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 to eliminate the overhead of one system call of read(2) per wakeup event. There are several renowned open-source projects relying on this feature for notification function (with eventfd): register eventfd with EPOLLET and avoid calling read(2) on the eventfd when there is wakeup event (eventfd being written). Examples: 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> --- Changes in v4: - Move the added sentence elsewhere to make more sense - Link to v3: https://lore.kernel.org/r/20240730-epoll-et-desc-v3-1-6aa81b1c400d@xxxxxxxxxx Changes in v3: - Updated the git commit description - Link to v2: https://lore.kernel.org/r/20240727-epoll-et-desc-v2-1-c99b2ac66775@xxxxxxxxxx Changes in v2: - Added the git commit description based on feedback - Link to v1: https://lore.kernel.org/r/20240727-epoll-et-desc-v1-1-390bafc678b9@xxxxxxxxxx --- man/man7/epoll.7 | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/man/man7/epoll.7 b/man/man7/epoll.7 index 951500131..a7235971d 100644 --- a/man/man7/epoll.7 +++ b/man/man7/epoll.7 @@ -121,7 +121,8 @@ .SS Level-triggered and edge-triggered meanwhile the remote peer might be expecting a response based on the data it already sent. The reason for this is that edge-triggered mode -delivers events only when changes occur on the monitored file descriptor. +delivers events only when changes occur on the monitored file descriptor, +that is, an event will only be generated upon each receipt of a chunk of data. So, in step .B 5 the caller might end up waiting for some data that is already present inside --- base-commit: cbc0a111e4dceea2037c51098de33e6bc8c16a5c change-id: 20240727-epoll-et-desc-04ea9a590f3b Best regards, -- Andy Pan <i@xxxxxxxxxx>
