[zeromq-dev] feedback on proposed Context API addition

Chuck Remes cremes.devlist at mac.com
Wed Feb 8 19:56:22 CET 2012

A user on irc (calvin) may post here (or on this thread) later. He popped into the channel asking about pinning a socket to a specific CPU. I pointed him at ZMQ_AFFINITY which is available through zmq_setsockopt(). However, as he pointed out, that only controls socket affinity with I/O threads which may still be scheduled on any CPU.

After a little back-and-forth, here's a proposal for an addition to the C API.

	void *zmq_init_with_affinity (int io_threads, char* cpu_bitmask_buffer, size_t bitmask_len);

Such a change would allow a programmer to create a context and specify which specific CPUs should have I/O threads pinned to them. We need to use a byte buffer to contain the bitmask and pass a length since systems with more than 64 CPUs and/or cores are already available.

For those cases where +io_threads+ is a number larger than the number of bits set in the bitmask, the function has three reasonable ways of treating it.

1. Round-robin assign I/O threads to the CPUs listed in the bitmask.

2. Pin the "excess" I/O threads to the last CPU listed in the bitmask.

3. Skip pinning the "excess" I/O threads to any CPU and let the scheduler float them around as needed.

This addition would not break existing code. Furthermore, we could implement zmq_init() internally with a call to zmq_init_with_affinity(). Passing a zero-length buffer would skip the CPU-affinity pinning functionality (much like option 3 above).



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