On a certain computer we estimate that, regardless of the OS it runs, thread scheduling costs about
50 ?s, a null procedure call 1 ?s, a context switch to the kernel 20 ?s and a domain transition 40
?s. For each of Mach and SPIN, estimate the cost to a client of calling a dynamically loaded null
procedure.

What will be an ideal response?

Mach, by default, runs dynamically loaded code in a separate address space. So invoking the code involves
control transfer to a thread in a separate address space. This involves four (context switch + domain transitions)
to and from the kernel as well as two schedulings (client to server thread and server thread to client thread) --
in addition to the null procedure itself.
Estimated cost: 4(20 + 40) + 2*50 + 1 = 341 ?s
In SPIN, the call involve two (context switch + domain transitions) and no thread scheduling.
Estimated cost: 2(20 + 40) + 1 = 121 ?s.