chapter04 cpu scheduling
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Chng 4 Lp lch CPU
Th.s: H L Hoi Trung
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2Khoa KTMT
n tp chng 3
Process
Trng thi process
To process
Hy Process
C ch giao tip process
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3Khoa KTMT
Mc tiu
Ti sao phi nh thi.
Gii thiu cc b lp lch trong h thng a
chng
M t cc gii thut ca b lp lch
Cc tiu chun nh gi b lp lch
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4Khoa KTMT
Ni Dung
Khai niem c ban
Cac bo nh thi
long-term, mid-term, short-term
Cac tieu chuan nh thi CPU
Cac giai thuat nh thi
First-Come, First-Served (FCFS) Shortest Job First (SJF) va Shortest Remaining
Time First (SRTF)
Priority Scheduling Round-Robin (RR) Highest Response Ratio Next (HRRN) Multilevel Queue Multilevel Feedback Queue
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5Khoa KTMT
Khai niem c ban
Trong cac he thong multitasking
Thc thi nhieu chng trnh ong thi lam tang hieu suat he
thong.
Tai moi thi iem, ch co mot process c thc thi. Do o,
can phai giai quyet van e phan chia, la chon process thc
thi sao cho c hieu qua nhat chien lc nh thi CPU.
nh thi CPU
Chon mot process (t ready queue) thc thi.
Vi mot multithreaded kernel, viec nh thi CPU la do OS
chon kernel thread c chiem CPU.
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6Khoa KTMT
Cac bo nh thi
ready
running
suspended
ready
suspended
blocked
new
terminatedblocked
Long-term
scheduling
Long-term
scheduling
Medium-term
scheduling
Medium-term
scheduling
Short-term
scheduling
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7Khoa KTMT
Cac bo nh thi
Long-term scheduling
Xac nh chng trnh nao c chap nhan nap vao he thong e thc thi
ieu khien mc o multiprogramming cua he thong
Long term scheduler thng co gang duy tr xen lan CPU-bound va I/O-bound process
Medium-term scheduling
Process nao c a vao (swap in), a ra khoi (swap out) bo nh chnh
c thc hien bi phan quan ly bo nh va c thao luan phan quan ly bo nh.
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8Khoa KTMT
Cac bo nh thi (tt)
Short term scheduling
Xac nh process nao trong ready queue se c chiem CPU
e thc thi ke tiep (con c goi la nh thi CPU, CPU
scheduling)
Short term scheduler con c goi vi ten khac la dispatcher
Bo nh thi short-term c goi moi khi co mot trong cac s
kien/interrupt sau xay ra:
Ngt thi gian (clock interrupt)
Ngat ngoai vi (I/O interrupt)
Li goi he thong (operating system call)
Signal
Chng nay se tap trung vao nh thi ngan han
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9Khoa KTMT
Dispatcher
Dispatcher se chuyen quyen ieu khien CPU ve cho
process c chon bi bo nh thi ngan han
Bao gom:
Chuyen ng canh (s dung thong tin ng canh trong PCB)
Chuyen che o ngi dung (user mode)
Nhay en v tr thch hp trong chng trnh ng dung e khi ong lai chng trnh (chnh la program counter trong PCB)
Cong viec nay gay ra ph ton
Dispatch latency: thi gian ma dispatcher dng mot process va khi ong mot process khac
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10Khoa KTMT
Cac tieu chuan nh thi CPU
User-oriented
Thi gian ap ng (Response time): khoang thi gian process
nhan yeu cau en khi yeu cau au tien c ap ng (time-
sharing, interactive system) cc tieu
Thi gian quay vong (hoan thanh) (Turnaround time): khoang thi
gian t luc mot process c nap vao he thong en khi process
o ket thuc cc tieu
Thi gian ch (Waiting time): tong thi gian mot process i trong
ready queue cc tieu
System-oriented
S dung CPU (processor utilization): nh thi sao cho CPU cang
ban cang tot cc ai
Cong bang (fairness): tat ca process phai c oi x nh nhau
Thong lng (throughput): so process hoan tat cong viec trong
mot n v thi gian cc ai.
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11Khoa KTMT
Hai yeu to cua giai thuat nh thi
Ham chon la (selection function): dung e chon
process nao trong ready queue c thc thi (thng
da tren o u tien, yeu cau ve tai nguyen, ac iem
thc thi cua process,), v duw = tong thi gian i trong he thong
e = thi gian a c phuc vu
s = tong thi gian thc thi cua process (bao gom ca e)
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12Khoa KTMT
Hai yeu to cua giai thuat nh thi (tt)
Che o quyet nh (decision mode): chon thi iem thc
hien ham chon la e nh thi. Co hai che o
Khong trng dung (Non-preemptive)
Khi trang thai running, process se thc thi cho en khi ket thuc hoac b blocked do yeu cau I/O
Trng dung (Preemptive)
Process ang thc thi (trang thai running) co the b ngat na chng va chuyen ve trang thai ready bi
he ieu hanh
Chi ph cao hn non-preemptive nhng anh oi lai bang thi gian ap ng tot hn v khong co trng
hp mot process oc chiem CPU qua lau.
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13Khoa KTMT
Preemptive va Non-preemptive
Hm nh thi c thc hin khi
(1) process Chuyn t trng thi running sang waiting
(2) process Chuyn t trng thi running sang ready
(3) process Chuyn t trng thi waiting, new sang ready
(4) process Kt thuc thc thi
1 v 4 khng cn la chn loi nh thi biu, 2 v 3 cn
Trng hp 1, 4 c gi l nh thi nonpreemptive
Trng hp 2, 3 c gi l nh thi preemptive
Thc hin c ch nao kho hn? Tai sao?
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14Khoa KTMT
Khao sat giai thuat nh thi
Service time = thi gian process can CPU trong mot chu ky
CPU-I/O
Process co service time ln la cac CPU-bound process
ProcessArrival
Time
Service
Time
1 0 3
2 2 6
3 4 4
4 6 5
5 8 2
load store
add store
read from file
wait for I/O
inc store
write to file
load store
add store
read from file
wait for I/O
wait for I/O
I/O burst
CPU burst
CPU burst
CPU burst
I/O burst
I/O burst
mot chu ky
CPU-I/O
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15Khoa KTMT
1. First-Come First-Served (FCFS)
Ham la chon: Tien trnh nao yeu cau CPU trc se c cap phat
CPU trc; Process se thc thi en khi ket thuc hoac b blocked do I/O
Che o quyet nh: non-preemptive algorithm
Hien thc : s dung hang i FIFO (FIFO queues)
Tien trnh i vao c them vao cuoi hang i
Tien trnh c la chon e x ly c lay t au cua queues
0 5 10 15 20
P1
P2
P3P4P5
addrun
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16Khoa KTMT
FCFS Scheduling
V du :
Process Burst Time
P1 24
P2 3
P3 3
Gia s th t vao cua
cac tien trnh la
P1, P2, P3
Thi gian ch
P1 = 0;
P2 = 24;
P3 = 27;
Thi gian ch trung
bnh
(0+24+27)/3 = 17
0 24 27 30
P1 P2 P3
Gantt Chart for Schedule
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17Khoa KTMT
FCFS Scheduling
V du:
Process Burst Time
P1 24
P2 3
P3 3
Gia s thi gian vao cua
cac tien trnh la
P2, P3, P1
Thi gian ch :
P1 = 6; P2 = 0; P3 = 3;
Thi gian ch trung bnh
(6+0+3)/3 = 3 , tot hn..
0 3 6 30
P1P2 P3
Gantt Chart for Schedule
Liu c xy ra trng hp tr hon v hn nh (starvation hay indefinte blocking) vi gii thut FCFS?
Nhn xt
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18Khoa KTMT
2. Shortest-Job-First(SJF) Scheduling
nh thi bieu cong viec ngan nhat trc
Khi CPU c t do, no se cap phat cho tien trnh yeu cau t
thi gian nhat e ket thuc ( tien trnh ngan nhat)
Lien quan en chieu dai thi gian s dung CPU cho lan tiep
theo cua moi tien trnh. S dung nhng chieu dai nay e lap
lch cho tien trnh vi thi gian ngan nhat.
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19Khoa KTMT
2. Shortest-Job-First(SJF) Scheduling
Hai hnh thc (Schemes):
Scheme 1: Non-preemptive( tien trnh oc quyen CPU)
Khi CPU c trao cho qua trnh no khong nhng cho en khi no ket thuc chu ky x ly cua no
Scheme 2: Preemptive( tien trnh khong oc quyen)
Neu mot tien trnh CPU mi c a vao danh sach vi chieu dai s dung CPU cho lan tiep theo nho hn thi gian
con lai cua tien trnh ang x ly no se dng hoat ong tien
trnh hien hanh (hnh thc nay con goi la Shortest-
Remaining-Time-First (SRTF).)
SJF la toi u cho thi gian ch i trung bnh toi thieu vi mot tap tien trnh cho trc
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20Khoa KTMT
Non-Preemptive SJF Scheduling
V du :
Process Arrival TimeBurst Time
P1 0 7
P2 2 4
P3 4 1
P4 5 4
0 8 16
P1 P2P3
Gantt Chart for Schedule
P4
127
Average waiting time = (0+6+3+7)/4 = 4
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21Khoa KTMT
Preemptive SJF Scheduling
(hay Sortest Remaining Time First - SRTF)
V du 1:Process Arrival TimeBurst Time
P1 0 7
P2 2 4
P3 4 1
P4 5 4
0 7 16
P1 P2P3
Gantt Chart for Schedule
P4
115
Average waiting time =
(9+1+0+2)/4 = 3
P2 P1
2 4
Process Arrival TimeBurst Time
P1 0 8
P2 1 4
P3 2 9
P4 3 5
VD2:
Thc hin ch no?
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22Khoa KTMT
Nhan xet ve giai thuat SJF
Co the xay ra tnh trang oi (starvation) oi vi cac process co CPU-burst ln khi co nhieu process vi CPU-
burst nho en he thong.
C che non-preemptive khong phu hp cho he thong
time sharing (interactive)
Giai thuat SJF ngam nh ra o u tien theo burst time
Cac CPU-bound process co o u tien thap hn so vi
I/O-bound process, nhng khi mot process khong thc
hien I/O c thc thi th no oc chiem CPU cho en khi
ket thuc
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23Khoa KTMT
Nhan xet ve giai thuat SJF
Tng ng vi mi process cn c di ca CPU burst tip theo
Hm la chn: chn process c di CPU burst nh nht
Chng minh c: SJF ti u trong vic gim thi gian i trung
bnh
Nhc im: Cn phi c lng thi gian cn CPU tip theo ca
process
Gii php cho vn ny?
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24Khoa KTMT
Nhan xet ve giai thuat SJF
(Thi gian s dng CPU chnh l di ca CPU burst)
Trung bnh tt c cc CPU burst o c trong qu kh
Nhng thng thng nhng CPU burst cng mi cng phn
nh ung hnh vi ca process trong tng lai
Mt k thut thng dng l s dng trung bnh ham m
(exponential averaging)
n+1 = a tn + (1 - a) n , 0 a 1
n+1 = a tn + (1- a) a tn-1 ++ (1- a)jan-j ++ (1- a)
n+1a0
Nu chn a = th c ngha l tr o c tn v tr d on nc xem quan trng nh nhau.
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25Khoa KTMT
D on thi gian s dng CPU
di CPU bursto c
di CPU burst d on,
vi a = v 0 = 10
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26Khoa KTMT
3. Priority Scheduling
Moi process se c gan mot o u tien
CPU se c cap cho process co o u tien cao nhat
nh thi s dung o u tien co the:
Preemptive hoac
Nonpreemptive
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27Khoa KTMT
Gan o u tien
SJF la mot giai thuat nh thi s dung o
u tien vi o u tien la thi-gian-s-dung-
CPU-d-oan.
Gan o u tien con da vao:
Yeu cau ve bo nh
So lng file c m
T le thi gian dung cho I/O tren thi gian s dung CPU
Cac yeu cau ben ngoai v du nh: so tien ngi dung tra khi thc thi cong viec
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28Khoa KTMT
3. Priority Scheduling
Van e: tr hoan vo han nh process co o u tien thap co the khong bao gi c thc thi
Giai phap: lam mi (aging) o u tien cua process se tang theo thi gian
Vd:
IBM, MIT 1973 process 1967.
0 127. Sau 15 tng u tin 1 ln khong bao lu th process c thc thi.
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29Khoa KTMT
V d
Thc hin cho cc gii thut FCFS, SJF, priority.
Non preemptive
Preemptive
Process Arrival time Burst time Priority
P1 0 10 3
P2 4 6 2
P3 8 3 1
P4 12 5 4
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30Khoa KTMT
4. nh thi luan phien (Round Robin -RR)
Moi process nhan c mot n v nho thi gian CPU
(time slice, quantum time), thong thng t 10-100 msec
e thc thi. Sau khoang thi gian o, process b oat
quyen va tr ve cuoi hang i ready.
Neu co n process trong hang i ready va quantum time
= q th khong co process nao phai ch i qua (n 1)q n v thi gian.
Hieu suat
Neu q ln: RR FCFS
Neu q nho (q khong c qua nho bi v phai ton chi ph chuyen ng canh)
Thi gian ch i trung bnh cua giai thuat RR thng
kha ln nhng thi gian ap ng nho
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31Khoa KTMT
Example of RR with Time Quantum = 20 Example: Process Burst Time Remaining Time
P1 53 53
P2 8 8
P3 68 68
P4 24 24
The Gantt chart is:
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32Khoa KTMT
Example of RR with Time Quantum = 20 Example: Process Burst Time Remaining Time
P1 53 33
P2 8 8
P3 68 68
P4 24 24
The Gantt chart is:
P1
0 20
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33Khoa KTMT
Example of RR with Time Quantum = 20 Example: Process Burst Time Remaining Time
P1 53 33
P2 8 0
P3 68 68
P4 24 24
The Gantt chart is:
P1
0 20
P2
28
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34Khoa KTMT
Example of RR with Time Quantum = 20 Example: Process Burst Time Remaining Time
P1 53 33
P2 8 0
P3 68 48
P4 24 24
The Gantt chart is:
P1
0 20
P2
28
P3
48
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35Khoa KTMT
Example of RR with Time Quantum = 20 Example: Process Burst Time Remaining Time
P1 53 33
P2 8 0
P3 68 48
P4 24 4
The Gantt chart is:
P1
0 20
P2
28
P3
48
P4
68
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36Khoa KTMT
Example of RR with Time Quantum = 20 Example: Process Burst Time Remaining Time
P1 53 13
P2 8 0
P3 68 48
P4 24 4
The Gantt chart is:
P1
0 20
P2
28
P3
48
P4
68
P1
88
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37Khoa KTMT
Example of RR with Time Quantum = 20 Example: Process Burst Time Remaining Time
P1 53 13
P2 8 0
P3 68 28
P4 24 4
The Gantt chart is:
P1
0 20
P2
28
P3
48
P4
68
P1
88
P3
108
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38Khoa KTMT
Example of RR with Time Quantum = 20 Example: Process Burst Time Remaining Time
P1 53 0
P2 8 0
P3 68 0
P4 24 0
The Gantt chart is:
P1
0 20
P2
28
P3
48
P4
68
P1
88
P3
108
P4
112
P1
125
P3
145
P3
153
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39Khoa KTMT
V du Round Robin
Time Quantum = 20
Process Burst Time
P1 53
P2 17
P3 68
P4 24
0
P1 P4P3
Gantt Chart for Schedule
P1P2
20
P3 P3 P3P4 P1
37 57 77 97 117 121 134 154 162
turnaround time trung bnh ln hn SJF, nhng ap ng tot hn
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40Khoa KTMT
Vi du
SJF - Preemptive
Round Robin - q = 2
Process Arrival
Time
Service
Time
1 0 3
2 2 6
3 4 4
4 6 5
5 8 2
Quantumn time = 3
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41Khoa KTMT
RR vi time quantum = 1
Thi gian turn-around trung bnh cao hn so vi SJF nhng
co thi gian ap ng trung bnh tot hn.
u tien CPU-bound process
I/O-bound process thng s dung rat t thi gian cua
CPU, sau o phai blocked i I/O
CPU-bound process tan dung het quantum time, sau o
quay ve ready queue c xep trc cac process b
blocked
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42Khoa KTMT
Time quantum va context switch
Process time = 10quantum
context
switch
0 1 2 3 4 5 6 7 8 9 10
0 106
0 10
12
6
1
0
1
9
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43Khoa KTMT
Thi gian hon thnh v quantum time
Thi gian hon thnh trung bnh (average turnaround time) khng
chc s c ci thin khi quantum ln
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44Khoa KTMT
Quantum va response time
Quantum time phai ln
hn thi gian dung e
x ly clock interrupt
(timer) va thi gian
dispatching
Nen ln hn thi gian
tng tac trung bnh
(typical interaction)
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45Khoa KTMT
Quantum time cho Round Robin*
Khi thc hin process switch th OS s s dng CPU ch khng phi process
ca ngi dng (OS overhead)
Dng thc thi, lu tt c thng tin, np thng tin ca process sp thc thi
Performance ty thuc vo kch thc ca quantum time (cn gi l time slice),
v hm ph thuc ny khng n gin
Time slice ngn th p ng nhanh
Vn : c nhiu chuyn ng cnh. Ph tn s cao.
Time slice di hn th throughput tt hn (do gim ph tn OS overhead) nhng
thi gian p ng ln
Nu time slice qu ln, RR tr thnh FCFS.
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46Khoa KTMT
Quantum time cho Round Robin
Quantum time v thi gian cho process switch:
Nu quantum time = 20 ms v thi gian cho process switch = 5 ms, nh vy
ph tn OS overhead chim 5/25 = 20%
Nu quantum = 500 ms, th ph tn ch cn 1%
Nhng nu c nhiu ngi s dng trn h thng v thuc loi interactive th s thy p ng rt chm
Ty thuc vo tp cng vic m la chn quantum time
Quantum time nn ln trong tng quan so snh vi thi gian cho process
switch
V d vi 4.3 BSD UNIX, quantum time l 1 giy
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47Khoa KTMT
Round Robin
Nu c n process trong hng i ready, v quantum time l q, nh
vy mi process s ly 1/n thi gian CPU theo tng khi c kch
thc ln nht l q
S khng c process no ch lu hn (n - 1)q n v thi gian
RR s dng mt gi thit ngm l tt c cc process u c tm
quan trng ngang nhau
Khng th s dng RR nu mun cc process khc nhau c u tin khc
nhau
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48Khoa KTMT
Round Robin: nhc im
Cc process dng CPU-bound vn cn c u tin
V d:
Mt I/O-bound process s dng CPU trong thi gian ngn hn quantum time v b blocked i
I/O. V
Mt CPU-bound process chy ht time slice v li quay tr v hng i ready queue ( pha trc cc
process b blocked)
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49Khoa KTMT
5. Highest Response Ratio Next
Chon process ke tiep co gia tr RR (Response ratio) ln
nhat.
Cac process ngan c u tien hn (v service time nho)
timeservice expected
timeservice expected ingspent wait time RR
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50Khoa KTMT
Highest Response Ratio Next
Process Arrival
Time
Service
Time
1 0 3
2 2 6
3 4 4
4 6 5
5 8 2
Quantumn time = 3
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51Khoa KTMT
6. Multilevel Queue Scheduling
Hang i ready c chia thanh nhieu hang i rieng
biet theo mot so tieu chuan nh
ac iem va yeu cau nh thi cua process
Foreground (interactive) va background process,
Process c gan co nh vao mot hang i, moi hang
i s dung giai thuat nh thi rieng
He ieu hanh can phai nh thi cho cac hang i.
Fixed priority scheduling: phuc vu t hang i co o u tien cao en thap. Van e: co the co starvation.
Time slice: moi hang i c nhan mot khoang thi gian chiem CPU va phan phoi cho cac process trong hang i khoang thi
gian o. V du:
80% cho hang i foreground nh thi bang RR.
20% cho hang i background nh thi bang giai thuat FCFS.
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52Khoa KTMT
Multilevel Queue Scheduling*
V d phn nhm cc qu trnh
System Processes
Interactive Processes
Batch Processes
Student Processes
u tin thp nht
u tin cao nht
-
53Khoa KTMT
7. Hang i phan hoi a cap
Multilevel Feedback Queue
Van e cua multilevel queue
process khong the chuyen t hang i nay sang hang i khac khac phuc bang c che feedback: cho phep process di chuyen mot cach thch hp gia cac hang i
khac nhau.
Multilevel Feedback Queue
Phan loai processes da tren cac ac tnh ve CPU-burst
S dung decision mode preemptive
Sau mot khoang thi gian nao o, cac I/O-bound process va interactive process se cac hang i co o u tien cao
hn con CPU-bound process se cac queue co o u tien
thap hn.
Mot process a ch qua lau mot hang i co o u tien thap co the c chuyen en hang i co o u tien cao
hn (c che nien han, aging).
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54Khoa KTMT
7. Multilevel Feedback Queue
V du: Co 3 hang i
Q0 , dung RR vi quantum 8 ms
Q1 , dung RR vi quantum 16 ms
Q2 , dung FCFS
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55Khoa KTMT
V d
iu phi u tin nhiu cp xoay vng, s dng 2 cp: Cp 1 s
dng gii thut robin round vi quantumn = 3ms. Cp 2 s dng
gii thut SRTF. Mt process nu cp I 5ms s c chuyn
xung cp II nu ang trng thi waiting cn nu ang trng
thi running th sau khi ra khi s chuyn. Ngc li mt process
ang cp II sau khong thi gian 10ms s c chuyn ln I.
Khi cc process vo b nh chnh th iu vo hng i cp I
-
56Khoa KTMT
7. Multilevel Feedback Queue (tt)
nh thi dung multilevel feedback queue oi hoi phai
giai quyet cac van e sau
So lng hang i bao nhieu la thch hp?
Dung giai thuat nh thi nao moi hang i?
Lam sao e xac nh thi iem can chuyen mot
process en hang i cao hn hoac thap hn?
Khi process yeu cau c x ly th a vao hang i
nao la hp ly nhat?
-
57Khoa KTMT
So sanh cac giai thuat
Giai thuat nh thi nao la tot nhat?
Cau tra li phu thuoc cac yeu to sau:
Tan xuat tai viec (System workload)
S ho tr cua phan cng oi vi dispatcher
S tng quan ve trong so cua cac tieu chuan nh thi nh response time, hieu suat CPU, throughput,
Phng phap nh lng so sanh
-
58Khoa KTMT
c thm
Policy v Mechanism
nh thi trn h thng multiprocessor
nh gi gii thut nh thi CPU
nh thi trong mt s h iu hnh thng dng
Ngun:
Operating System Concepts. Sixth Edition. John Wiley & Sons, Inc.
2002. Silberschatz, Galvin, Gagne
-
59Khoa KTMT
Bi tp
S dng cc gii thut FCFS, SJF, SRTF, Priority tnh cc gi tr thi gian i, thi gian p ng v thi gian hon thnh trung bnh
-
60Khoa KTMT
Bi tp (tt)
S dng cc gii thut FCFS, SJF, SRTF, Priority tnh cc gi tr thi gian i, thi gian p ng v thi gian hon thnh trung bnh
-
61Khoa KTMT
Bi tp
Xt 1 tp cc process sau c thi gian thc thi CPU tnh bng mili
giy:
Gi s th t n thc thi ca cc process l P1, P2, P3, P4, P5.
(tt c process ny u n ti thi im bng 0).
1. V s Gautt thc thi ca cc process theo gii thut nh thi:
FCFS, SJF, RR (quantumn = 1), Priority (cng nh cng u tin,
cng u tin xt Pi v Pj, Pi u tin hn nu i < j).
2. Thi gian i ca cc process trong tng gii thut nh thi?
Process Burst - time Priority
P1 10 3
P2 1 1
P3 2 3
P4 1 4
p5 5 2
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62Khoa KTMT
Bi tp
3. Tnh thi gian hon thnh(turnaround time) ca tng process cho
tng gii thut?
Trong cc gii thut sau, gii thut no c th gy ra trng hp 1
process c th khng bao gi c thc thi:
1. First come, first serve
2. Shortest job first
3. Round robin
4. Priority.
Gii thch l do ti sao xy ra trng hp trn?
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63Khoa KTMT
Bi tp
1. RR vi q = 2
2. Preemptive Priority vi s cng ln cng u tin
3. iu phi u tin nhiu cp xoay vng, s dng 2 cp: Cp 1 s
dng gii thut robin round vi quantumn = 3ms. Cp 2 s dng
gii thut SRTF. Mt process nu cp I 5ms s c chuyn
xung cp II nu ang trng thi waiting cn nu ang trng
thi running th sau khi ra khi s chuyn. Ngc li mt process
ang cp II sau khong thi gian 10ms s c chuyn ln I.
Khi cc process vo b nh chnh th iu vo hng i cp I.
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64Khoa KTMT
Bi tp
Cho 4 tin trnh A, B, C, D vi thi gian vo ready list v thigian cn CPU cho cc ln th 1, th 2, th 3 v thi gian thc hinI/O tng ng nh bn sau:
V gin Gantt, Tnh thi gian i trung bnh, Thi gian p ngtrung bnh, Thi gian lu li trong h thng trung bnh cho ccgii thut (cu 1, 2 ch dng thi gian thc thi 1st execution time nh l burst time)
1. FCFS
2. RR vi q = 3
3. SRFT cho c 3 ln exec
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