Usually, it is not a good idea to "request penultimate priority for" whatever-it-is that you are doing.
The scheduler's job is "to be as fair as possible," and "to keep all of the computer's hardware as busy as possible," but without "starving" any of the units-of-work that are contending for it. (In other words, "if there is something for (say ...) the CPU to do right now, the CPU will be doing it.")
Priority-numbers, therefore, are merely a hint to the scheduler about "what to do, 'all other things being equal' ... which, usually, they are not, in which case the priority has very little effect at all. Nonetheless, reckless assignment of priorities can result in starvation. Therefore, Don't.