Understanding epoll Programming and Its Use in Redis Server

Network programming typically follows steps: create a listening socket, bind an IP and port, listen, accept client connections in a loop, and handle each connection in a child process or thread.

listen_fd = socket();</code>
<code>bind(listen_fd, ip_and_port);</code>
<code>listen(listen_fd);</code>
<code>while (1) { new_client_fd = accept(); /* spawn child or thread */ }

Because creating a process or thread per connection does not scale to the C10K problem, I/O multiplexing such as epoll is used.

epoll provides three system calls: epoll_create, epoll_ctl, and epoll_wait.

int epoll_create(int size);</code>
<code>int epoll_ctl(int epfd, int op, int fd, struct epoll_event *event);</code>
<code>int epoll_wait(int epfd, struct epoll_event *events, int maxevents, int timeout);

A typical epoll server creates an epoll instance, registers the listening socket with edge‑triggered read events, then loops on epoll_wait to dispatch ready events.

epoll_fd = epoll_create(MAXEPOLLSIZE);
 ev.events = EPOLLIN | EPOLLET;
 ev.data.fd = listen_fd;
 epoll_ctl(epoll_fd, EPOLL_CTL_ADD, listen_fd, &ev);
 while (1) {
     int n = epoll_wait(epoll_fd, fired_events, MAXEPOLLSIZE, -1);
     for (int i = 0; i < n; ++i) {
         if (fired_events[i].data.fd == listen_fd) {
             /* accept new client */
         } else {
             /* read/write handling */
         }
     }
 }

Redis 5.0 uses epoll on Linux. Its event loop (aeEventLoop) wraps epoll through functions aeApiCreate, aeApiAddEvent, aeApiDelEvent, and aeApiPoll, which map directly to the three epoll calls.

int aeApiCreate(aeEventLoop *eventLoop);</code>
<code>int aeApiAddEvent(aeEventLoop *eventLoop, int fd, int mask);</code>
<code>void aeApiDelEvent(aeEventLoop *eventLoop, int fd, int delmask);</code>
<code>int aeApiPoll(aeEventLoop *eventLoop, struct timeval *tvp);

The event loop stores an array indexed by file descriptor; each entry ( aeFileEvent) holds a mask and pointers to read/write handler functions. During initialization Redis creates the loop with aeCreateEventLoop(server.maxclients+CONFIG_FDSET_INCR) and registers handlers: acceptTcpHandler for listening sockets and readQueryFromClient for client sockets.

aeCreateFileEvent(server.el, server.ipfd[j], AE_READABLE, acceptTcpHandler, NULL);
 aeCreateFileEvent(server.el, fd, AE_READABLE, readQueryFromClient, client);

Debugging can be performed with gdb by setting breakpoints on acceptTcpHandler and readQueryFromClient, then inspecting the aeEventLoop structures to see how epoll events are dispatched.