- Information
- AI Chat
1. FCFS Scheduling
Operating Systems Principles (CSCI 340)
Queens College CUNY
Recommended for you
Preview text
(FCFS) First Come First Serve ~ Non-Preemptive ~ CPU Scheduling Algorithm
Let Pi be = Process i where ( i = 0, 1, 2, 3, ...) AT be = Arrival Time BT be = Burst Time CT be = Completion Time TAT be = Turnaround Time WT be = Waiting Time Starting Time is always zero to find CT.
Draw Gantt Chart. Calculate CT , TAT , WT.
P AT BT CT TAT WT
P 0 0 5???
P 1 1 3???
P 2 2 8???
P 3 3 6???
Drawing Gantt Chart:
Step 0: Start with an empty Gantt Chart.
t = 012345678910111213141516171819202122
P 0
P 1
P 2
P 3
Step 1: When t = 0 , P 0 arrives and is the only process available. Since this algorithm is non-preemptive, this process will not be paused and will entirely execute.
t = 012345678910111213141516171819202122
P 0 0
P 1
P 2
P 3
Step 2: When t = 5 , P 0 completes and three processes ( P 1 , P 2 , P 3 ) already arrived and are available. Since this algorithm is FCFS, P 1 will execute first by following the order of the process. Also, since this is non-preemptive algorithm, this process will not be paused and will entirely execute.
t = 012345678910111213141516171819202122
P 0 0
P 1 1
P 2
P 3
Step 3: When t = 8 , P 1 completes and two processes ( P 2 , P 3 ) are available. Between P 2 and P 3 , P 2 will go first under FCFS algorithm. P 2 will execute entirely without being paused.
t = 012345678910111213141516171819202122
P 0 0
P 1 1
P 2 2
P 3
Step 4: When t = 16 , P 2 completes and only one process ( P 3 ) is available. P 3 will execute entirely without being paused.
t = 012345678910111213141516171819202122
P 0 0
P 1 1
P 2 2
P 3 3
Calculating CT , TAT , WT :
Let Completion Time ( CT ) be =
for each millisecond
For P 3 : Burst Time ( BT ) is 6 milliseconds ( ms ) and Arrival Time ( AT ) is at t = 3.
t = 012345678910111213141516171819202122
P 3 3
CT 22
TAT 19
WT 13
Completion Time ( CT ) Turnaround Time ( TAT ) Waiting Time ( WT )
=
=
=
Finishing Time – Starting Time Completion Time – Arrival Time Turnaround Time – Burst Time
⇒
⇒
⇒
CT = 22 − 0 = 22
TAT = 22 − 3 = 19
WT = 19 − 6 = 13
P AT BT CT TAT WT
P 0 0 5 5 5 0
P 1 1 3 8 7 4
P 2 2 8 16 14 6
P 3 3 6 22 19 13
Made By: asifroni673@gmail asif@qmail.cuny
1. FCFS Scheduling
Course: Operating Systems Principles (CSCI 340)
University: Queens College CUNY
