Boost asio async UDP socket has high latency

Question

I'm having difficulty with the delay when starting async asio udp socket. I do not have the same problems with an identical application using synchronous udp. I am communicating with a physical device that allows a maximum latency of 5 ms for each UDP packet, and currently I am getting 8 ms latency with std dev of 0.065 ms, which is surprisingly accurate.

I am on Mac OS X 10.10.4 with Xcode 6.3.2 (6D2105).

Here are some key information about this issue:

Sync UDP Test Program

• working, remaining under 5 ms latency 100% of the time for an hour
• Code
  • KukaFRIClientDataTest.cpp main file
  • UDP socket connection class KukaFriClientData.hpp
• Key Rows
  • calls socket.receive_from() , then socket.send() to the lowest level.

Async UDP Testing Program

• high latency error, 8 ms with 0.065 ms std dev
• Code
  • KukaFRITest.cpp main file
  • KukaFRIThreadSeparator.hpp High-Level Shell Class
  • The low-level shell class KukaFRI.hpp .
• Key Rows
  • calls socket_.async_receive_from() , then this handler calls socket_.async_send_to() , which calls the final callback with the results.

Does anyone know why this lag issue can occur in Async UDP version? Thanks.