Comparison of select, poll, and epoll: Mechanisms, Complexity, and Practical Usage

select

select monitors multiple file descriptors by linearly scanning the entire fd set, resulting in O(n) time complexity; it is limited by FD_SETSIZE (typically 1024 on 32‑bit systems) and incurs significant copy overhead between user and kernel space.

poll

poll works similarly to select but stores the fd list in a linked list, eliminating the hard limit on the number of descriptors; however, it still suffers from O(n) linear scanning and copies the entire fd array between user and kernel space on each call.

epoll

epoll provides O(1) event notification by registering interest in file descriptors once via epoll_ctl and then waiting for readiness with epoll_wait ; it supports edge‑triggered (EPOLLET) and level‑triggered (EPOLLLT) modes, removes the descriptor limit, and reduces copying by using memory‑mapped buffers.

Key Differences

Maximum connections: select is limited by FD_SETSIZE, poll has no hard limit, epoll can handle tens of thousands of connections depending on system memory.

Scalability with many fds: select and poll degrade linearly as the fd count grows, while epoll processes only active fds, avoiding linear slowdown.

Message passing: select and poll copy data between kernel and user space; epoll shares a memory region, reducing copy overhead.

Implementation Highlights

select copies the fd_set from user to kernel, registers a callback with __pollwait , traverses all fds, and copies the result back; poll follows the same steps but uses a pollfd array and a linked‑list storage.

epoll registers each fd once via epoll_ctl , stores callbacks in the kernel, and epoll_wait simply checks a ready‑list, dramatically lowering CPU usage when many fds are idle.

When to Use Which

For a small number of connections or when all sockets are highly active, select or poll may perform adequately; for large‑scale servers with many idle connections, epoll offers superior efficiency and scalability.

References

https://www.cnblogs.com/zhaodahai/p/6831456.html https://www.cnblogs.com/sky-heaven/p/7011684.html