[zeromq-dev] zmq_poll performance question

[Francis Le Bourse]

On 11/6/2014 3:18 PM, Pieter Hintjens wrote:
> Oh, ok. Sounds like you have a good candidate for some before/after
> measurement and optimization. Are you going to try to make a patch for
> this?
Yes, but I am not sure about the way to proceed.
I have already tried to:
     - remove the call to zmq_getsockopt(., ZMQ_EVENTS, ., .). Doesn't work.
     - call zmq_getsockopt() only if pollfds[i].revents != 0. Works well 
a few minutes.
Will it be necessary to add a zmq_getsockopt_ex() and a lot of other 
_ex() methods, passing the already set pollfds[i] result to avoid 
calling poll() in zmq::signaler_t::wait() ?

Cheers,
Francis

>
> On Thu, Nov 6, 2014 at 2:09 PM, Francis Le Bourse
> <zno-reply-francis.lebourse at sfr-sh.fr> wrote:
>> On 11/6/2014 11:47 AM, Pieter Hintjens wrote:
>>> A simple optimization is, when you are polling sockets for input, to
>>> continue reading from an active socket using a non-blocking read. So
>>> you process all waiting messages on a socket and then only switch back
>>> to poll when needed.
>> Thank you for you quick reply.
>>
>> Yes, but the question was more about the zmq_poll() internals.
>> For 600+ file descriptors, zmq_poll() calls poll() a huge number of times
>> for only a few that will trigger a POLLIN and the relevant information is
>> already known / present in the pollfds array. The performance hit is there.
>>
>> Cheers,
>> Francis
>>
>>
>>
>>> On Thu, Nov 6, 2014 at 11:28 AM, Francis Le Bourse
>>> <zno-reply-francis.lebourse at sfr-sh.fr> wrote:
>>>> Hi,
>>>>
>>>> I am looking at a performance issue in zmq, when the number of zsockets /
>>>> file descriptors becomes large.
>>>> The relevant calls are:
>>>>       poll+0x57
>>>>       zmq_poll+0x2e3
>>>>       zloop_start+0x1e8
>>>>       main+0xb40
>>>>       __libc_start_main+0xfd
>>>> immediately followed by a loop of
>>>>       poll+0x57
>>>>       zmq::signaler_t::wait(int)+0x33
>>>>       zmq::mailbox_t::recv(zmq::command_t*, int)+0x78
>>>>       zmq::socket_base_t::process_commands(int, bool)+0xbe
>>>>       zmq::socket_base_t::getsockopt(int, void*, unsigned long*)+0x135
>>>>       zmq_getsockopt+0x75
>>>>       zmq_poll+0x3da
>>>>       zloop_start+0x1e8
>>>>       main+0xb40
>>>>       __libc_start_main+0xfd
>>>>
>>>> The code in the loop is executed once per file descriptor in the initial
>>>> pollarray, redoing a poll() system call each time.
>>>> Is there a reason to proceed that way ?
>>>> Would be possible to reuse the results of the first poll() in order to
>>>> bypass the second set of system calls ?
>>>>
>>>> Cheers,
>>>> Francis
>>>>
>>>>
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>>>>
>>