[zeromq-dev] User-space polling of ZMQ sockets

Francesco francesco.montorsi at gmail.com
Thu Apr 12 22:39:00 CEST 2018


Hi Luca, Hi Bill,
thanks for the answers.
Actually I verified that the configure script of zmq seems to be using
epoll:

   configure: Choosing polling system from 'kqueue epoll devpoll pollset
poll select'...
   configure: Using 'epoll' polling system with CLOEXEC

and indeed inside platform.hpp I found:

  /* Use 'epoll' polling system */
  #define ZMQ_USE_EPOLL 1

  /* Use 'epoll' polling system with CLOEXEC */
  #define ZMQ_USE_EPOLL_CLOEXEC 1

I also read the links you sent me Bill: actually since I'm monitoring only
2 file descriptors (2 zmq sockets) I think that poll() and epoll()
difference is hardly measurable.
My feeling is just that whatever code ends up calling poll() so often will
pay a lot of


http://arkanis.de/weblog/2017-01-05-measurements-of-system-call-performance-and-overhead






2018-04-11 21:05 GMT+02:00 Bill Torpey <wallstprog at gmail.com>:

> Well, a little googling found this, which is a pretty good writeup:
> https://jvns.ca/blog/2017/06/03/async-io-on-linux--
> select--poll--and-epoll/
>
>
> On Apr 11, 2018, at 2:42 PM, Bill Torpey <wallstprog at gmail.com> wrote:
>
> So, are there any benchmark tests that can be used to quantify the
> overhead of zmq_poll?  It seems like this question keeps coming up, and it
> would certainly be nice to have some real numbers (and the code used to
> generate them).
>
> Having said that, there are several mechanisms that zmq_poll can use, and
> there are apparently significant performance differences between them.  My
> understanding of the conventional wisdom is that epoll is preferable to
> poll, which is preferable to select — but I don’t have any data to back
> that up.
>
> In any event, you can examine the output of the libzmq build to see which
> mechanism is being used:
>
> -- Looking for kqueue
> -- Looking for kqueue - not found
> -- Looking for epoll_create
> -- Looking for epoll_create - found
> -- Looking for epoll_create1
> -- Looking for epoll_create1 - found
> *-- Detected epoll polling method*
>
> I’ve found one (very old!) post that discusses these differences:
> https://cs.uwaterloo.ca/~brecht/servers/epoll/.  If anyone can suggest
> additional sources, please do.
>
>
> On Apr 11, 2018, at 1:46 PM, Luca Boccassi <luca.boccassi at gmail.com>
> wrote:
>
> On Wed, 2018-04-11 at 18:47 +0200, Francesco wrote:
>
> Hi all,
>
> I'm using zmq_poller_wait_all() API in one of my threads because I
> need to
> poll over 2 ZMQ sockets.
> I'm receiving a lot of traffic on both these sockets.
> I think the performance of my software is bad and that may be due
> IMHO to
> the huge amount of poll() syscalls that my thread does... I think the
> overhead of doing the system call is what is killing me...
>
> so my question is: when zmq_poller_wait_all() polls zmq FDs it is
> actually
> polling the real socket FD or rather some ZMQ internal structure?
> If the latter is true, do I have a way to poll more than 1 zmq socket
> without doing a system call?
>
>
> THanks,
> Francesco
>
>
> I'm not familiar with the zmq_poller API so someone else might help -
> in general, a common optimisation is to drain the socket using non-
> blocking receives when poll wakes up (taking care of using heuristic to
> avoid starving the other sockets), so that you can process multiple
> messages per poll
>
> --
> Kind regards,
> Luca Boccassi_______________________________________________
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> zeromq-dev at lists.zeromq.org
> https://lists.zeromq.org/mailman/listinfo/zeromq-dev
>
>
>
>
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