ZeroMQ-线程数

Question

Q1: How many threads are created when using ZeroMQ?

According to ( output from $ ps -o nlwp <pid> ) the following program creates 3 threads.

Q2: Why?

Q3: Shouldn't this be just 2 ( one for main, one for I/O-thread, given num_zmq_io_threads == 1 )?

Q4: Is this documented anywhere?

#include <zmq.hpp>

int main( int argc, char * argv[] )
{
    const int num_zmq_io_threads = 1;        
    zmq::context_t context( num_zmq_io_threads );

    zmq::socket_t socket1( context, ZMQ_PUB );
    socket1.bind( "tcp://127.0.0.1:5555" );

    for(;;)
    {
        // spin
    }

    return 0;
}

Answer 1

ZeroMQ will create a reaper thread to handle socket shutdown, in addition to the number of I/O-threads called for. If you ask for zero I/O-threads, possible when using only intra-processess messages, there will still be a reaper thread. This is why you appear to get one more thread than asked for.

The reason you see no threads when the socket is not created is because ZeroMQ doesn't create the threads on context creation; they are created later. This is more apparent with the ZeroMQ C API, where there are no options when creating the context with zmq_ctx_new() , and then options like number and priority of I/O-threads must be specified after the context is created with zmq_ctx_set() .

Creating the socket triggers ZMQ to create the threads. I don't know where it's documented when exactly ZMQ will create threads. Ie, does it always create all threads when the first socket is created on a context or is it more complex than that? For instance, does it depend on when the socket created has some I/O-operation for a thread to perform?