What is Scheduling?

Pre-emptive scheduling refers to schemes such as SRTF (Shortest remaining time first), In which a process can be "pre-empted" (having the CPU time taken off it) in the middle of a burst. For example under SRTF:

Process | Arrival Time(nanoseconds) | Burst Time(nanoseconds)

P1 0 30

P2 10 5

For the first 10ns P1 will be running, but at 10s, P2 arrives, because it only has 5 nanoseconds of CPU time remaining till the completion of the burst as opposed to 20 for P1, P1 will be pre-empted, and P2 will run instead. P1 will then start running again at 15ns.

|< (P1 is pre-empted)

| P1 | P2 | P1 |

0ns 10ns 15ns