一、最大連接數

1、select

select在單進程中最多同時監聽1024個fd；要想實現百萬并發需要一千個進程，并且性能會很差、內存消耗巨大。所以select只適用于連接數在一千個以下的場景。

2、epoll

epoll本身不限制連接數，但是連接數會受到系統限制。

1）文件句柄限制

Linux系統限制單進程可以打開文件的數量默認是1024，可以使用ulimit -n命令查看。

$ ulimit -n
1024

臨時修改：ulimit -n [文件句柄數量]
永久修改：編輯 /etc/security/limits.conf 文件， 修改后內容為

* 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

文件句柄數設置多大合適
這個值在kernel的文檔里意思是file-max一般為內存大小（KB）的10%來計算，如果使用shell，可以這樣計算：

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

* 表示匹配所有用戶
參考博客：
【 Linux 】單臺服務器上并發TCP連接數
網絡編程釋疑之：單臺服務器上的并發TCP連接數可以有多少

2）端口號不會限制連接數

參考博客：
網絡編程釋疑之：單臺服務器上的并發TCP連接數可以有多少

3）內存限制

4G內存極限值52萬。
參考博客：
4G內存服務器epoll并發量最大能達到多少

二、epoll原理

參考博客：
高并發網絡編程之epoll詳解

三、epoll編程

參考博客：
Linux下Socket編程(四)——epoll的使用
linux高效率編程：epoll和多線程

1、作者：sparkling

//   
// a simple echo server using epoll in linux  
//   
// 2009-11-05  
// 2013-03-22:修改了幾個問題，1是/n格式問題，2是去掉了原代碼不小心加上的ET模式;
// 本來只是簡單的示意程序，決定還是加上 recv/send時的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;  
}