lecture 6 cpu scheduling - nus computing
TRANSCRIPT
Lecture 6CPU Scheduling
16 September, 2011
1
new
ready running
blocked exit
2
batchvs.
interactivevs.
real-time3
CPU-boundvs.
I/O-bound
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pre-emptivevs.
non pre-emptive
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(process states)
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7
8
PA
PB
PC
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B B B BC C AC A A
(CPU states)
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CPU Utilization
B B B BC C AC A A
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Throughput
B B B BC C AC A A
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Response Time
B B B BC C AC A A
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Turnaround Time
B B B BC C AC A A
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Fairness
C C CB B B BA A A
B B B BC C AC A A
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Balance
B B B BC C AC A A
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Scheduling Algorithms
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FCFS(non-preemptive)
Ready Queue
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PA
PB
PC
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AA B C
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Round Robin(preemptive)
Ready Queue
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PA
PB
PC
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A B C A A B C A A
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Long Time Quantumvs.
Short Time Quantum
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Time Quantum20 - 50ms
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Shortest Job First(non-preemptive)
Priority Queue
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A B C D
AB C D
FCFS
SJF
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Priority Queue
Shortest Remaining Time First(preemptive)
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EstimatingRemaining Time
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Priority Scheduling(preemptive)
k
1
0
:
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Staticvs.
Dynamic Priority
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Guidelines for
Good Schedulers
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1. interactive jobs should
have higher priority than CPU bound jobs
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2. CPU intensive jobs should
be given larger time quantum
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3. round robin is good for
response time but bad for turnaround time
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4. shortest job first reduces
the turnaround time
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(how to know if a job is interactive? remaining
time on the job?)
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Multilevel Feedback Queue
k
1
0
:
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new
ready running
blocked exit
join highestlevel queue
stay at same/higher
level
join lower level but
higher quanta
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interactive jobs remains on top
CPU intensive jobs sink to bottom
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1. interactive jobs should
have higher priority than CPU bound jobs
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2. CPU intensive jobs should have be given larger time
quantum
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3. round robin is good for
response time but bad for turnaround time
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4. shortest job first reduces
the turnaround time
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Linux Scheduler
139
1
0
:
139
1
0
:
ExpiredActive
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new
ready running
blocked exit
join active list at default priority stay in active list
at same priority
move toexpired list and
recompute priority
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Linux Static Priority0
99
139
“real time” jobs
user jobs
default priority 120
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Linux Static Priority0
99
139
“real time” jobs
user jobs
nice
-20
+19
time quantum800ms
5ms48
Linux Dynamic Priority
slee
p tim
e -
run
time
-5
+549
Swap Active <-> Expired
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1
0
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139
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0
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ExpiredActive
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