Shortest-Job-First Scheduling, SJF

  
Shortest-Job-First (SJR) Scheduling

Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time.
Two schemes:
1.  non pre- emptive – once CPU given to the process it cannot be preempted until completes its CPU burst.
2. preemptive – if a new process arrives with CPU burst length less than remaining time of current executing
                            process, preempt. This scheme is know as the Shortest-Remaining-Time-First (SRTF).
SJF is optimal – gives minimum average waiting time for a given set of processes.
 
                Process          Arrival Time     Burst Time
                    P1                    0.0                         7
                    P2                    2.0                         4
                    P3                    4.0                         1
                    P4                    5.0                         4
SJF (non-preemptive)
 
P1
P3
P2
P4
0                                  7                               8                         12                                     16
 
Average waiting time = [0 +(8-2)+(7-4) +(12-5)] /4 =4
 
Example of Preemptive SJF
 
Proces                        Arrival  Time               Burst Time
P1                                    0.0                               7
P2                                    2.0                               4
P3                                    4.0                               1
P4                                     5.0                              4
 SJF (preemptive)
 
P1
P2
P3
P2
P4
P1
0                   2                       4                     5                         7                      11               16
 
         Average waiting time = (9 + 1 + 0 +2)/4 =3
       Determining Length of Next CPU Burst
 
Can only estimate the length.
Can be done by using the length of previous CPU bursts, using exponential averaging.
 
Prediction of the Length of the Next CPU Burst
             Pn+1 = a tn +(1-a)Pn 
             This formula defines an exponential average
              Pn stores the past history
              tn contents are most recent information
              the parameter “a “controls the relative weight of recent  and past history of  in our prediction
              If a =0 then Pn +1 =Pn
             That is prediction is constant
              If a = 1 then Pn +1 = tn

              Prediction is last cpu burst

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