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Latest release of nginx mainline (1.21.6) works around the terrible load balancing of Linux's EPOLLEXCLUSIVE: nginx.org/en/CHANGES trac.nginx.org/nginx/ticket/2 github.com/nginx/nginx/co Sad that this is still a problem for Linux years after the kernel feature was added in 2016.

github.com

Events: fixed balancing between workers with EPOLLEXCLUSIVE. · nginx/nginx@96c342e

Linux with EPOLLEXCLUSIVE usually notifies only the process which was first to add the listening socket to the epoll instance. As a result most of the connections are handled by the first worker p...

Daniel Micay

@DanielMicay

Feb 20

They've been using EPOLLEXCLUSIVE since shortly after it became available. It replaced needing accept_mutex to avoid unnecessary wakeups. Linux could have used round-robin load balancing for EPOLLEXCLUSIVE as they do for REUSEPORT instead of pathological behavior being standard.

Daniel Micay

@DanielMicay

REUSEPORT isn't a strict upgrade over EPOLLEXCLUSIVE. It distributes the connections (TCP) / messages (UDP) via round-robin without needing synchronization across them. EPOLLEXCLUSIVE avoids giving work to a worker that's currently not waiting but isn't as ridiculously scalable.

9:49 PM · Feb 20, 2022Twitter Web App

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It would help if nginx finally got upstream support for async open. REUSEPORT will work increasingly well as the remaining blocking goes away. They still don't use io_uring but mostly because thread pool AIO works well enough that io_uring would mostly just reduce CPU use a bit.