Take a look at this question . "Real time" (for a process) refers to the scheduling or thinking algorithm that the kernel makes when it decides which process starts. In real time, preempt all other processes (with less scheduling weight) when an interrupt is received and must be started.
A program that simply accepts user input goes into sleep mode (block) while waiting for input, or even between keystrokes (depending). Such a program should not have such a high planning priority. RT processes must be run ahead of all other processes in the system. This may be due to the fact that the process is crucial for some important purpose or high-performance timers are needed (in this case, you need a real-time OS in which Linux is not standard).
, SCHED_FIFO SCHED_RR . , . , , , , RT, . - , , .
, , , , SCHED_FIFO RT. , , , Linux .
, nice .