syscall tracing overheads: utrace vs. kprobes

[Roland McGrath]

> Certainly, in general.  But in this specific test, only the under-test
> system calls occurred in essnetially the whole system, so the overhead
> measurements were in a way the bare minimums imposed by the kprobes
> vs. utrace callback infrastructure itself.

Yes.  That's why I meant to explain how these numbers are true but not
necessarily the numbers that matter.

> > In real situations [...]  the distribution of overheads will be
> > quite different.
> 
> Or rather, the basic overhead quanta measured above may be multiplied
> along several different axes.

Yes.