Messages in this thread | | | Date | Wed, 26 Apr 2006 23:20:56 +0200 | From | Willy Tarreau <> | Subject | Re: accept()ing socket connections with level triggered epoll |
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Hello,
On Wed, Apr 26, 2006 at 02:55:57PM -0600, kyle@pbx.org wrote: > Hello, > > I think I may have found a bug in Linux's implementation of epoll. My > program creates a server socket that listens for incoming SOCK_STREAM > connections. It uses epoll to wait for notification of a new connection > (and also to handle the client sockets). While the client sockets use edge > triggered epoll, for performance reasons, the server socket uses level > triggered epoll. > > I have found that when I open connections to my program very quickly, it is > sometimes possible to call accept more than once before reaching the point > where no more connections are available and EAGAIN is returned. If I return > to epoll_wait without accepting all of the available connections, I should > immediately be notified that a read is still available on the server socket, > since I am using level triggered epoll for that descriptor (at least that is > my understanding of how all of this is supposed to work ;). However, epoll > does not make this notification. Even if the program accepts further > incoming connections, the missed connection is never accepted, and > eventually times out on the client side.
I find this very strange because if your program accepts other connections, I don't see how it could "select" some connections and ignore others. The accept() call returns the next connection(s) in the listen queue. Stupid question : are you sure that you don't miss anything in the loop around accept() ? eg: reinitialise one error code or anything which could prevent accept() from being further called after you have successfully done several accept() at once ? I'm personnally using epoll in level triggered mode in haproxy, which often does multiple accept() per call on very high loads (>10k sessions/s), and although I've encountered difficult beginnings with epoll, it's rock solid now.
> Kernel version is 2.6.9. I can provide test code if needed.
I would suggest trying 2.6.16 first to see if it may be related to a bug which has been fixed since then, and otherwise, some test code would help.
> Thanks, > Kyle Cronan > <kyle@pbx.org>
Cheers, Willy
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