Using libev with pthreads and one listening socket

Marc Lehmann schmorp at schmorp.de
Wed Mar 30 10:47:40 CEST 2011


On Wed, Mar 30, 2011 at 03:27:04PM +1100, Arlen Cuss <celtic at sairyx.org> wrote:
> I enjoyed reading your (many) comments in the documentation talking
> about the same :-P, particularly "a frankenpoll" re: epoll.

It's a direct consequence of my many frustrated workarounds and insightful
moments ("ugh").

> This will be deployed on FreeBSD, so the performance characteristics may
> end up varying anyway.

It would be interesting how though - kqueeu "suffers" from the same
inefficiency as epoll in this respect, but there is a big difference:
everything is done by passing a big changelist into a single syscall,
while for epoll, each change is one extra syscall.

This difference can make it work fine.

> This seems to be the case. Right now I'm still employing the (possibly
> criminal) method of having all threads listen on the fd - it saves on
> the syscalls -- maybe I don't need to worry about that anyway -- but
> it's also a simpler design.

That it is indeed, and you only need to be fast enough after all.

Whether it's "criminal" or not mostly depends on what the kernel does or not.
If your threads are all very busy with other stuff, then you either get no
wakeup at all (because the kernel checks *again* to see if there are any new
conenctions) or you get a spurious wakeup, but also some work to do.

So under enough load, doing it in all threads might just work out.

-- 
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