DTrace landed in FreeBSD 7.1 in 2009. It offers a view into the operation of programs, and is an excellent tool for debugging and performing analysis of complex software. It is a powerful tool for observing what the operating system is up to. Using the full power of FDT probes requires familiar knowledge with how things are implemented. SDT probes and specific providers like ip, tcp and udp offer insights into the internal behavior of the kernel without requiring a high level of knowledge of the underlying code. This is an interesting article on the subject.