Chuong08 - VirtualMemory

Chng 8

Bo Nh Ao

Khoa KTMT 2

Noi dung trnh bay

Tong quan ve bo nh ao

Cai at bo nh ao : demand paging

Cai at bo nh ao : Page Replacement

Cac giai thuat thay trang (Page Replacement Algorithms)

Van e cap phat Frames

Van e Thrashing

Cai at bo bo nh ao : Demand Segmentation

Khoa KTMT 3

1. Tong quan bo nh ao

Nhan xet: khong phai tat ca cac phan cua mot process

can thiet phai c nap vao bo nh chnh tai cung mot

thi iem

V du

oan ma ieu khien cac loi hiem khi xay ra

Cac arrays, list, tables c cap phat bo nh (cap phat tnh)

nhieu hn yeu cau thc s

Mot so tnh nang t khi c dung cua mot chng trnh

Ca chng trnh th cung co oan code cha can dung

Bo nh ao (virtual memory): Bo nh ao la mot ky thuat

cho phep x ly mot tien trnh khong c nap toan bo

vao bo nh vat ly

Khoa KTMT 4

1. Bo nh ao (tt)

u iem cua bo nh ao

So lng process trong bo nh nhieu hn

Mot process co the thc thi ngay ca khi kch thc cua no ln

hn bo nh thc

Giam nhe cong viec cua lap trnh vien

Khong gian trao oi gia bo nh chnh va bo nh

phu(swap space).

V du:

swap partition trong Linux

file pagefile.sys trong Windows

Khoa KTMT 5

2. Cai at bo nh ao

Co hai ky thuat:

Phan trang theo yeu cau (Demand Paging)

Phan oan theo yeu cau (Segmentation Paging)

Phan cng memory management phai ho tr paging

va/hoac segmentation

OS phai quan ly s di chuyen cua trang/oan gia bo

nh chnh va bo nh th cap

Trong chng nay,

Ch quan tam en paging

Phan cng ho tr hien thc bo nh ao

Cac giai thuat cua he ieu hanh

Khoa KTMT 6

2.1.Phan trang theo yeu cau

demand paging

Demand paging: cac trang cua qua trnh ch c nap

vao bo nh chnh khi c yeu cau.

Khi co mot tham chieu en mot trang ma khong co

trong bo nh chnh (valid bit) th phan cng se gay ra

mot ngat (goi la page-fault trap) kch khi page-fault

service routine (PFSR) cua he ieu hanh.

PFSR:

1. Chuyen process ve trang thai blocked

2. Phat ra mot yeu cau oc a e nap trang c tham chieu vao

mot frame trong; trong khi i I/O, mot process khac c cap

CPU e thc thi

3. Sau khi I/O hoan tat, a gay ra mot ngat en he ieu hanh;

PFSR cap nhat page table va chuyen process ve trang thai

ready.

Khoa KTMT 7

2.2. Loi trang va cac bc x ly

Khoa KTMT 8

2.3. Thay the trang nh

Bc 2 cua PFSR gia s phai thay trang v khong tm

c frame trong, PFSR c bo sung nh sau

1. Xac nh v tr tren a cua trang ang can

2. Tm mot frame trong:

a. Neu co frame trong th dung no

b. Neu khong co frame trong th dung mot giai thuat thay trang

e chon mot trang hy sinh (victim page)

c. Ghi victim page len a; cap nhat page table va frame table

tng ng

3. oc trang ang can vao frame trong (a co c t bc 2);

cap nhat page table va frame table tng ng.

Khoa KTMT 9

2.3. Thay the trang nh (tt)

Khoa KTMT 10

2.4. Cac thuat toan thay the trang

Hai van e chu yeu:

Frame-allocation algorithm

Cap phat cho process bao

nhieu frame cua bo nh thc?

Page-replacement algorithm

Chon frame cua process se

c thay the trang nh

Muc tieu: so lng page-fault

nho nhat

c anh gia bang cach thc

thi giai thuat oi vi mot chuoi

tham chieu bo nh (memory

reference string) va xac nh

so lan xay ra page fault

V du

Th t tham chieu cac a ch

nh, vi page size = 100:

0100, 0432, 0101, 0612, 0102,

0103, 0104, 0101, 0611, 0102,

0103, 0104, 0101, 0610, 0102,

0103, 0104, 0101, 0609, 0102,

0105

cac trang nh sau c tham

chieu lan lt = chuoi tham

chieu bo nh (trang nh)

1, 4, 1, 6, 1,

1, 1, 1, 6, 1,

1, 1, 1, 6, 1,

1, 1, 1, 6, 1,

1

Khoa KTMT 11

a) Giai thuat thay trang FIFO

Cac d lieu can biet ban au:

So khung trang

Tnh trang ban au

Chuoi tham chieu

Trang nh c nht s c thay th

B nh chnh cp cho process 3 frame

Khoa KTMT 12

Nghch ly Belady

Khoa KTMT 13

Nghch ly Belady

Bat thng (anomaly) Belady: so page fault tang mac dau qua trnh

a c cap nhieu frame hn.

Khoa KTMT 14

2.4 b)Giai thuat thay trang OPT(optimal)

Giai thuat thay trang OPT

Thay the trang nh se c tham chieu tre nhat trong tng lai

Kh hin thc?

V du: mot process co 7 trang, va c cap 3 frame

Khoa KTMT 15

c) Giai thuat lau nhat cha s dung

Least Recently Used (LRU)

V du:

Moi trang c ghi nhan (trong bang phan trang) thi iem c

tham chieu trang LRU la trang nh co thi iem tham chieu nho

nhat (OS ton chi ph tm kiem trang nh LRU nay moi khi co page fault)

Do vay, LRU can s ho tr cua phan cng va chi ph cho viec tm

kiem. t CPU cung cap u s ho tr phan cng cho giai thuat LRU.

Khoa KTMT 16

LRU va FIFO

So sanh cac giai thuat thay trang LRU va FIFO

chuoi tham chieu

trang nh

Khoa KTMT 17

Gii thut c hi th hai

S dng cc bit tham kho ti nhng khon thi gian u n

Dng mt byte cho mi trang trong mt bng nm trong b nh

Dng mt thanh ghi dch cha lch s tham kho trong 8 ln gn nht

VD: 00110101, 00000000, 11111111

L gii thut thay th FIFO, trc khi thay th mt trang xem xt bit tham

kho ca n

i khi s dng hai bit: tham kho v sa i nh mt cp (x,x):

(0,0) khng c dng mi y v khng c sa i-l trang tt nht

thay th.

(0,1) khng c dng mi y nhng c sa i-khng tht tt v

trang cn c vit ra trc khi thay th.

(1,0) c dng mi y nhng khng c sa i-n c th s nhanh chng c

dng li.

(1,1) c dng mi y v c sa i-trang c th s nhanh chng c dng li

v trang s cn c vit ra a trc khi n c th c thay th.

Khoa KTMT 18

Gii thut c hi th hai (tt)

Khoa KTMT 19

2.5.So lng frame cap cho process

OS phai quyet nh cap cho moi process bao nhieu

frame.

Cap t frame nhieu page fault

Cap nhieu frame giam mc o multiprogramming

Chien lc cap phat tnh (fixed-allocation)

So frame cap cho moi process khong oi, c xac nh vao thi

iem loading va co the tuy thuoc vao tng ng dung (kch thc

cua no,)

Chien lc cap phat ong (variable-allocation)

So frame cap cho moi process co the thay oi trong khi no chay

Neu ty le page-fault cao cap them frame

Neu ty le page-fault thap giam bt frame

OS phai mat chi ph e c nh cac process

Khoa KTMT 20

a) Chien lc cap phat tnh

Cap phat bang nhau: V du, co 100 frame va 5

process moi process c 20 frame

Cap phat theo t le: da vao kch thc process

Cap phat theo o u tien

mS

spa

m

sS

ps

iii

i

ii

for allocation

frames ofnumber total

process of size

5964137

127

564137

10

127

10

64

2

1

2

a

a

s

s

m

i

V du:

Khoa KTMT 21

3. Tr tren toan bo he thong

Thrashing

Neu mot process khong co u so frame can thiet th t so

page faults/sec rat cao.

Thrashing: hien tng cac trang nh cua mot process b

hoan chuyen vao/ra lien tuc.

Khoa KTMT 22

a)Mo hnh cuc bo (Locality)

e han che thrashing, he ieu hanh phai cung cap cho

process cang u frame cang tot. Bao nhieu frame th u cho mot process thc thi hieu qua?

Nguyen ly locality (locality principle)

Locality la tap cac trang c tham chieu gan nhau

Mot process gom nhieu locality, va trong qua trnh thc thi,

process se chuyen t locality nay sang locality khac

V sao hien tng thrashing xuat hien?

Khi size of locality > memory size

Khoa KTMT 23

b) Giai phap tap lam viec (working set)

c thiet ke da tren nguyen ly locality.

Xac nh xem process thc s s dung bao nhieu

frame.

nh ngha:

WS(t) - so lng cac tham chieu trang nh cua process gan

ay nhat can c quan sat trong khong thi gian .

- khoang thi gian tham chieu

V du:

2 4 5 6 9 1 3 2 6 3 9 2 1 4

thi iem t1

D = 4

chuoi tham khao

trang nh

Khoa KTMT 24


nh ngha: working set cua process Pi, ky hieu WS

i, la tap gom D

cac trang c s dung gan ay nhat.

Nhan xet:

D qua nho khong u bao phu toan bo locality.

D qua ln bao phu nhieu locality khac nhau.

D = bao gom tat ca cac trang c s dung.

Dung working set cua mot process e xap x locality cua no.

chuoi tham khao trang

V du: D = 10 v

Khoa KTMT 25


nh ngha WSSila kch thc cua working set cua P

i:

WSSi= so lng cac trang trong WS

i

chuoi tham khao trang

WSS(t1) = 5 WSS(t2) = 2

V du (tiep): D = 10 v

Khoa KTMT 26


at D = WSSi= tong cac working-set size cua moi

process trong he thong.

Nhan xet: Neu D > m (so frame cua he thong) se xay ra thrashing.

Giai phap working set:

Khi khi tao mot qua trnh: cung cap cho qua trnh so lng

frame thoa man working-set size cua no.

Neu D > m tam dng mot trong cac process.

Cac trang cua qua trnh c chuyen ra a cng va cac frame cua no c thu hoi.

Khoa KTMT 27


WS loai tr c tnh trang tr tre ma van am bao mc

o a chng

Theo vet cac WS? => WS xap x (oc them trong sach)

oc them:

He thong tap tin

He thong nhap xuat

He thong phan tan

Khoa KTMT 28

Bai tap

Bi 01: Mt my tnh 32-bit a ch, s dng mt bng trang nh cp.

a ch o c phn b nh sau : 9 bit dnh cho bng trang cp 1, 11

bit cho bng trang cp 2, v cho offset. Cho bit kch thc mt trang

trong h thng, v a ch o c bao nhiu trang ?

Bi 02: Xt chui truy xut b nh sau:

1, 2 , 3 , 4 , 3 , 5 , 1 , 6 , 2 , 1 , 2 , 3 , 7 , 5 , 3 , 2 , 1 , 2 , 3 , 6

C bao nhiu li trang xy ra khi s dng cc thut ton thay th sau

y, gi s c 4 khung trang v ban u cc khung trang u trng ?

a) LRU

b) FIFO

c) Optimal

d) C hi th 2

Bi tp

Cho mt process c s frame truy cp nh

sau:

1,2,4,3,5,2,3,5,6,7,8,9,1,2,3,4,5,2,3,7,6,4,

5,6,7,9,8,1,2,5

Cho bit c biu nhiu tp lm vic

(working set)?

Lit k cc frame trong tng working set?

Bit D = 7.

Khoa KTMT 29

Bi tp

H thng ti mt thi im c 5 process:

process 1 cn 5 frame, process 2 cn 15

frame, process 3 cn 10 frame, process 4

cn 9 frame, process 5 cn 11 frame. H

thng hin c 30 frame. Tnh xem mi

process c cp bao nhiu frame?

Phn b u?

Theo t l?

Khoa KTMT 30

Documents

Chuong08 - VirtualMemory