Could you recommend some guides about Epoll on Linux

Here's an introduction to Epoll, a pretty basic tutorial: http://blog.kovyrin.net/2006/04/13/epoll-asynchronous-network-programming/

A more complete example can be found here: https://banu.com/blog/2/how-to-use-epoll-a-complete-example-in-c/

Also, the man pages

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <sys/epoll.h>

#define PORT 1500 

#define MAX_CON (1200)

static struct epoll_event *events;

int main(int argc, char *argv[])
{
    fd_set master;
    fd_set read_fds;
    struct sockaddr_in serveraddr;
    struct sockaddr_in clientaddr;
    int fdmax;
    int listener;
    int newfd;
    char buf[1024];
    int nbytes;
    int addrlen;
    int yes;
    int epfd = -1;
    int res = -1;
    struct epoll_event ev;
    int i=0;
    int index = 0;
    int client_fd = -1;

    int SnumOfConnection = 0;
    time_t Sstart, Send;

    if((listener = socket(AF_INET, SOCK_STREAM, 0)) == -1)
    {
            perror("Server-socket() error lol!");
            exit(1);
    }

    if(setsockopt(listener, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(int)) == -1)
    {
            perror("Server-setsockopt() error lol!");
            exit(1);
    }
    serveraddr.sin_family = AF_INET;
    serveraddr.sin_addr.s_addr = INADDR_ANY;
    serveraddr.sin_port = htons(PORT);
    memset(&(serveraddr.sin_zero), '\0', 8);
    if(bind(listener, (struct sockaddr *)&serveraddr, sizeof(serveraddr)) == -1)
    {
            perror("Server-bind() error lol!");
            exit(1);
    }
    if(listen(listener, 10) == -1)
    {
            perror("Server-listen() error lol!");
            exit(1);
    }
    fdmax = listener; /* so far, it's this one*/

    events = calloc(MAX_CON, sizeof(struct epoll_event));
    if ((epfd = epoll_create(MAX_CON)) == -1) {
            perror("epoll_create");
            exit(1);
    }
    ev.events = EPOLLIN;
    ev.data.fd = fdmax;
    if (epoll_ctl(epfd, EPOLL_CTL_ADD, fdmax, &ev) < 0) {
            perror("epoll_ctl");
            exit(1);
    }
    //time(&start);
    for(;;)
    {
            res = epoll_wait(epfd, events, MAX_CON, -1);
            client_fd = events[index].data.fd;

            for (index = 0; index < MAX_CON; index++) {
                    if(client_fd == listener)
                    {
                            addrlen = sizeof(clientaddr);
                            if((newfd = accept(listener, (struct sockaddr *)&clientaddr, &addrlen)) == -1)
                            {
                                    perror("Server-accept() error lol!");
                            }
                            else
                            {
                            //      printf("Server-accept() is OK...\n");
                                    ev.events = EPOLLIN;
                                    ev.data.fd = newfd;
                                    if (epoll_ctl(epfd, EPOLL_CTL_ADD, newfd, &ev) < 0) {
                                            perror("epoll_ctl");
                                            exit(1);
                                    }
                            }
                            break;
                    }
                    else
                    {
                            if (events[index].events & EPOLLHUP)
                            {
                                    if (epoll_ctl(epfd, EPOLL_CTL_DEL, client_fd, &ev) < 0) {
                                            perror("epoll_ctl");
                                    }
                                    close(client_fd);
                                    break;
                            }
                            if (events[index].events & EPOLLIN)  {
                                    if((nbytes = recv(client_fd, buf, sizeof(buf), 0)) <= 0)
                                    {
                                            if(nbytes == 0) {
                                            //      printf("socket %d hung up\n", client_fd);
                                            }
                                            else {
                                                    printf("recv() error lol! %d", client_fd);
                                                    perror("");
                                            }

                                            if (epoll_ctl(epfd, EPOLL_CTL_DEL, client_fd, &ev) < 0) {
                                                    perror("epoll_ctl");
                                            }
                                            close(client_fd);
                                    }
                                    else
                                    {
                                            if(send(client_fd, buf, nbytes, 0) == -1)
                                                    perror("send() error lol!");
                                    }
                                    break;
                            }
                    }
            }
    }
    return 0;
}

I wrote this program for testing and I was able to connect more than 80k connections and I find average system load only to 0.27%.