nh thi CPU 1

Mc tiu

n SV s gii thch c

l Ti sao cn phi nh thi

l Cc tiu ch nh thi

l Mt s gii thut nh thi

nh thi CPU 2

Vn cn gii quyt

n V d: ti a li CPU trong h thng multiprogramming / multitasking

l Trong b nh c nhiu process ng thi

l Khi mt process phi i (vd i hon tt yu cu I/O), h iu hnh cp pht CPU cho mt qu trnh khc thay v CPU chy khng (idle)

n Vn : la chon process thc thi sao cho c hiu qu nht --- ti u i vi mt tiu ch cho trc. Cn c chin lc nh thi CPU

nh thi CPU 3

Phn loi qu trnh

n Qu trnh thc thi qua nhiu chu k thc thi CPU v i I/O

l CPU burst

l I/O burst

n CPU-bound process c thi gian s dng CPU nhiu hn thi gian s dng I/O

n I/O-bound process dng phn ln thi gian i I/O

nh thi CPU 4

n CPU-bound process: CPU burst di

n I/O-bound process: CPU burst ngn

Nhn ma trn

emacs emacs

nh thi CPU 5

Cc trng thi ca qu trnh nhc li

n S chuyn trng thi ca qu trnh s 5 trng thi

ready running

dispatch

interrupt

I/O or event

completion

I/O or event

wait

new terminated

waiting

admit exit

nh thi CPU 6

M rng cc trng thi ca qu trnh

n Lm th no iu chnh s lng qu trnh ang trong b nh chnh?

l Swap out / swap in qu trnh (cn phn bit vi paging!) -- C ch

Swap out: em qu trnh t b nh chnh ra a

Swap in: em qu trnh t a vo b nh chnh

l Chon qu trnh no swap out / swap in -- Chnh sch

l Hai trng thi mi cho qu trnh

suspended ready

suspended blocked

nh thi CPU 7

M rng cc trng thi ca qu trnh

ready

running

suspended

ready

suspended

blocked

new

terminated blocked

nh thi CPU 8

Phn loi cc hot ng nh thi (1/2)

ready

running

suspended

ready

suspended

blocked

new

terminated blocked

Long-term

scheduling

Medium-term

scheduling

Medium-term

scheduling

Short-term

scheduling

nh thi CPU 9

Phn loi cc hot ng nh thi (2/2)

n nh thi di hn (long-term scheduling): xc nh cc process mi (new) no c a vo hng i sn sng

l Thng ch c trong batch system

n nh thi trung hn (medium-term scheduling): xc nh process no c a ra khi (swap out) hay c a vo (swap in) b nh chnh

l Swap out / in c th tn n vi giy thi gian chu k nh thi trung hn c th l vi pht

n nh thi ngn hn (short-term scheduling): xc nh process no c thc thi tip theo

nh thi CPU 10

nh thi di hn

n nh hng n -a-lp-trnh (degree of multiprogramming: s lng qu trnh ang trong b nh)

n Nu cng nhiu process ang trong b nh th kh nng moi process b block c xu hng gim

l S dng CPU hiu qu hn

l Nhng mi process c phn chia khong thi gian s dng CPU nh hn

n Thng c xu hng a vo mt tp ln ln cc CPU-bound process v I/O-bound process

nh thi CPU 11

nh thi trung hn

n i khi h iu hnh (nh time-sharing system) c thm medium-term scheduling

l Medium-term scheduler quyt nh vic a process (khng phi process trng thi new)

t b nh chnh ra a (swap out) hay

t a vo b nh chnh (swap in)

memory

memory

nh thi CPU 12

nh thi trung hn

n Ph thuc vo yu cu qun l vic a-lp-trnh (multiprogramming)

l Cho php b nh thi di hn chp nhn (admit) nhiu process hn s lng process m c tng kch thc c cha va trong b nh chnh ( k thut b nh o)

l Nhng nu c qu nhiu process th s lm tng vic truy xut a ( thrashing), do cn phi la chon -a-lp-trnh cho ph hp

n c thc hin bi phn mm qun l b nh

nh thi CPU 13

nh thi trung hn

n Ty theo chin lc s dng b nh, medium-term scheduler c th quyt nh

l swap out qu trnh chim khi lng ln b nh, hay c u tin thp

l swap in qu trnh khi c b nh

nh thi CPU 14

nh thi ngn hn

n Xc nh process no c thc thi tip theo, cn goi l nh thi CPU

n Ty h thng ( nh thi non-preemptive, preemptive) m c kch hot khi c mt s kin dn n kh nng chon mt process thc thi

l Ngt thi gian (clock interrupt)

l Ngt ngoi vi (I/O interrupt)

l Li goi h thng (operating system call)

l Signal

Chng ny s tp trung vo nh thi ngn hn

nh thi CPU 15

Ni dung cn quan tm

n nh thi trn h thng c mt processor (uniprocessor scheduling): quyt nh vic s dng (mt) CPU cho mt tp cc process trong h thng

n Tiu ch no?

nh thi CPU 16

Tiu ch nh thi (1/4)

n li CPU (CPU utilization)

l Khong thi gian CPU bn tnh ton cho ng dng, t 0% n 100%

n Thi gian ch (waiting time)

l Thi gian mt process trong hng i ready

nh thi CPU 17

Tiu ch nh thi (2/4)

n Thng nng (throughput)

l S lng process hon tt trong mt n v thi gian

n Thi gian p ng (response time)

l Thi gian t lc c yu cu ca ngi dng (user request) n khi c p ng u tin n ngi dng

l Thng l vn vi cc I/O-bound process

nh thi CPU 18

Tiu ch nh thi (3/4)

n Thi gian quay vng (turnaround time)

l Thi gian mt process hon tt, k t lc vo h thng (submission) n lc kt thc (termination)

l L mt tr c trng cn quan tm i vi cc process thuc dng CPU-bound

n Thi gian quay vng trung bnh (average turnaround time)

nh thi CPU 19

Tiu ch nh thi (4/4)

n li CPU

l Ti a ha gi CPU cng bn tnh ton cho ng dng cng tt

n Thng nng s lng process kt thc vic thc thi trong mt n v thi gian

l Ti a ha

n Turnaround time thi gian k t lc a vo (submission) n lc kt thc

l Ti thiu ha

n Thi gian ch thi gian mt process ch trong hng i ready

l Ti thiu ha

n Thi gian p ng thi gian t khi a yu cu n khi c p ng u tin

l Ti thiu ha

nh thi CPU 20

C th lm c?

n Tt c cc tiu ch khng th c ti u ng thi v c mt s tiu ch lin quan nhau

nh thi CPU 21

Tiu ch nh thi t cc gc nhn (1/2)

n Hng n ngi s dng (user-oriented)

l Thi gian quay vng

Thi gian t lc submission n lc process kt thc

Cn quan tm i vi cc h thng x l b (batch system)

l Thi gian p ng

Cn quan tm i vi cc h thng tng tc (interactive system)

nh thi CPU 22

Tiu ch nh thi t cc gc nhn (2/2)

n Hng n h thng (system-oriented)

l li CPU

l Cng bng (fairness)

l Thng nng: s process hon tt trong mt n v thi gian

nh thi CPU 23

Hai thnh phn ca chin lc nh thi (1/2)

n Hm la chn (selection function)

l Xc nh process no trong ready queue s c thc thi tip theo. Thng theo cc tiu ch nh

w = tng thi gian i trong h thng

e = thi gian c phc v

s = tng thi gian thc thi ca process (bao gm c tr e)

nh thi CPU 24

Hai thnh phn ca chin lc nh thi (2/2)

n Ch quyt nh (decision mode)

Xc nh thi im hm la chon c thc thi. Hai ch quyt

nh

l Non-preemptive

Mt process s trng thi running cho n khi n b block hoc n kt thc

l Preemptive

Process ang thc thi c th b ngt v chuyn v trng thi ready

Trnh trng hp mt process c chim CPU

nh thi CPU 25

Thi im thc thi hm la chon

ready running

(2)

(3) (1)

new terminated

waiting

(3) (4)

nh thi CPU 26

Non-preemption v preemption (1/2)

n Hm la chon c th c thc thi khi c qu trnh

(1) chuyn t trng thi running sang waiting

(2) chuyn t trng thi running sang ready

(3) chuyn t trng thi waiting, new sang ready

(4) kt thc thc thi

n Chin lc nh thi non-preemptive: ch thc thi hm la chon trong trng hp 1 v 4 (qu trnh running nu b ngt s tip tc running sau )

l MS Windows 3.x

n Chin lc nh thi preemptive: ngoi trng hp 1 v 4 cn thc thi thm hm la chon trong trng hp 2 hoc 3 (hoc c hai)

l Windows 95 (hay tr hn)

C qu trnh tr nn ready

nh thi CPU 27

Non-preemption v preemption (2/2)

n Hin thc ch quyt nh no kh hn? Ti sao?

n Preemptive scheduling: khng phi lc no preempt qu trnh cng c, cn ty thuc vo kernel

l Non-preemptive kernel

Li goi h thng ca qu trnh c thc thi n khi xong (tr v user space) hoc block

from Isaac POS(0230A)

nh thi CPU 28

Non-preemption v preemption (2/2)

n (tt)

l Preemptive kernel

C th preempt qu trnh khi kernel ang thc thi mt li goi h thng ca n

Vn : gi nht qun cc d liu trong kernel (v d cc hng i I/O)

l Linux

Non-preemptive kernel: Linux 2.4

Preemptive kernel: Linux 2.6

nh thi CPU 29

Dispatcher

n Chc nng: chuyn quyn iu khin CPU sang process c chon bi b nh thi ngn hn

n Cng vic bao gm:

l Chuyn ng cnh (s dng thng tin ng cnh trong PCB)

l Chuyn v user mode

l Nhy n a ch thch hp (chnh l program counter trong PCB) trong vng code ca qu trnh c tip tc thc thi

n Cng vic ny gy ra ph tn

l Dispatch latency: thi gian t lc dng mt process n lc mt process khc tip tc chy

nh thi CPU 30

First Come First Served (FCFS) (1/5)

n Hm la chon: chon process trong hng i ready lu nht

n Ch quyt nh: non-preemptive

l Mt process s c thc thi cho n khi n block hoc kt thc

n FCFS thng c hin thc bng mt FIFO queue

head end

Ready queue

nh thi CPU 31

First Come First Served (2/5)

Process Burst time (ms)

P1 24

P2 3

P3 3 n Gi s cc process n theo th t P1 , P2 , P3 n Gin Gantt cho vic nh thi l:

n Thi gian i cho P1 = 0, P2 = 24, P3 = 27

l Thi gian i trung bnh: (0 + 24 + 27) / 3 = 17

P1 P2 P3

24 27 30 0

nh thi CPU 32

First Come First Served (3/5)

n Thi gian phc v trung bnh =

n Thng nng =

n Thi gian quay vng =

P1 P2 P3

24 27 30 0

nh thi CPU 33

First Come First Served (4/5)

n Gi s cc process n theo th t:

P2 , P3 , P1

n Gin Gantt cho vic nh thi l:

n Thi gian i ca P1 = 6, P2 = 0, P3 = 3

l Thi gian i trung bnh: (6 + 0 + 3) / 3 = 3

Tt hn rt nhiu so vi trng hp trc

P1 P3 P2

6 3 30 0

nh thi CPU 34

First Come First Served (5/5)

Nhn xt

n FCFS khng cng bng vi process c CPU burst ngn v qu trnh phi ch trong thi gian di (so vi thi gian m n cn phc v) th mi c s dng CPU. iu ny ng ngha vi vic FCFS u tin cc process thuc dng CPU bound.

n Cu hi: Liu c xy ra trng hp tr hon v hn nh (starvation hay indefinite blocking) vi gii thut FCFS?

n FCFS thng c s dng trong cc h thng b (batch system)

nh thi CPU 35

V d thc t

n Vic phc v khch trong nh hng

l Thc khch s n v goi mn n cho mnh

l Mi mn n cn thi gian chun b khc nhau

n Mc tiu:

l Gim thi gian i trung bnh ca cc thc khch

n Cch lm no s ph hp?

l Thng thng cc nh hng s phc v theo kiu FCFS (!)

nh thi CPU 36

Shortest Job First (SJF) (1/3)

n i vi mi process, cn bit di ca CPU burst

n Hm la chon: chon process c di CPU burst nh nht

n Ch quyt nh: non-preemptive

n Chng minh c: SJF l ti u trong vic gim thi gian i trung bnh

n Vn : Cn phi c lng CPU burst tip theo ca process

l Gii php?

nh thi CPU 37

Shortest Job First (2/3)

Process Thi im n Burst time (ms)

P1 0,0 7

P2 2,0 4

P3 4,0 1

P4 5,0 4

n Gin Gantt khi nh thi theo SJF

n Thi gian i trung bnh = (0 + 6 + 3 + 7)/4 = 4

P1 P3 P2

7 3 16 0

P4

8 12

nh thi CPU 38

Shortest Job First (3/3)

n Thi gian phc v trung bnh =

n Thng nng =

n Thi gian quay vng =

P1 P3 P2

7 3 16 0

P4

8 12

nh thi CPU 39

D on thi gian s dng CPU (1/2)

Phng php: Ly trung bnh c trong s cc CPU burst o c trong qu kh

n Gi thit: nhng CPU burst cng mi cng phn nh gn hnh vi ca process trong tng lai

n Phng php trung bnh hm m (exponential averaging)

l n+1 = atn + (1 a)n , 0 a 1

t l tr o c, l tr d on

l Suy ra:

n+1 = atn + (1 a)atn1 ++ (1 a)jatnj ++ (1 a)

n+1a0

l V d: Nu chon a = th c ngha l tr o c tn v tr d on n c xem quan trong nh nhau

nh thi CPU 40

D on thi gian s dng CPU (2/2)

di CPU burst o c

di CPU burst d on,

vi a = v 0 = 10

nh thi CPU 41

Shortest Job First

Nhn xt

n SJF s dng u tin ngm nh: cng vic ngn nht c u tin trc

l Nhng cng vic thuc loi I/O bound thng c CPU burst ngn

n Process c thi gian thc thi di c th b tr hon v hn nh nu cc process c thi gian thc thi ngn lin tc vo

n Khng thch hp cho mi trng tng tc

l Cc CPU bound process c u tin thp, nhng c th chim CPU lu mt khi n c la chon thc thi

nh thi CPU 42

Shortest Remaining Time First (SRTF) (1/4)

n Ch quyt nh ca SJF: non-preemptive

n Phin bn preemptive ca SJF:

l Nu mt process mi n m c di CPU burst nh hn thi gian cn CPU cn li ca process ang thc thi, th preempt process ang thc thi

l Cch lm ny cn c goi l Shortest-Remaining-Time-First (SRTF)

nh thi CPU 43

Shortest Remaining Time First (2/4)

(V d ging vd cho SJF)

Process Thi im n Burst time (ms)

P1 0,0 7

P2 2,0 4

P3 4,0 1

P4 5,0 4

n Gin Gantt khi nh thi theo SRTF

n Thi gian i trung bnh = (9 + 1 + 0 + 2) / 4 = 3

l Tt hn gii thut SJF (4)

P1 P3 P2

4 2 11 0

P4

5 7

P2 P1

16

nh thi CPU 44

Shortest Remaining Time First (3/4)

n Thi gian phc v trung bnh =

n Thng nng =

n Thi gian quay vng =

P1 P3 P2

4 2 11 0

P4

5 7

P2 P1

16

nh thi CPU 45

Shortest Remaining Time First (4/4)

Nhn xt

n Trnh trng hp process c thi gian thc thi di c chim CPU

n Cn phi qun l thi gian thc thi cn li ca cc process

n C thi gian quay vng tt hn SJF

n Process c thi gian thc thi ngn c u tin cao

n C th dn n starvation

nh thi CPU 46

Priority Scheduling

n Mi process c gn mt u tin

n CPU s c cp cho process c u tin cao nht

n Ch quyt nh:

l Preemptive

l Non-preemptive

nh thi CPU 47

Gn u tin

n SJF: u tin l thi-gian-s-dng-CPU-d-on

n Gn u tin cn c th da vo:

l Yu cu v b nh

l S lng file c m

l T l thi gian dng cho I/O trn thi gian s dng CPU

l Cc yu cu bn ngoi nh: s tin ngi dng tr khi thc thi cng vic

nh thi CPU 48

Priority Scheduling

n Vn : tr hon v hn nh process c u tin thp c th khng bao gi c thc thi

n Gii php: aging u tin ca process c tng theo thi gian

nh thi CPU 49

Round Robin (RR) (1/4)

n Hm la chon: ging FCFS

2 1

3

4

5 6

7

8

nh thi CPU 50

Round Robin (2/4)

n Ch quyt nh: preemptive

l Khong thi gian ti a cho php (quantum time, thng 10 - 100 ms) c m bo bng vic s dng timer interrupt

l Process ang chy, khi ht thi gian (quantum time) s c chuyn v cui ca hng i ready

nh thi CPU 51

Round Robin (3/4)

Process Burst time (ms)

P1 53

P2 17

P3 68

P4 24

n Quantum time = 20 ms

n Gin Gantt:

n Thng c thi gian quay vng cao hn SJF, nhng li c thi gian p ng tt hn

P1 P2 P3 P4 P1 P3 P4 P1 P3 P3

0 20 37 57 77 97 117 121 134 154 162

nh thi CPU 52

Round Robin (4/4)

n Thi gian phc v trung bnh =

n Thng nng =

n Thi gian quay vng =

P1 P2 P3 P4 P1 P3 P4 P1 P3 P3

0 20 37 57 77 97 117 121 134 154 162

nh thi CPU 53

Quantum time v chuyn ng cnh

n Quantum time cng nh th cng c nhiu ln chuyn ng cnh (context switch) trong khi thc thi

S ln ngng/tip

tc qu trnh

nh thi CPU 54

Quantum time v thi gian quay vng

n Thi gian quay vng trung bnh khng chc s c ci thin khi quantum ln

nh thi CPU 55

Quantum time cho Round Robin

Nhn xt

n Khi thc hin process switch th OS s s dng CPU ch khng phi process ca ngi dng (OS overhead, ph tn):

l Dng thc thi process, lu tt c thng tin, np thng tin ca process sp thc thi

n Performance ty thuc vo kch thc ca quantum time (cn goi l time slice), v hm ph thuc ny khng n gin

l Time slice ngn th p ng nhanh

Vn : c nhiu chuyn ng cnh. Ph tn s cao

l Time slice di hn th throughput tt hn (do gim OS overhead) nhng thi gian p ng ln

l Nu time slice qu ln, RR tr thnh FCFS

nh thi CPU 56

Quantum time cho Round Robin

n Quantum time v thi gian cho process switch: v d thi gian cho process switch = 5 ms

l Nu quantum time = 20 ms, th ph tn (OS overhead) chim 5 / (20 + 5) = 20%

l Nu quantum = 500 ms, th ph tn ch cn 1%

Nhng nu c nhiu ng dng interactive th s thy p ng rt chm

n Ty thuc vo tp cng vic m la chon quantum time

n Quantum time nn ln trong tng quan so snh vi thi gian cho process switch

l V d vi 4.3 BSD UNIX, quantum time l 1 s

nh thi CPU 57

Round Robin

n Nu c n process trong hng i ready, v quantum time l q, nh vy mi process s c cp pht thi gian CPU theo tng khi c kch thc ln nht l q

l S khng c process no ch lu hn (n 1) q n v thi gian

n RR s dng mt gi thit ngm l tt c cc process u c tm quan trong ngang nhau

l Khng s dng RR nu mun cc process khc nhau c u tin khc nhau

head end

Ti a n 1 PCBs

Ready queue

nh thi CPU 58

Round Robin: nhc im

n Cc process dng CPU-bound vn cn c u tin

l V d:

Mt I/O-bound process s dng CPU trong thi gian ngn hn quantum time v block i I/O. V

Mt CPU-bound process chy ht time slice v lin tc quay tr v hng i ready, thng trc cc I/O bound process ang block.

nh thi CPU 59

Multilevel Queue Scheduling (1/3)

c p dng trong trng hp cc qu trnh c th c phn thnh

nhm, v d: interactive v batch

n Hng i ready s c chia thnh nhiu hng i ring r

l V d: foreground (cho cng vic cn tng tc) v background (cho cng vic dng b)

l Khng di chuyn qu trnh sang hng i ready khc

n Mi hng i ready s c gii thut nh thi ring. V d:

l foreground: dng RR

l background: dng FCFS

nh thi CPU 60

Multilevel Queue Scheduling (2/3)

n nh thi trong Multilevel Queue Scheduling cn cn phi thc hin gia cc hng i vi nhau

l Theo cch c nh (fixed priority scheduling) v d: phc v tt c cc process ca foreground ri mi n background

Process hng i cao preempt process hng i thp

C kh nng xy ra tr hon v hn nh (starvation) i vi process trong hng i c u tin thp

l Chia thi gian (time slice) mi hng i s c ly mt khong s dng CPU nht nh nh thi cho cc process ca mnh. V d:

80% cho foreground (dng RR)

20% cho background (dng FCFS)

nh thi CPU 61

Multilevel Queue Scheduling (3/3)

n V d phn nhm cc qu trnh

System processes

Interactive processes

Batch processes

Student processes

u tin thp nht

u tin cao nht

nh thi CPU 62

Multilevel Feedback Queue (1/3)

n Trong h thng s dng Multilevel Feedback Queue, b nh thi c th di chuyn process gia cc ready queue ty theo c tnh ca n c quan st.

V d:

l Nu mt process s dng CPU qu lu, n s b di chuyn sang mt hng i c u tin thp hn

l Nu mt process ch qu lu trong mt hng i c u tin thp, n s c di chuyn ln hng i c u tin cao hn (aging, gip trnh starvation)

n V d: Windows NT/XP/Vista

nh thi CPU 63

Multilevel Feedback Queue (2/3)

n V d: C 3 hng i

l Q0 , dng RR vi q = 8 ms

l Q1 , dng RR vi q = 16 ms

l Q2 , dng FCFS

n Gii thut

l Process mi s vo hng i Q0 . Khi n lt, process s c mt quantum l 8 ms. Nu khng tr CPU trong vng 8 ms, process s c a xung cui hng i Q1

l Ti Q1 , process s c cho mt quantum l 16 ms. Nu n khng tr CPU trc khi ht quantum th s b chuyn xung Q2

l Cng vic hng i cao preempt cng vic hng i thp

nh thi CPU 64

Multilevel Feedback Queue (3/3)

n Multilevel Feedback Queue c xc nh bi cc thng s

l C bao nhiu hng i?

l Vi mi queue s dng gii thut nh thi no?

l Khi no thng cp mt process?

l Khi no ging cp mt process?

nh thi CPU 65

Chnh sch v c ch (1/2)

Mt nguyn l thit k h iu hnh: Tch bit chnh sch (policy) v c ch (mechanism)

n Chnh sch

l Lc ra quyt nh cn phi lm g (v d ti u mt tiu ch no )

n C ch

l Cng c hin thc cc chnh sch

nh thi CPU 66

Chnh sch v c ch (2/2)

n nh thi qu trnh

l Chnh sch

Khi no th nh thi qu trnh?

Qu trnh no c chy?

l C ch

Lm th no chuyn ng cnh? Dispatcher

n V d:

l Chnh sch: FCFS

l C ch: dispatcher

n u im: v d

l Thay i loi ti cng vic (workload) th ch cn thay i chnh sch

l Thay i HW platform th ch cn thay i dispatcher

nh thi CPU 67

Tng kt

n S thc thi ca mt process

n B nh thi chon mt process t hng i ready

l Dispatcher thc hin switching

n Cc tiu ch nh thi (thng xung t nhau)

l li CPU, thi gian ch, thi gian p ng, thng nng,

n Cc gii thut nh thi

l FCFS, SJF, Priority, RR, Multilevel Feedback Queue

nh thi CPU 68

Bi tp 1

Process Burst Time

P1 10

P2 29

P3 3

P4 7

P5 12

n Tt c u n thi im 0

n Xt cc gii thut FCFS, SJF, v RR vi quantum time = 10

n Gii thut no cho

l thi gian i trung bnh nh nht?

l thng nng cao nht?

l thi gian quay vng trung bnh ca process nh nht?

nh thi CPU 69

Bi tp 2

n FCFS: thi gian i trung bnh l 28 ms, hy tnh cc thng s khc

nh thi CPU 70

Bi tp 3

n SJF (non-preemptive): thi gian i trung bnh l 13 ms, hy tnh cc thng s khc

nh thi CPU 71

Bi tp 4

n RR: thi gian i trung bnh l 23 ms, hy tnh cc thng s khc

nh thi CPU 72

Bi tp 5

Bi tp 5

nh thi CPU 73

Process Burst Time Arrival time

P1 10 0

P2 29 2

P3 3 5

P4 7 2

P5 12 6

Xt cc gii thut FCFS, SJF, SRTF, v RR vi quantum time = 10

Gii thut no cho

thi gian i trung bnh nh nht?

thng nng cao nht?

thi gian quay vng trung bnh ca process nh nht?