SO_REUSEPORT: strange kernel balancer behaviour

John-Mark Gurney jmg at
Mon Jul 22 20:02:13 UTC 2013

trafdev wrote this message on Mon, Jul 15, 2013 at 13:04 -0700:
> Yep I think it's wasting of resources, poll manager should somehow be 
> configured to update only one process/thread.
> Anyone know how to do that?

This isn't currently possible w/o a shared kqueue, since the event is
level triggered, not edge..  You could do it w/ a shared kqueue using
_ONESHOT (but then you'd also have a shared listen fd which obviously
isn't what the OP wants)...

I guess it wouldn't be too hard to do a wake one style thing, where
kqueue only deliveres the event once per "item/level", but right now
kqueue doesn't know anything about the format of data (which would be
number of listeners waiting)...  Even if it did, there would be this
dangerous contract that if an event is returned that the user land
process would handle it...  How is kqueue suppose to handle a server
that crashes/dies between getting the event and accepting a connection?
How is userland suppose to know that an event wasn't handled, or is
just taking a long time?

Sadly, if you want to avoid the thundering heards problem, I think
blocking on accept is the best method, or using a fd passing scheme
where only on process accept's connections...

> On Mon Jul 15 12:53:55 2013, Adrian Chadd wrote:
> >i've noticed this when doing this stuff in a threaded program with
> >each thread listening on the same port.
> >
> >All threads wake up on each accepted connection, one thread wins and
> >the other threads get EAGAIN.
> >
> >
> >
> >-adrian
> >
> >On 15 July 2013 12:31, trafdev <trafdev at> wrote:
> >>Thanks for reply.
> >>
> >>This approach produces lot of "resource temporary unavailable" (eagain) on
> >>accept-ing connections in N-1 processes.
> >>Is this possible to avoid this by e.g. tweaking kqueue?
> >>
> >>
> >>On Sun Jul 14 19:37:59 2013, Sepherosa Ziehau wrote:
> >>>
> >>>On Sat, Jul 13, 2013 at 1:16 PM, trafdev <trafdev at> wrote:
> >>>>
> >>>>Hello.
> >>>>
> >>>>Could someone help with following problem of SO_REUSEPORT.
> >>>
> >>>
> >>>The most portable "load balance" between processes listening on the
> >>>same TCP addr/port probably is:
> >>>
> >>>s=socket();
> >>>bind(s);
> >>>listen(s);
> >>>/* various socketopt and fcntl as you needed */
> >>>pid=fork();
> >>>if (pid==0) {
> >>>      server_loop(s);
> >>>      exit(1);
> >>>}
> >>>server_loop(s);
> >>>exit(1);
> >>>
> >>>Even in Linux or DragonFly SO_REUSEPORT "load balance" between
> >>>processes listening on the same TCP addr/port was introduced recently,
> >>>so you probably won't want to rely on it.

  John-Mark Gurney				Voice: +1 415 225 5579

     "All that I will do, has been done, All that I have, has not."

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