCB));
if (idle == NULL) return FALSE;
idle-PID = -1;
idle-Prior = -1;
idle-Sts = SUSPEND;
idle-Time = -1;
idle-Next = NULL;
if (ProcQueue == NULL)
{
ProcQueue = idle;
return TRUE;
}
while(now-Next != NULL)
{
now = now-Next;
}
now-Next = idle;
return TRUE;
}
int SortProcQueue()
{ /*冒泡排序*/
PCB *last, *now;
int b = FALSE;/*上次遍历是否无交换产生*/
if (ProcQueue==NULL || ProcQueue-Next==NULL)/*如果链表中无进程或只有一个进程*/
return FALSE;
while (!b)
{
b = TRUE;
last=ProcQueue;
now=last-Next;
if (last-Prior now-Prior)
{
ProcQueue = now;
last-Next = now-Next;
now-Next = last;
b = FALSE;
last = ProcQueue;
now = last-Next;
}
while (now-Next!=NULL)
{
if ((now-Prior)(now-Next-Prior))
{
last-Next = now-Next;
now-Next = now-Next-Next;
last-Next-Next = now;
b = FALSE;
}
else
last = last-Next;
now = last-Next;
}
}
return TRUE;
}
int AddBatch()
{
Batch *bt = (Batch*)malloc(sizeof(Batch));
if (bt==NULL) return FALSE;
bt-Next = BatchQueue;
BatchQueue = bt;
bt-pcb = NULL;
return (InsertIDLE());
}
int DeleteBatch()
{
Batch *bt = BatchQueue;
PCB *last, *now;
if (BatchQueue==NULL || BatchQueue-Next==NULL)/*如果只剩最后一道则不删除*/
return FALSE;
if (ProcQueue==NULL || ProcQueue-Next==NULL)/*如果只有最后一个后备空闲进程*/
return FALSE;/**/
last = ProcQueue;
now = last-Next;
while (now-Next != NULL)/*查找到最后一个进程，该进程必定是后备空闲进程*/
{
last = now;
now = last-Next;
}
if (now==NULL || now-PID=0)/*未查找到后备进程*/
return FALSE;/**/
free(now);
last-Next = NULL;
BatchQueue = BatchQueue-Next;
free(bt);
return TRUE;
}
int UnSuspendProc(int id)
{
PCB *now = ProcQueue;
if (ProcQueue==NULL) return FALSE;
while (now != NULL)
{
if (now-PID == id)
{
now-Sts = READY;
return TRUE;
}
}
return FALSE;
}
int UpdateBatchs()
{
Batch *bt = BatchQueue;
PCB *last = ProcQueue, *now;
while (bt != NULL)
{
bt-pcb = NULL;
bt = bt-Next;
}
if (ProcQueue == NULL) return TRUE;
while (last-Sts==RUN last-PID=0 last-Time=0)
{
ProcQueue = ProcQueue-Next;
free(last);
last = ProcQueue;
}
now = last-Next;
while (now != NULL)
{
if (now-Sts==RUN now-PID=0 now-Time=0)/*如果该进程是运行中的一般进程并已执行完毕*/
{
last-Next = now-Next;
free(now);
}
else
last = last-Next;
now = last-Next;
}
bt = BatchQueue;
now = ProcQueue;
while (bt != NULL now != NULL)
{
bt-pcb = now;
now-Sts = RUN;
bt = bt-Next;
now = now-Next;
}
while (now != NULL)
{
if (now-Sts == RUN)
{
now-Sts = SUSPEND;
}
now = now-Next;
}
return TRUE;
}
int PassSeconds(int n)
{
static int time = 0;
int i=0, ProcEnd = FALSE;
PCB *pcb = ProcQueue;
Batch *bt = BatchQueue;
if (bt == NULL) return FALSE;
time += n;
if (time=TIME)
{
i = time/TIME;/*经过多少时间段*/
time = time%TIME;
}
while (bt != NULL)/*更新进程运行时间*/
{
if (bt-pcb-PID=0)
{
bt-pcb-Time -= n;
if (bt-pcb-Time = 0)/*进程结束*/
{
ProcEnd = TRUE;
}
}
bt = bt-Next;
}
if (i 0)
{
while (pcb != NULL)/*更新进程优先权（动态优先权）*/
{
if (pcb-Sts == RUN pcb-PID=0)/*运行的进程优先权降低*/
{
pcb-Prior -= i;
if (pcb-Prior 0)
pcb-Prior = 0;
}
else if (pcb-Sts == SUSPEND pcb-PID=0)/*挂起的进程优先权升高*/
pcb-Prior += i;
pcb = pcb-Next;
}
}
if (i0)
SortProcQueue();/*如果优先级有变动则重新排序*/
if (ProcEnd || i0)
{
UpdateBatchs();/*更新多道进程*/
}
return TRUE;
}
/****************************************************************/
/*图形界面相关函数*/
/****************************************************************/
/*表格的单位宽度和高度*/
#define WIDTH 64
#define HEIGHT 12
void *black=NULL;/*背景色方格，使用它擦出表格中的图形*/
int InitGraph()/*初始化图形界面*/
{
int GraphDriver; /* The Graphics device driver */
int GraphMode; /* The Graphics mode value */
int ErrorCode;
GraphDriver = DETECT; /* Request auto-detection */
initgraph( GraphDriver, GraphMode, "" );
ErrorCode = graphresult(); /* Read result of initialization*/
if( ErrorCode != grOk )
{ /* Error occured during init */
printf(" Graphics System Error: %s\n", grapherrormsg( ErrorCode ) );
getch();
return FALSE;
}
cleardevice();
black = (void*)malloc(imagesize(1,1,WIDTH-1,HEIGHT-1));
getimage(1,1,WIDTH-1,HEIGHT-1,black);
DrawFrame();
DrawData();
return TRUE;
}
int DrawFrame()/*画边框和表头*/
{
settextjustify(CENTER_TEXT, CENTER_TEXT);
gprintf(320, HEIGHT/2-1, "Multi-Batch System Emulation");
settextjustify(LEFT_TEXT, TOP_TEXT);
moveto(0,HEIGHT);
lineto(0,479);
lineto(639,479);
lineto(639,HEIGHT);
lineto(0,HEIGHT);
line(WIDTH*4,HEIGHT,WIDTH*4,479);
line(WIDTH*7,HEIGHT,WIDTH*7,479);
line(0,HEIGHT*2,639,HEIGHT*2);
line(0,HEIGHT*3,639,HEIGHT*3);
gprintf(HEIGHT*0+2,HEIGHT*1+2,"System Batchs");/*系统多道列表头*/
gprintf(HEIGHT*0+2,HEIGHT*2+2,"Batch");
gprintf(WIDTH*1+2,HEIGHT*2+2,"ProcID");
gprintf(WIDTH*2+2,HEIGHT*2+2,"Time");
gprintf(WIDTH*3+2,HEIGHT*2+2,"Prior");
gprintf(WIDTH*4+2,HEIGHT*1+2,"Suspended Processes");/*挂起队列列表头*/
gprintf(WIDTH*4+2,HEIGHT*2+2,"ProcID");
gprintf(WIDTH*5+2,HEIGHT*2+2,"Time");
gprintf(WIDTH*6+2,HEIGHT*2+2,"Prior");
gprintf(WIDTH*7+2,HEIGHT*1+2,"Ready Processes");/*就绪队列列表头*/
gprintf(WIDTH*7+2,HEIGHT*2+2,"ProcID");
gprintf(WIDTH*8+2,HEIGHT*2+2,"Time");
gprintf(WIDTH*9+2,HEIGHT*2+2,"Prior");
}
int DrawData()/*绘制系统数据*/
{
int numRun=0, numSus=0, numRed=0;/*运行挂起和就绪的进程各有多少*/
PCB* now = ProcQueue;
int x=0, y=0;
while (now != NULL)
{
switch(now-Sts)
{
case RUN:
x = WIDTH*1;
y = HEIGHT*(3+(numRun++));
break;
case SUSPEND:
x = WIDTH*4;
y = HEIGHT*(3+(numSus++));
break;
case READY:
x = WIDTH*7;
y = HEIGHT*(3+(numRed++));
break;
}
if (now-Sts==RUN)/*该进程为正在运行的进程*/
{
putimage(x-WIDTH+1,y+1,black,COPY_PUT);
gprintf(x-WIDTH+2,y+2,"%d",numRun);
}
if (now-PID=0)/*该进程不是后备进程*/
{
putimage(x+1,y+1,black,COPY_PUT);
gprintf(x+2,y+2,"%d",now-PID);
putimage(x+1+WIDTH,y+1,black,COPY_PUT);
gprintf(x+WIDTH+2,y+2,"%d",now-Time);
putimage(x+1+WIDTH*2,y+1,black,COPY_PUT);
gprintf(x+WIDTH*2+2,y+2,"%d",now-Prior);
}
else
{
putimage(x+1,y+1,black,COPY_PUT);
putimage(x+1+WIDTH,y+1,black,COPY_PUT);
putimage(x+1+WIDTH*2,y+1,black,COPY_PUT);
gprintf(x+2,y+2,"system idle process");
}
now = now-Next;
}
}
int DlgGetNum(char *buf,int l,int t,int r,int b,int gettime)
{
char ch;
int pos=0;
bar(l,t,r,b);
gprintf(l+10,t+5,"Add new Process");
if (gettime)
gprintf(l+10,t+20,"input the time:");
else
gprintf(l+10,t+20,"input the priority:");
while (1)
{
ch = getch();
if (ch == ENTER)/*如果输入了回车键*/
{
if(pos!=0)/*如果位置不在第一位(回车键不准第一个输入)*/
{
buf[pos]='\0';
break;
}
}
else if (ch='0' ch='9')
{
buf[pos++]=ch;
buf[pos]='\0';
}
else if (ch == ESC)
{
return FALSE;
}
else/*其他按键均按BackSpace处理*/
{
--pos;
buf[pos]='\0';
}
if (pos0)
{ pos=0; buf[pos]='\0';}
else if (pos4)/*最多输入4位数*/
{ pos=4; buf[pos]='\0';}
bar(l,t+35,r,t+47);
gprintf(l+50,t+35,buf);
}
return TRUE;
}
int NewProcDlg()
{
int l=200,t=150,r=380,b=200,pos=0,prior=0,time=0;
char buf[5],ch;
PCB *pcb;
void* bg = (void*)malloc(imagesize(l,t,r,b));
getimage(l,t,r,b,bg);
setfillstyle(1,2);
flushall();
/*******输入优先级**********/
if (!DlgGetNum(buf,l,t,r,b,FALSE))
goto exit;
prior = atoi(buf);
/*******输入时间**********/
pos=0;
buf[pos]='\0';
if (!DlgGetNum(buf,l,t,r,b,TRUE))
goto exit;
time = atoi(buf);
InsertProc(prior, time);
exit:
putimage(l,t,bg,COPY_PUT);
free(bg);
return TRUE;
}
int gprintf( int xloc, int yloc, char *fmt, ... )/*图形系统中的格式输出*/
{
va_list argptr; /* Argument list pointer */
char str[140]; /* Buffer to build sting into */
int cnt; /* Result of SPRINTF for return */
va_start( argptr, format ); /* Initialize va_ functions */
cnt = vsprintf( str, fmt, argptr ); /* prints string to buffer */
outtextxy( xloc, yloc, str ); /* Send string in graphics mode */
va_end( argptr ); /* Close va_ functions */
return( cnt ); /* Return the conversion count */
}
/****************************************************************/
/*main函数*/
int main()
{
clock_t start=0, end=0;
char kb;
InitBatchs(3);/*初始化为3道系统*/
InitGraph();
while (1)
{
start = end = clock();
while (!kbhit())
{
start = clock();
if ((start-end)/18.2 = 1)/*时间过了一秒*/
{
start = end = clock();
PassSeconds(1);
cleardevice();
DrawFrame();
DrawData();
}
}
kb = getch();
switch (kb)
{
case ESC:
closegraph();
return 0;
case 'w':
AddBatch();
UpdateBatchs();
cleardevice();
DrawFrame();
DrawData();
break;
case 's':
DeleteBatch();
UpdateBatchs();
cleardevice();
DrawFrame();
DrawData();
break;
case 'i':
NewProcDlg();
UpdateBatchs();
DrawFrame();
DrawData();
break;
}
}
return 0;
}
