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4. RR Scheduling
Operating Systems Principles (CSCI 340)
Queens College CUNY
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(RR) Round Robin ~ Preemptive ~ CPU Scheduling Algorithm
Let
P
i
be = Process i where ( i = 0, 1, 2, 3, ...)
q be = Quantum Time for each
P
i
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
with q = 3 milliseconds, ( ms ).
Calculate CT , TAT , WT.
P AT BT CT TAT WT
P
0
0 7???
P
1
2 4???
P
2
4 1???
Drawing Gantt Chart:
Step 0: Start with an empty Gantt Chart. Let RT be =
P
i
’s Remaining Burst Time
and a data structure Queue called PQueue with type
P
i
with q = 3 ms.
Note: The process is enqueue to
PQueue in orderly :
(1) the new process appears at or after its arrival time
(2) the process has remaining burst time (incomplete execution).
The process is dequeue from PQueue to be selectively picked for runtime.
t = 0123456789101112
RT
P
0
7
P
1
4
P
2
1
PQueue →enqueue ⋯ dequeue→
Step 1: When t = 0 ,
P
0
arrives and is the only process available. Following RR algorithm,
the CPU will check if there is a new process and any other available process. The
new process is
P
0
and there are no other available processes.
P
0
is enqueued.
PQueue P
0
Then the CPU will selectively pick the process to be executed from the Queue. By
dequeuing,
P
0
will be the next process to be executed. And because this algorithm
is preemptive and is RR, this process can only run partial burst time for 3
ms ,
q = 3 , before being paused and resume to complete the remaining duration at
another time.
t = 0123456789101112
RT
P
0
0 7
→
4
P
1
4
P
2
1
PQueue P
0
Step 2: When
t = 3 ,
P
1
has already arrived and one process (
P
0
) has remaining service time.
Following RR algorithm, the CPU will check if there is a new process and any
other available process. The new process is
P
1
and the other available process is
P
0
.
P
1
and
P
0
are enqueued in order.
PQueue P
0
P
1
P
0
Then the CPU will selectively pick the process to be executed from the Queue. By
dequeuing,
P
1
will be the next process to be executed. And because this algorithm
is preemptive and is RR, it can only run partial burst time for 3
ms ,
q = 3 , before
being paused and resume to complete the remaining duration at another time.
t = 0123456789101112
RT
P
0
0 4
P
1
1 4 → 1
P
2
1
PQueue P
0
P
1
P
0
Step 3: When
t = 6 ,
P
2
has already arrived and one process (
P
1
) has remaining service time.
Following RR algorithm, the CPU will check if there is a new process and any
other available process. The new process is
P
2
and the other available process is
P
1
.
P
2
and
P
1
are enqueued in order.
Note:
P
0
is not available since it is in the
PQueue unless it is dequeued.
PQueue P
1
P
2
P
0
P
1
P
0
t = 0123456789101112
RT
P
0
0 0 1
P
1
1 1 1 → 0
P
2
2 0
PQueue P
0
P
1
P
2
P
0
P
1
P
0
Step 6: When t = 11 ,
P
1
completes and there is no other process available. By dequeuing,
P
0
will be the next process to be executed. And because this algorithm is RR, it can
run the whole duration since its’ burst time is less than or equal to 3
ms (
≤ 3
).
By then,
P
0
completes its service time.
t = 0123456789101112
RT
P
0
0 0 0 1
→
0
P
1
1 1 0
P
2
2 0
PQueue P
0
P
1
P
2
P
0
P
1
P
0
The Complete Gantt Chart
t = 0123456789101112
RT
P
0
0 0 0 0
P
1
1 1 0
P
2
2 0
PQueue P
0
P
1
P
2
P
0
P
1
P
0
Calculating
CT
,
TAT
,
WT
:
Let
Completion Time (
CT
) be
=
Turnaround Time (
TAT
) be =
Waiting Time ( WT ) be =
for each ms = Finishing Time – Starting Time
for each
ms = Completion Time – Arrival Time
for each ms = Turnaround Time – Burst Time
For
P
0
: Burst Time (
BT
) is 7 milliseconds and Arrival Time (
AT
) is at
t = 0 .
For
P
1
: Burst Time (
BT
) is 4 milliseconds and Arrival Time (
AT
) is at
t = 2 .
For
P
2
: Burst Time ( BT ) is 1 milliseconds and Arrival Time ( AT ) is at t = 4.
t = 0123456789101112
P
0
0 0 0
CT = 12
TAT = 12
WT = 5
P
1
1 1
CT = 11
TAT = 9
WT = 5
P
2
2
CT = 10
TAT = 6
4. RR Scheduling
Course: Operating Systems Principles (CSCI 340)
University: Queens College CUNY
