select模型和Epoll的对比:
1)select模型有最大并发数限制,因为一个进程打开的FD是有限制的,(FD_SETSIZE默认为1024)。很多人提到增加FD_SETSIZE,这显然是不科学的。因为每一次调用select都会线性的扫描全部的FD集合,效率直线下降
2)epoll没有最大并发数限制,它所支持的FD上限是最大可以打开文件的数(一般与系统内存有关)。epoll只会遍历那些被内核IO事件异步唤醒而加入Ready队列的描述符集合
Epoll 关键数据结构
前面提到 Epoll 速度快和其数据结构密不可分,其关键数据结构就是:
struct epoll_event {
__uint32_t events; // Epoll events
epoll_data_t data; // User data variable
};
typedef union epoll_data {
void *ptr;
int fd;
__uint32_t u32;
__uint64_t u64;
} epoll_data_t;
可见 epoll_data 是一个 union 结构体 , 借助于它应用程序可以保存很多类型的信息 :fd 、指针等等。有了它,应用程序就可以直接定位目标了。
使用 Epoll
既然 Epoll 相比 select 这么好,那么用起来如何呢?会不会很繁琐啊 … 先看看下面的三个函数吧,就知道 Epoll 的易用了。
int epoll_create(int size);
生成一个 Epoll 专用的文件描述符,其实是申请一个内核空间,用来存放你想关注的 socket fd 上是否发生以及发生了什么事件。 size 就是你在这个 Epoll fd 上能关注的最大 socket fd 数,大小自定,只要内存足够。
int epoll_ctl(int epfd, int op, int fd, struct epoll_event *event );
控制某个 Epoll 文件描述符上的事件:注册、修改、删除。其中参数 epfd 是 epoll_create() 创建 Epoll 专用的文件描述符。相对于 select 模型中的 FD_SET 和 FD_CLR 宏。
int epoll_wait(int epfd,struct epoll_event * events,int maxevents,int timeout);
等待 I/O 事件的发生;参数说明:
epfd: 由 epoll_create() 生成的 Epoll 专用的文件描述符;
epoll_event: 用于回传代处理事件的数组;
maxevents: 每次能处理的事件数;
timeout: 等待 I/O 事件发生的超时值;
返回发生事件数。
相对于 select 模型中的 select 函数。
例子程序
下面是一个简单 Echo Server 的例子程序,麻雀虽小,五脏俱全,还包含了一个简单的超时检查机制,简洁起见没有做错误处理。
#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;
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;
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]\n", ev->fd);
else
printf("Event Add OK[fd=%d]\n", ev->fd);
}
// 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: bad accept", __func__);
}
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
if(fcntl(nfd, F_SETFL, O_NONBLOCK) < 0) break;
// add a read event for receive data
EventSet(&g_Events[i], nfd, RecvData, &g_Events[i]);
EventAdd(g_epollFd, EPOLLIN|EPOLLET, &g_Events[i]);
printf("new conn[%s:%d][time:%d]\n", inet_ntoa(sin.sin_addr), ntohs(sin.sin_port), g_Events[i].last_active);
}while(0);
}
// 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, sizeof(ev->buff)-1, 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|EPOLLET, ev);
}
else if(len == 0)
{
close(ev->fd);
printf("[fd=%d] closed gracefully.\n", fd);
}
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->len, 0);
ev->len = 0;
EventDel(g_epollFd, ev);
if(len > 0)
{
// change to receive event
EventSet(ev, fd, RecvData, ev);
EventAdd(g_epollFd, EPOLLIN|EPOLLET, ev);
}
else
{
close(ev->fd);
printf("recv[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|EPOLLET, &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)
{
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;
}