【网络编程】epoll 笔记

一、最大连接数

1、select

select在单进程中最多同时监听1024个fd；要想实现百万并发需要一千个进程，并且性能会很差、内存消耗巨大。所以select只适用于连接数在一千个以下的场景。

2、epoll

epoll本身不限制连接数，但是连接数会受到系统限制。

1）文件句柄限制

Linux系统限制单进程可以打开文件的数量默认是1024，可以使用ulimit -n命令查看。

$ ulimit -n
1024

临时修改：ulimit -n [文件句柄数量]
永久修改：编辑 /etc/security/limits.conf 文件， 修改后内容为

* soft nofile 1000000
* hard nofile 1000000

全局限制：修改/etc/sysctl.conf 文件:

fs.file-max = 1000000
net.ipv4.ip_conntrack_max = 1000000
net.ipv4.netfilter.ip_conntrack_max = 1000000

文件句柄数设置多大合适
这个值在kernel的文档里意思是file-max一般为内存大小（KB）的10%来计算，如果使用shell，可以这样计算：

    grep -r MemTotal /proc/meminfo | awk '{printf("%d",$2/10)}'  

* 表示匹配所有用户
参考博客：
【 Linux 】单台服务器上并发TCP连接数
网络编程释疑之：单台服务器上的并发TCP连接数可以有多少

2）端口号不会限制连接数

参考博客：
网络编程释疑之：单台服务器上的并发TCP连接数可以有多少

3）内存限制

4G内存极限值52万。
参考博客：
4G内存服务器epoll并发量最大能达到多少

二、epoll原理

参考博客：
高并发网络编程之epoll详解

三、epoll编程

参考博客：
Linux下Socket编程(四)——epoll的使用
linux高效率编程：epoll和多线程

1、作者：sparkling

//   
// a simple echo server using epoll in linux  
//   
// 2009-11-05  
// 2013-03-22:修改了几个问题，1是/n格式问题，2是去掉了原代码不小心加上的ET模式;
// 本来只是简单的示意程序，决定还是加上 recv/send时的buffer偏移
// by sparkling  
//   
#include <sys/socket.h>  
#include <sys/epoll.h>  
#include <netinet/in.h>  
#include <arpa/inet.h>  
#include <fcntl.h>  
#include <unistd.h>  
#include <stdio.h>  
#include <errno.h>  
#include <iostream>  
using namespace std;  
#define MAX_EVENTS 500  
struct myevent_s  
{  
    int fd;  
    void (*call_back)(int fd, int events, void *arg);  
    int events;  
    void *arg;  
    int status; // 1: in epoll wait list, 0 not in  
    char buff[128]; // recv data buffer  
    int len, s_offset;  
    long last_active; // last active time  
};  
// set event  
void EventSet(myevent_s *ev, int fd, void (*call_back)(int, int, void*), void *arg)  
{  
    ev->fd = fd;  
    ev->call_back = call_back;  
    ev->events = 0;  
    ev->arg = arg;  
    ev->status = 0;
    bzero(ev->buff, sizeof(ev->buff));
    ev->s_offset = 0;  
    ev->len = 0;
    ev->last_active = time(NULL);  
}  
// add/mod an event to epoll  
void EventAdd(int epollFd, int events, myevent_s *ev)  
{  
    struct epoll_event epv = {0, {0}};  
    int op;  
    epv.data.ptr = ev;  
    epv.events = ev->events = events;  
    if(ev->status == 1){  
        op = EPOLL_CTL_MOD;  
    }  
    else{  
        op = EPOLL_CTL_ADD;  
        ev->status = 1;  
    }  
    if(epoll_ctl(epollFd, op, ev->fd, &epv) < 0)  
        printf("Event Add failed[fd=%d], evnets[%d]\n", ev->fd, events);  
    else  
        printf("Event Add OK[fd=%d], op=%d, evnets[%0X]\n", ev->fd, op, events);  
}  
// delete an event from epoll  
void EventDel(int epollFd, myevent_s *ev)  
{  
    struct epoll_event epv = {0, {0}};  
    if(ev->status != 1) return;  
    epv.data.ptr = ev;  
    ev->status = 0;
    epoll_ctl(epollFd, EPOLL_CTL_DEL, ev->fd, &epv);  
}  
int g_epollFd;  
myevent_s g_Events[MAX_EVENTS+1]; // g_Events[MAX_EVENTS] is used by listen fd  
void RecvData(int fd, int events, void *arg);  
void SendData(int fd, int events, void *arg);  
// accept new connections from clients  
void AcceptConn(int fd, int events, void *arg)  
{  
    struct sockaddr_in sin;  
    socklen_t len = sizeof(struct sockaddr_in);  
    int nfd, i;  
    // accept  
    if((nfd = accept(fd, (struct sockaddr*)&sin, &len)) == -1)  
    {  
        if(errno != EAGAIN && errno != EINTR)  
        {  
        }
        printf("%s: accept, %d", __func__, errno);  
        return;  
    }  
    do  
    {  
        for(i = 0; i < MAX_EVENTS; i++)  
        {  
            if(g_Events[i].status == 0)  
            {  
                break;  
            }  
        }  
        if(i == MAX_EVENTS)  
        {  
            printf("%s:max connection limit[%d].", __func__, MAX_EVENTS);  
            break;  
        }  
        // set nonblocking
        int iret = 0;
        if((iret = fcntl(nfd, F_SETFL, O_NONBLOCK)) < 0)
        {
            printf("%s: fcntl nonblocking failed:%d", __func__, iret);
            break;
        }
        // add a read event for receive data  
        EventSet(&g_Events[i], nfd, RecvData, &g_Events[i]);  
        EventAdd(g_epollFd, EPOLLIN, &g_Events[i]);  
    }while(0);  
    printf("new conn[%s:%d][time:%d], pos[%d]\n", inet_ntoa(sin.sin_addr),
            ntohs(sin.sin_port), g_Events[i].last_active, i);  
}  
// receive data  
void RecvData(int fd, int events, void *arg)  
{  
    struct myevent_s *ev = (struct myevent_s*)arg;  
    int len;  
    // receive data
    len = recv(fd, ev->buff+ev->len, sizeof(ev->buff)-1-ev->len, 0);    
    EventDel(g_epollFd, ev);
    if(len > 0)
    {
        ev->len += len;
        ev->buff[len] = '\0';  
        printf("C[%d]:%s\n", fd, ev->buff);  
        // change to send event  
        EventSet(ev, fd, SendData, ev);  
        EventAdd(g_epollFd, EPOLLOUT, ev);  
    }  
    else if(len == 0)  
    {  
        close(ev->fd);  
        printf("[fd=%d] pos[%d], closed gracefully.\n", fd, ev-g_Events);  
    }  
    else  
    {  
        close(ev->fd);  
        printf("recv[fd=%d] error[%d]:%s\n", fd, errno, strerror(errno));  
    }  
}  
// send data  
void SendData(int fd, int events, void *arg)  
{  
    struct myevent_s *ev = (struct myevent_s*)arg;  
    int len;  
    // send data  
    len = send(fd, ev->buff + ev->s_offset, ev->len - ev->s_offset, 0);
    if(len > 0)  
    {
        printf("send[fd=%d], [%d<->%d]%s\n", fd, len, ev->len, ev->buff);
        ev->s_offset += len;
        if(ev->s_offset == ev->len)
        {
            // change to receive event
            EventDel(g_epollFd, ev);  
            EventSet(ev, fd, RecvData, ev);  
            EventAdd(g_epollFd, EPOLLIN, ev);  
        }
    }  
    else  
    {  
        close(ev->fd);  
        EventDel(g_epollFd, ev);  
        printf("send[fd=%d] error[%d]\n", fd, errno);  
    }  
}  
void InitListenSocket(int epollFd, short port)  
{  
    int listenFd = socket(AF_INET, SOCK_STREAM, 0);  
    fcntl(listenFd, F_SETFL, O_NONBLOCK); // set non-blocking  
    printf("server listen fd=%d\n", listenFd);  
    EventSet(&g_Events[MAX_EVENTS], listenFd, AcceptConn, &g_Events[MAX_EVENTS]);  
    // add listen socket  
    EventAdd(epollFd, EPOLLIN, &g_Events[MAX_EVENTS]);  
    // bind & listen  
    sockaddr_in sin;  
    bzero(&sin, sizeof(sin));  
    sin.sin_family = AF_INET;  
    sin.sin_addr.s_addr = INADDR_ANY;  
    sin.sin_port = htons(port);  
    bind(listenFd, (const sockaddr*)&sin, sizeof(sin));  
    listen(listenFd, 5);  
}  
int main(int argc, char **argv)  
{  
    unsigned short port = 12345; // default port  
    if(argc == 2){  
        port = atoi(argv[1]);  
    }  
    // create epoll  
    g_epollFd = epoll_create(MAX_EVENTS);  
    if(g_epollFd <= 0) printf("create epoll failed.%d\n", g_epollFd);  
    // create & bind listen socket, and add to epoll, set non-blocking  
    InitListenSocket(g_epollFd, port);  
    // event loop  
    struct epoll_event events[MAX_EVENTS];  
    printf("server running:port[%d]\n", port);  
    int checkPos = 0;  
    while(1){  
        // a simple timeout check here, every time 100, better to use a mini-heap, and add timer event  
        long now = time(NULL);  
        for(int i = 0; i < 100; i++, checkPos++) // doesn't check listen fd  
        {  
            if(checkPos == MAX_EVENTS) checkPos = 0; // recycle  
            if(g_Events[checkPos].status != 1) continue;  
            long duration = now - g_Events[checkPos].last_active;  
            if(duration >= 60) // 60s timeout  
            {  
                close(g_Events[checkPos].fd);  
                printf("[fd=%d] timeout[%d--%d].\n", g_Events[checkPos].fd, g_Events[checkPos].last_active, now);  
                EventDel(g_epollFd, &g_Events[checkPos]);  
            }  
        }  
        // wait for events to happen  
        int fds = epoll_wait(g_epollFd, events, MAX_EVENTS, 1000);  
        if(fds < 0){  
            printf("epoll_wait error, exit\n");  
            break;  
        }  
        for(int i = 0; i < fds; i++){  
            myevent_s *ev = (struct myevent_s*)events[i].data.ptr;  
            if((events[i].events&EPOLLIN)&&(ev->events&EPOLLIN)) // read event  
            {  
                ev->call_back(ev->fd, events[i].events, ev->arg);  
            }  
            if((events[i].events&EPOLLOUT)&&(ev->events&EPOLLOUT)) // write event  
            {  
                ev->call_back(ev->fd, events[i].events, ev->arg);  
            }  
        }  
    }  
    // free resource  
    return 0;  
}