[zeromq-dev] EAGAIN on ZMQ_STREAM

Sat Jun 18 18:54:45 CEST 2016

Hi,

I’m using the C++ binding from https://github.com/zeromq/cppzmq/blob/master/zmq.hpp. It is quite simple. The send method I use (there are several depending on the parameters) is:

        inline bool send (message_t &msg_, int flags_ = 0)

        {

            int nbytes = zmq_msg_send (&(msg_.msg), ptr, flags_);

            if (nbytes >= 0)

                return true;

            if (zmq_errno () == EAGAIN)

                return false;

            throw error_t ();

        }

In this case, the return value is false for EAGAIN and true otherwise. Other errors are handled as exceptions. 

I jut to ask under which circumstances EAGAIN is return for ZMQ_STREAM sockets. The HWM should be large enough for anything I send, but I will investigate more on Monday. Is it correct that HWM=0 disabled the limit completely and I should not expect any EAGAIN ever?

Cheers,

  Jens

Von: zeromq-dev [mailto:zeromq-dev-bounces at lists.zeromq.org] Im Auftrag von Doron Somech
Gesendet: Samstag, 18. Juni 2016 18:47
An: ZeroMQ development list
Betreff: Re: [zeromq-dev] EAGAIN on ZMQ_STREAM

Which language and binding are you using?

you are checking the return number from the send method, in most binding that return the number of bytes send and not the error code. It might be different with binding you are using but I'm not sure.

Anyway if that is the case it might be that the number of bytes just equal EAGAIN?

On Sat, Jun 18, 2016 at 11:59 AM, Jens Auer <jens.auer at betaversion.net> wrote:

Hi Doron,

I don’t think it is disconnect. I am testing it on the local network, so there is not much internet infrastructure in between. I am using a pseudo client which logs socket errors, connects and disconnects and I don’t see anything. On the server side, I have a socket monitor which logs socket events but this does not show anything. Finally, if the TCP connection disconnects the ZMQ_STREAM socket shall create the empty disconnect message which I do not receive, neither before or after the EAGAIN. 

Best wishes,

  Jens 

Von: zeromq-dev [mailto:zeromq-dev-bounces at lists.zeromq.org] Im Auftrag von Doron Somech
Gesendet: Freitag, 17. Juni 2016 22:14
An: ZeroMQ development list
Betreff: Re: [zeromq-dev] EAGAIN on ZMQ_STREAM

Why you can exclude the first case? TCP connection can get disconnect quite often.

Anyway from what you are describing it seems the connection got disconnected.

On Fri, Jun 17, 2016 at 8:05 PM, Jens Auer <jens.auer at betaversion.net> wrote:

Hi,

I am witnessing something in my system that I didn't expect. I am processing
data at a data rate of 100Mbps, with occasional peaks of 5s of data (=5 *
100Mbps) delivered as fast as possible because an internal buffer is
flushed. I use ZeroMQ internally, but I have to deliver the data on a TCP
plain socket at the endpoint of my system. Here, I use a ZMQ_STREAM socket
where clients connect. Basically, I keep track of connected clients and send
the data to all clients as a copy of the message. The main loop here looks
like
for(auto const& c: clientIdMessages) {
   bool eagain = !socket.send(c.copy(), ZMQ_DONTWAIT | ZMQ_SNDMORE);
   if (!eagain) {
      eagain = !socket.send(msg.copy(), ZMQ_DONTWAIT | ZMQ_SNDMORE);
   }
}

This usually works fine, but from time to time a message is skipped because
send returns with EAGAIN. If I understand the docs correctly, this happens
if the peer is unknown (ZMQ_STREAM behavior) or when the message cannot be
queued. I can exclude the first case, so I must a high HWM for sending
messages. However, I tried to increase the HWM to a huge value of 50 times
the expected message rate and it did not solve the problem. I also tried to
disable the HWM by setting it to 0, which means "no limit". Still, I am
getting an EAGAIN which I don't expect.

I can exclude any previous loss because I have counters showing me that the
full data stream is delivered to the ZMQ_STREAM socket code above. I also
added an assertion on !eagain after sending, and this fires. Are there any
other conditions when I can get EAGAIN from a ZMQ_STREAM socket?

Best wishes,
  Jens

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