Defined as time of completion - time of arrival
T = TCompletion - TArrival
Time of response = Time first run minus time of arrival.
T = TFirstRun - TArrival
First in first out: The first process to arrive gets executed first.
Problems:
Given 3 processes: A, B and C (which arrives at that order). A runs for 100 seconds and B and C for 10 seconds each.
Time of completion for: A = 100 secs, B = 110 and C = 120. Average 110 secs.
FIFO behaves poorly on processes of different lengths
The shorts job runs first, then the next one and so on.
Average turn around time for process A, B and C decreases from 110 secs to 50 secs.
Problems
If A arrives first and 10 seconds later B and C arrive, we will get similar turnaround time than in FIFO.
The shortest job to completion runs.
If process A which takes 100 seconds it's running and a process B which takes 5 seconds arrives 10 seconds later A starts running, we switch to run process B because it will end before.
Bad response time.
Runs a jobs for a time slice
The time slice must be a multiple of the timer-interrupt period
Better response time
Shorter time slice makes RR perform better on the response time metric
Too short makes context switching slow down the systems
Fair scheduling policies perform poorly on metrics like turn around time but good on metrics like response time. (SJF, STCF)
Unfair scheduling policies perform poorly on response time and better on turn around time. (RR)
If process A has I/O operations: |AAAAA|I/O|AAAAA|
It spits the process in two, the I/O it's the delimiter
When the I/O is in progress another process can run (i.e B)