Concurrency

Epoll

Linux IO multiplexing — register fds, get notifications on readiness.

In plain terms

Replaces select/poll which were O(N) per call. epoll_wait is O(1). The foundation of every async Linux network server.

Origin

Added to Linux 2.5.45 (2002) by Davide Libenzi. Solved the C10K problem by giving O(1) ready-fd notification instead of O(N) per call. Kqueue (FreeBSD, 2000) was the inspiration.

Where it shows up in production

Node.js libuv epoll on Linux, kqueue on macOS, IOCP on Windows. All same shape.
Go netpoller Goroutines blocked on network I/O park on epoll readiness; the scheduler reschedules them when ready.
Nginx, Redis Single-threaded event loops on epoll. The reason both scale to 100k+ connections per process.

Sources & further reading

Docs epoll(7) man page
Article Kegel — The C10K problem

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