ZeroMQ (clrzmq4) polling issue

Question

What I'm trying to accomplish is to implement reading a message from one of two sockets, wherever it arrives first. As far as I understand polling (zmq_poll) is the right thing to do (as demonstrated in mspoller in guide). Here I'll provide small pseudo-code snippet:

TimeSpan timeout = TimeSpan.FromMilliseconds(50);

using (var receiver1 = new ZSocket(ZContext.Current, ZSocketType.DEALER))
using (var receiver2 = new ZSocket(ZContext.Current, ZSocketType.PAIR))
{
    receiver1.Bind("tcp://someaddress");
    // Note that PAIR socket is inproc:
    receiver2.Connect("inproc://otheraddress");

    var poll = ZPollItem.CreateReceiver();

    ZError error;
    ZMessage msg;

    while (true)
    {
        if (receiver1.PollIn(poll, out msg, out error, timeout))
        {
            // ...
        }

        if (receiver2.PollIn(poll, out msg, out error, timeout))
        {
            // ...
        }
    }
}

As you can see it is actually the same exact implementation as in mspoller in guide.

In my case receiver2 (PAIR socket) should receive a large number of messages. In fact I've created a test in which number of messages sent to it is always greater than the number of messages it is capable to receive (at least in demonstrated implementation).

I've run the test for 2 seconds, and I was very surprised with results:

• Number of messages sent to receiver2: 180 (by "sent" I mean that they are handed out to another PAIR socket not shown in the previous snippet);
• Number of messages received by receiver2: 21 ??? Only 21 messages in 2 seconds??? 10 messages per second???

Then I've tried to play with different timeout values and I've found out that it significantly influences the number of messages received. Duration (2 seconds) and number of messages sent (180) remain the same. The results are:

• timeout value of 200 milliseconds - number of messages received drops to 10 (5 per second);
• timeout value of 10 milliseconds - number of messages received rises to 120 (60 per second).

The results are telling me that polling simply does not work. If polling were working properly, as far as I understand the mechanism, timeout should not have any influence in this scenario. No matter if we set timeout to 1 hour or 5 milliseconds - since there are always messages to receive there's nothing to wait for, so the loop should work with the same speed.

My another big concern is the fact that even with very small timeout value receiver2 is not capable to receive all 180 messages. I'm struggling here to accomplish receiving rate of 100 messages per second, although I've selected ZeroMQ which should be very fast (benchmarks are mentioning numbers as 6 million messages per second).

So my question is obvious: am I doing something wrong here? Is there a better way to implement polling?

By browsing clrzmq4 code I've noticed that there's also possibility to call pollIn method on enumeration of sockets ZPollItems.cs, line 151, but I haven't found any example anywhere!

Can this be the right approach? Any documentation anywhere?

Thanks

Answer 1

I've found the problem / solution for this. Instead using PollIn method on each socket separately we should use PollIn method on array of sockets. Obviously the example from the guide is HUGELY MISLEADING. Here's the correct approach:

TimeSpan timeout = TimeSpan.FromMilliseconds(50);

using (var receiver1 = new ZSocket(ZContext.Current, ZSocketType.DEALER))
using (var receiver2 = new ZSocket(ZContext.Current, ZSocketType.PAIR))
{
    receiver1.Bind("tcp://someaddress");
    receiver2.Connect("inproc://otheraddress");

    // We should "remember" the order of sockets within the array
    // because order of messages in the received array will correspond to it.
    ZSocket[] sockets = { receiver1, receiver2 };

    // Note that we should use two ZPollItem instances:
    ZPollItem[] pollItems = { ZPollItem.CreateReceiver(), ZPollItem.CreateReceiver() };

    ZError error;
    ZMessage[] msg;

    while (true)
    {
        if (sockets.PollIn(pollItems, out msg, out error, timeout))
        {
            if (msg[0] != null)
            {
                // The first message gotten from receiver1
            }

            if (msg[1] != null)
            {
                // The second message gotten from receiver2
            }
        }
    }
}

Now receiver2 reaches 15,000 received messages per second, no matter timeout value, and no matter number of messages received by receiver1.

UPDATE: Folks from clrzmq4 have acknowledged this issue, so probably the example will be corrected soon.