Question

Processing times on a single machine for 3 jobs are given below. All the jobs are available at time t = 0.

Job	1	2	3
Processing time (minute)	15	3	6

The mean flow time (in minute) as per the shortest processing time (SPT) sequence is _______.

Answer 1

Concept: 

Shortest processing time: In the SPT rule, jobs are sequenced in increasing order of their processing time i.e. minimum time first and maximum time in last. Note: If two jobs have the same process time then job having less due date will be processed first.

Calculation:

Given:

The jobs are to be scheduled using the shortest processing time (SPT) sequence.

Job	Processing times (minutes)	Flow time (minutes)
2	3	3
3	6	6 + 3 = 9
1	15	15 + 9 = 24

∴ The total flow time = 3 + 9 + 24 = 36 minutes

∴ The mean flow time = 36/3 = 12 minutes

∴ The mean flow time as per the shortest processing time (SPT) sequence is 12 minutes.