Blocking, non blocking, IO multiplexing select\poll\epoll

Preface

linux There are five kinds of io Model , Respectively :
 1. Blocking
 2. Non blocking
 3.io Multiplexing
 4. Event driven
 5. asynchronous

picture source

One 、 Blocking IO

The first mode is blocking , It means that the user performs IO Blocked during operation ,

  The server sends out read request ,read Is to see if there is client The user made a data request . issue read Then the operation is handed over to the kernel , Will be waiting for the requested data to return , The server can continue to perform the following operations .

Two 、 Non blocking

In order to solve the problem that the blocking mode has been waiting , Can be socket Set to NONBLOCK,

  When the user level thread of the server sends a request, it will not be blocked even if there is no data , It's about going back to , But it needs to be felt constantly read request , Know that the data is read , The advantage of this pattern over non blocking is simply that you can immediately go back and do something else , But for the data read It needs to be sent all the time . Consume a lot CPU resources .

3、 ... and 、 IO Multiplexing

   What are the main disadvantages of the above model , High concurrency is not possible , Because querying a network IO when , If there is no data, it will be blocked here all the time , Or use the polling method to query all the time , When there are other network requests , How can the server discover and handle it in time ？

This requires IO Multiplexing technology , The three methods should be introduced one by one ：

---

select

sockfd=socket(AF_INET,SOCK_STREAM,0);
memset(&addr,0,sizeof(addr));
addr.sin_family=AF_INET;
addr,sin_port=htons(2000);
addr.sin_addr.s_addr=INADDR_ANY;
bind(sockfd,(struct sockaddr*)&addr,sizeof(addr));
listen(sockfd,5);

for(int i=0;i<5;++i){
    
	memset(&client,0,sizeof(client));
	addrlen=sizeof(client);
	fds[i]=accept(sockfd,(struct sockfd,(struct sockaddr*)&client,&addrlen));
	if(fds[i]>max)
		max=fds[i];
}
//--------------------------------------------------------------------------------------//
while(1){
    
	FD_ZERO(&rset);
	for(int i=0;i<5;++i){
    
		FD_SET(fds[i],&rset);
	}
	puts("round again");
	select(max+1,&rest,NULL,NULL,NULL);// Inquire about IO
	// Reading data 
	for(int i=0;i<5;++i){
    
		if(FD_ISSET(fds[i],&rset)){
    
			memset(buffer,0,MAXBUF);
			read(fds[i],buffer,MAXBUF);
			puts(buffer);
		}
	}
}

Above the dotted line are all networks scoket Set up ,fds Arrays are what we store 5 individual client Port number ,rset It's a bitmap Data of type , If client The port numbers of are ：1、2、5、7、9, that FD_SET Is to put the corresponding rset Set up ：

0110010101000...

The point is select function , How does he work ？ In fact, threads will block select here , But different from the above blocking ,select Cycle pair 5 Access ports to see if there are requests ,

1、 If not, it will be blocked all the time select here , Always query ;
2、 If you have any , This time f The query loop has the requested corresponding flag bit Set up , Then return ;

shortcoming ：
1、 Passed in rset Parameters need to be copied from user mode into kernel mode ;
2、rset This type of bitmap yes 1024 position ;
3、rset Assign values again after each use ;
4、select After returning or O(n) Time complexity traversal rset, See which has a request ;

poll

poll Instead of using bitmap The way of setting ,

struct pollfd{
    
    int fd;
    short events;
    short revents;
};

for(int i=0;i<5;++i){
    
   memset(&client,0,sizeof(client));
   addrlen=sizeof(client);
   pollfds[i]=accept(sockfd,(struct sockfd,(struct sockaddr*)&client,&addrlen));
   pollsfds[i].events=POLLIN;
}
sleep(1);
while(1){
    
    puts("round again");
    poll(pollfds,5,5000);
    // Reading data 
    for(int i=0;i<5;++i){
    
        if(FD_ISSET(fds[i],&rset)){
    
            memset(buffer,0,MAXBUF);
            read(fds[i],buffer,MAXBUF);
            puts(buffer);
        }
    }
}

  pollfd Structure of the fd Used to store port number ,events Used to set whether to read or output ,revents It is used to record whether the port has data requests ;

This method is similar to select, Just use structure instead of bitmap, take revents Set to judge , It's just solved bitmap Can only 1024 A place , And the operations that need to be re assigned after each return .

epoll

for(int i=0;i<5;++i){
    
   memset(&client,0,sizeof(client));
   addrlen=sizeof(client);
   pollfds[i]=accept(sockfd,(struct sockfd,(struct sockaddr*)&client,&addrlen));
   pollsfds[i].events=POLLIN;
}
sleep(1);
while(1){
    
    puts("round again");
    poll(pollfds,5,5000);
    // Reading data 
    for(int i=0;i<5;++i){
    
        if(FD_ISSET(fds[i],&rset)){
    
            memset(buffer,0,MAXBUF);
            read(fds[i],buffer,MAXBUF);
            puts(buffer);
        }
    }
}

   epfd Is a blank area of memory requested ,epoll_event The structure array of is used to represent the status of each port , All port statuses will be recorded in epfd in ;

   When there is a data request , It is still the setting of the corresponding structure , But after setting, it will be rearranged and placed at the head of the team , So from epoll There is no need to read data after returning O(N) The traversal ;

   In addition, the epfd It is shared in user mode and kernel mode , No copy operation is required .