DTrace is a facility that dynamically adds probes to a running system, both in user processes and in the kernel.
These probes can be queried via the D programming language to determine an astonishing amount about the kernel,
the system state, and process activities. For example, Figure 2.20 follows an application as it executes a system call (ioctl()) and shows the functional
calls within the kernel as they execute to perform the system call. Lines ending with “U” are executed in user mode, and lines ending in “K” in kernel mode.