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 nonblockingint 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 datalen = 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 eventEventDel(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;
}