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CGS 3763 Operating Systems Concepts

Spring 2013

Dan C. MarinescuOffice: HC 30!

Office "ours: M#$% 11:30 # 12:30 &M



'ast time:

(o%ay: ("rea% coor%ination) 'oc*s

+e,t time

'ecture - Mon%ay) /anuary 2) 2013

12116 2



("rea% coor%ination

Critica section co%e t"at accesses a s"are% resource

ace con%itions t4o or more t"rea%s access s"are% %ata an% t"e resut

%epen%s on t"e or%er in 4"ic" t"e t"rea%s access t"e s"are% %ata.

Mutua e,cusion ony one t"rea% s"ou% e,ecute a critica section at any

one time.

Sc"e%uing agorit"ms %eci%e 4"ic" t"rea% to c"oose 4"en mutipe

t"rea%s are in a 5++&' state 8O first in first out

'8O ast in first out

9riority sc"e%uing

D eariest %ea%ine first

9reemption aiity to stop a running acti;ity an% start anot"er one 4it" a

"ig"er priority. Si%e effects of t"rea% coor%ination

Dea%oc*

9riority in;ersion a o4er priority acti;ity is ao4e% to run efore one 4it" a "ig"er priority

'ecture 1 3



Soutions to t"rea% coor%ination proems must satisfy a

set of con%itions

'ecture 1 4

1. Safety: ("e re<uire% con%ition 4i ne;er e ;ioate%.

2. 'i;eness: ("e system s"ou% e;entuay progress irrespecti;e of contention.

3. ree%om rom Star;ation: +o process s"ou% e %enie% progress for e;er. ("at

is) e;ery process s"ou% ma*e progress in a =nite time.

!. oun%e% $ait: ;ery process is assure% of not more t"an a =,e% numer of

o;erta*es y ot"er processes in t"e system efore it ma*es progress.>. airness: %epen%ent on t"e sc"e%uing agorit"m

? 8O: +o process 4i e;er o;erta*e anot"er process.

? '5: ("e process 4"ic" recei;e% t"e ser;ice east recenty gets t"e ser;ice ne,t.

or e,ampe for t"e mutua e,cusion proem t"e soution s"ou% guarantee t"at:

Safety t"e mutua e,cusion property is ne;er ;ioate%

'i;eness a t"rea% 4i access t"e s"are% resource in a =nite time

ree%om for star;ation a t"rea% 4i access t"e s"are% resource in a =nite time

oun%e% 4ait a t"rea% 4i access t"e s"are% resource at east after a =,e% numer of

accesses y ot"er t"rea%s.



("rea% coor%ination proems

Dining p"iosop"ers Critica section

'ecture 1 5



& soution to critica section proem

&ppies ony to t4o t"rea%s (i an% ( @ 4it" i)@ AB0)1 4"ic" s"are integer turn if turn=i t"en it is t"e turn of ( i to enter t"e critica section

boolean flag[2] if flag[i]= TRUE t"en (i is rea%y to enter t"e critica section

(o enter t"e critica section t"rea% (i

sets flag[i]= TRUE

sets turn=j

8f ot" t"rea%s 4ant to enter t"en turn 4i en% up 4it" a ;aue of eit"er i or j

an% t"e correspon%ing t"rea% 4i enter t"e critica section.

(i enters t"e critica section ony if eit"er flag[j]= FALSE or turn=i

("e soution is correct Mutua e,cusion is guarantee%

("e i;einess is ensure%

("e oun%e%#4aiting is met

ut t"is soution may not 4or* as oa% an% store instructions can e

interrupte% on mo%ern computer arc"itectures

'ecture 1 6



'ecture 1 7



'oc*s efore#or#&fter actions

'oc*s s"are% ;ariaes 4"ic" acts as a fag to coor%inate access to a

s"are% %ata. Manipuate% 4it" t4o primiti;es &CE58

'&S

Support impementation of efore#or#&fter actions ony one t"rea% can

ac<uire t"e oc*) t"e ot"ers "a;e to 4ait.

& t"rea%s must oey t"e con;ention regar%ing t"e oc*s. ("e t4o operations &CE58 an% '&S must e atomic.

Har%4are support for impementation of oc*s SM ea% an% Set Memory

CM9 Compare an% S4ap

SM Fmem

8f mem=LOCKED t"en SM returns r=LOCKED an% sets mem=LOCKED 8f mem=ULOCKED t"e SM returns r=LOCKED an% !et! mem=LOCKED

'ecture 1 8



'ecture 1 9



'ecture 1 10



Dea%oc*s

Happen <uite often in rea ife an% t"e propose% soutions are not

a4ays ogica: $"en t4o trains approac" eac" ot"er at a crossing)

ot" s"a come to a fu stop an% neit"er s"a start up again unti

t"e ot"er "as gone.I a pear from Jansas egisation. Dea%oc* @ury.

Dea%oc* egisati;e o%y.

'ecture 1 11



,ampes of %ea%oc*

(raffic ony in one %irection.

Soution one car ac*s up Fpreempt resources an% roac*. Se;era

cars may "a;e to e ac*e% up .

Star;ation is possie.

'ecture 1 12



'ecture 1 13



("rea% %ea%oc*

Dea%oc*s pre;ent sets of concurrent t"rea%sprocesses fromcompeting t"eir tas*s.

Ho4 %oes a %ea%oc* occur a set of oc*e% t"rea%s eac"

"o%ing a resource an% 4aiting to ac<uire a resource "e% y

anot"er t"rea% in t"e set.

,ampe

oc*s A an% ") initiaiKe% to 1

# 0 # 1

4ait F& 4aitF

4ait F 4aitF&

&im pre;ent or a;oi% %ea%oc*s

'ecture 1 14



System mo%e

esource types R 1) R 2) . . .) R m $C#U %&%le!' memor& !(a%e' )*O+e,i%e!-

ac" resource type R i "as . i instances.

esource access mo%e:

re<uest

use reease

'ecture 1 15



Simutaneous con%itions for %ea%oc*

Mutua e,cusion: ony one process at a time can use a resource. Ho% an% 4ait: a process "o%ing at east one resource is 4aiting to

ac<uire a%%itiona resources "e% y ot"er processes. +o preemption: a resource can e reease% ony ;ountariy y t"e

process "o%ing it Fpresumay after t"at process "as finis"e%.

Circuar 4ait: t"ere e,ists a set B# 0) # 1) L) # 0 of 4aiting processes suc"

t"at # 0 is 4aiting for a resource t"at is "e% y # 1) # 1 is 4aiting for a

resource t"at is "e% y # 2) L) # n 1 is 4aiting for a resource t"at is "e% y

# n) an% # 0 is 4aiting for a resource t"at is "e% y # 0.

'ecture 1 16



$ait for grap"s

'ecture 1 17



("e 4ait an% signa operations

9 Fs F4ait

B

8f s.C 0 t"en s.C N N

ese @oin s.'

Fs Fsigna

B

8f s.' is empty t"en s.C P P

ese reease a process from s.'

'ecture 1 19



Monitors

Semap"ores can e use% incorrecty

mutipe t"rea%s may e ao4e% to enter t"e critica section guar%e% y t"e semap"ore may cause %ea%oc*s

("rea%s may access t"e s"are% %ata %irecty 4it"out c"ec*ing t"e semap"ore.

Soution encapsuate s"are% %ata 4it" access met"o%s to operate on t"em.

Monitors an astract %ata type t"at ao4s access to s"are% %ata 4it" specific

met"o%s t"at guarantee mutua e,cusion

'ecture 1 20



'ecture 1 21



&sync"ronous e;ents an% signas

Signas) or soft4are interrupts) 4ere originay intro%uce% in 5ni, to notify aprocess aout t"e occurrence of a particuar e;ent in t"e system.

Signas are anaogous to "ar%4are 8O interrupts: $"en a signa arri;es) contro 4i arupty s4itc" to t"e signa "an%er.

$"en t"e "an%er is =nis"e% an% returns) contro goes ac* to 4"ere it came from

&fter recei;ing a signa) t"e recei;er reacts to it in a 4e#%e=ne% manner. ("atis) a process can te t"e system FOS 4"at t"ey 4ant to %o 4"en signa

arri;es: 8gnore it.

Catc" it an% %ei;er it. 8n t"is case) it must specify Fregister t"e signa "an%ing

proce%ure. ("is proce%ure resi%es in t"e user space. ("e *erne 4i ma*e a ca to

t"is proce%ure %uring t"e signa "an%ing an% contro returns to *erne after it is%one.

Ji t"e process F%efaut for most signas.

,ampes: ;ent # c"i% e,it) signa # to parent. Contro signa from *eyoar%.

'ecture 1 22



Signas state an% impementation

& signa "as t"e foo4ing states: Signa sen% # & process can sen% signa to one of its group memer process

Fparent) siing) c"i%ren) an% furt"er %escen%ants.

Signa %ei;ere% # Signa it is set.

9en%ing signa # %ei;ere% ut not yet recei;e% Faction "as not een ta*en.

Signa ost # eit"er ignore% or o;er4ritten. 8mpementation: ac" process "as a *erne space Fcreate% y %efaut

cae% signa %escriptor "a;ing its for eac" signa. Setting a it is %ei;ering

t"e signa) an% resetting t"e it is to in%icate t"at t"e signa is recei;e%. &

signa cou% e oc*e%ignore%. ("is re<uires an a%%itiona it for eac"

signa. Most signas are system controe% signas.

'ecture 1 23

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