I'd like to enable dtrace by default on i386, amd64, and arm -- that is, setting MKDTRACE=yes in make for the userland tools, and enabling options KDTRACE_HOOKS in the kernel for the hooks.
The overhead of KDTRACE_HOOKS in the kernel is a predicted not-taken branch in a handful of places. I have not measured the speed impact but I expect it to be very small. All logic related to dtrace is relegated to a module, so unless you load dtrace.kmod, there's negligible memory impact. The overhead of MKDTRACE=yes in the userland is about 15 MB of dtrace tools and libraries on amd64. The benefits are manifold: low-overhead flexible diagnostics for performance and debugging issues in the field with no kernel rebuild or reboot. In the past few months while trying to help people debug things, my first question has often been `do you have dtrace?'. Objections?
