页面调度算法 FIFO，LRU，OPT，及C++代码

页面调度算法 FIFO，LRU，OPT

介绍了三种页面调度算法，给出了C++代码

1.FIFO

先来先去算法这个非常好理解，给出分析图

可以看出，缺页次数为8次，缺页中断率为8/12=66.7%，依次置换的页面为：1,2,5,3,4

C++代码在最后给出

2.LRU

LRU，Least Recently Used 近期最少使用算法，先给出分析图

这个也不难理解，我们只需要从之前的一个页面开始往左寻找就可以了，比如在第一次遇到页面3时，我们发现3左边是1,再左边是5，所以1,5近期使用了，就把1,5留下，另外一个页面置换掉，所以此时内存中的页面2就被置换了。

C++代码在最后给出

3.OPT

最佳置换算法，是一种理论算法，对未来的页面检测从而判断置换页。分析图如下

OPT跟LRU非常相似，LRU是往左找，OPT就是往右找啦，比如第一次遇到页面3时，往右依次是2,5，所以2,5页面保留，把内存中的页面1置换掉。

4. 三种算法的C++代码

FIFO：

//FILENAME: FIFO.cpp#include<iostream>using namespace std;int main(){    int memory[3]={-1,-1,-1}; //-1 means no page in this memory page    int page[12]={1,2,5,1,3,2,5,4,1,4,5,2};//the test pages    int pm=0; //pointer of memory    int count=0;//missing page count    int replace[12];    int pr=0;    cout<<"######FIFO#######"<<endl;    //search begin    for (int i=0;i<12;i++)    {        //check if there is page[i] in memory        bool exist = false;        for (int j=0;j<3;j++)        {            if (page[i]==memory[j])            {                exist=true;                break;            }        }        //not exist , replace this memory page        if (exist==false)        {            if(memory[pm]!=-1)            {                replace[pr]=memory[pm];                pr++;            }            count++;            memory[pm]=page[i];            pm++;            if (pm==3) pm=0;        }        //output        cout<<page[i]<<":  [ ";        for(int j=0;j<3;j++)        {            if (memory[j]==-1) cout<<"*  ";            else cout<<memory[j]<<"  ";        }        cout<<"]"<<endl;    }    //output    cout<<"######################"<<endl;    cout<<"the lack page count = " <<count<<endl;    cout<<"repalce pages are  : ";    for (int i=0;i<pr;i++)    {        cout<<replace[i]<<" ";    }    cout<<endl;    cout<<"the rate of page lack is "<<count/12.0*100<<"%"<<endl;    return 0;}

LRU：

//FILENAME: LRU.cpp#include<iostream>using namespace std;int main(){    int memory[3]={-1,-1,-1};    int page[12]={1,2,5,1,3,2,5,4,1,4,5,2};//the test pages    int count=0;//lack page count    int replace[12]; //replace page record    int pr=0;    //pointer of replace    cout<<"######LRU#######"<<endl;    //search begin    for (int i=0;i<12;i++)    {        //there are 3 memory pages in memory ,so if i<3,just put it in memory        if(i<3)        {            memory[i]=page[i];            count++;        }        else        {            //check if this page is in memory already            bool exist=false;            for(int j=0;j<3;j++)            {                if(page[i]==memory[j])                {                    exist=true;                    break;                }            }            if (exist==false)            {                //begin to choose a memory page to replace                int last=0;                bool ok[3];                for (int j=0;j<3;j++) ok[j]=false;                //check from i step -1 till 0                for(int j=i;j>=0;j--)                {                    for(int k=0;k<3;k++)                    {                        if (page[j]==memory[k])                        {                            ok[k]=true;                            last++;                            break;                        }                    }                    if (last==2)break;                }                //check which ok ==false                for (int j=0;j<3;j++)                {                    if (ok[j]==false)                    {                        //replace this memory[j]                        count++;                        replace[pr]=memory[j];                        pr++;                        memory[j]=page[i];                        break;                    }                }            }        }        //output        cout<<page[i]<<":  [ ";        for(int j=0;j<3;j++)        {            if (memory[j]==-1) cout<<"*  ";            else cout<<memory[j]<<"  ";        }        cout<<"]"<<endl;    }    //out put    cout<<"######################"<<endl;    cout<<"the lack page count = " <<count<<endl;    cout<<"repalce pages are  : ";    for (int i=0;i<pr;i++)    {        cout<<replace[i]<<" ";    }    cout<<endl;    cout<<"the rate of page lack is "<<count/12.0*100<<"%"<<endl;    return 0;}

OPT：

//FILENAME: OPT.cpp#include<iostream>using namespace std;int main(){    int memory[3]={-1,-1,-1};    int page[12]={1,2,5,1,3,2,5,4,1,4,5,2};//the test pages    int count=0;//missing page count    int replace[12]; //replace page record    int pr=0;  // pointer of replace    cout<<"######OPT#######"<<endl;    //search begin    for (int i=0;i<12;i++)    {        //there are 3 memory pages in memory ,so if i<3,just put it in memory        if(i<3)        {            memory[i]=page[i];            count++;        }        else        {            //check if this page is in memory already            bool exist=false;            for(int j=0;j<3;j++)            {                if(page[i]==memory[j])                {                    exist=true;                    break;                }            }            if (exist==false)            {                //###############################                //begin to choose a memory page to replace                int later=0;                bool ok[3];                for (int j=0;j<3;j++) ok[j]=false;                //check from i step -1 till 0                for(int j=i+1;j<12;j++)                {                    for(int k=0;k<3;k++)                    {                        if (page[j]==memory[k])                        {                            ok[k]=true;                            later++;                            break;                        }                    }                    if (later==2)break;                }                //check which ok ==false                for (int j=0;j<3;j++)                {                    if (ok[j]==false)                    {                        //replace this memory[j]                        count++;                        replace[pr]=memory[j];                        pr++;                        memory[j]=page[i];                        break;                    }                }                //#############################            }        }        //output        cout<<page[i]<<":  [ ";        for(int j=0;j<3;j++)        {            if (memory[j]==-1) cout<<"*  ";            else cout<<memory[j]<<"  ";        }        cout<<"]"<<endl;    }    cout<<"######################"<<endl;    cout<<"the lack page count = " <<count<<endl;    cout<<"repalce pages are  : ";    for (int i=0;i<pr;i++)    {        cout<<replace[i]<<" ";    }    cout<<endl;    cout<<"the rate of page lack is "<<count/12.0*100<<"%"<<endl;    return 0;}