From: Andy Pan <i@xxxxxxxxxx> --- For the moment, the edge-triggered epoll generates an event for each receipt of a chunk of data, that is to say, epoll_wait() will return and tell us a monitored file descriptor is ready whenever there is a new activity on that FD since we were last informed about that FD. This is not a real _edge_ implementation for epoll, but it's been working this way for years and plenty of projects are relying on it. There are several renowned open-source projects relying on this feature for notification function (with eventfd), such as nginx [1], netty [2], tokio [3], libevent [4], ect. [5] These projects are widely used in today's Internet infrastructures. Thus, changing this behavior of epoll ET will fundamentally break them and cause a significant negative impact. Linux has changed it for pipe before [6], breaking some Android libraries, which had got "reverted" somehow. [7] [8] Nevertheless, the paragraph in the manual pages describing this characteristic of epoll ET seems ambiguous, I think a more explict sentence should be used to clarify it. We're improving the notification mechanism for libuv recently by exploiting this feature with eventfd, which brings us a significant performance boost. [9] Therefore, we (as well as the maintainers of nginx, netty, tokio, etc.) would have a sense of security to build an enhanced notification function based on this feature if there is a guarantee of retaining this implementation of epoll ET for the backward compatibility in the man pages. [1]: https://github.com/nginx/nginx/blob/efc6a217b92985a1ee211b6bb7337cd2f62deb90/src/event/modules/ngx_epoll_module.c#L386-L457 [2]: https://github.com/netty/netty/pull/9192 [3]: https://github.com/tokio-rs/mio/blob/309daae21ecb1d46203a7dbc0cf4c80310240cba/src/sys/unix/waker.rs#L111-L143 [4]: https://github.com/libevent/libevent/blob/525f5d0a14c9c103be750f2ca175328c25505ea4/event.c#L2597-L2614 [5]: https://github.com/libuv/libuv/pull/4400#issuecomment-2123798748 [6]: https://lkml.iu.edu/hypermail/linux/kernel/2010.1/04363.html [7]: https://github.com/torvalds/linux/commit/3a34b13a88caeb2800ab44a4918f230041b37dd9 [8]: https://github.com/torvalds/linux/commit/3b844826b6c6affa80755254da322b017358a2f4 [9]: https://github.com/libuv/libuv/pull/4400#issuecomment-2103232402 Signed-off-by: Andy Pan <i@xxxxxxxxxx> --- man/man7/epoll.7 | 1 + 1 file changed, 1 insertion(+) diff --git a/man/man7/epoll.7 b/man/man7/epoll.7 index 951500131..361d9db99 100644 --- a/man/man7/epoll.7 +++ b/man/man7/epoll.7 @@ -172,6 +172,7 @@ .SS Level-triggered and edge-triggered Since even with edge-triggered .BR epoll , multiple events can be generated upon receipt of multiple chunks of data, +that is, an event will be generated upon each receipt of a chunk of data, the caller has the option to specify the .B EPOLLONESHOT flag, to tell --- base-commit: cbc0a111e4dceea2037c51098de33e6bc8c16a5c change-id: 20240727-epoll-et-desc-04ea9a590f3b Best regards, -- Andy Pan <i@xxxxxxxxxx>
