Monitor Examples

7/25/2019 Monitor Examples

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Monitor Solutions to Classical

Problems


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Announcements

CS 415 Projects graded.

Mean 80.! "ig# $0 out o% $0

CS 414 "ome&or' due Monda(.


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*oals %or +oda(

Continue &it# S(nc#roni,ation Abstractions

Monitors and condition -ariables

Producer Consumer &it# Monitors

eaders/riters roblem and solution &it# Monitors


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e-ie& Monitors

Monitors reresent t#e logic o% t#e rogram ait i% necessar( Signal en c#ange somet#ing so an( &aiting t#reads can

roceed

3asic structure o% monitor/based rogram

lockwhile (need to wait) { unlock

condvar.wait();

lock}

do something so no need to wait

condvar.signal();

unlock

Check and/or updatestate variablesWait if necessary

Check and/or updatestate variables


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e-ie& Producer/consumer &it#

a bounded bu%%er Problem e%inition

Producer uts t#ings into a s#ared bu%%er &ait i% %ull6 Consumer ta'es t#em out &ait i% emt(6 7se a %ied/si,e bu%%er bet&een t#em to a-oid loc'ste

9eed to s(nc#roni,e access to t#is bu%%er

Correctness Constraints

Consumer must &ait %or roducer to %ill bu%%ers! i% none %ull sc#eduling constraint Producer must &ait %or consumer to emt( bu%%ers! i% all %ull

sc#eduling constraint

:nl( one t#read can maniulate bu%%er ;ueue at a time mutual eclusion

emember ( &e need mutual eclusion 3ecause comuters are stuid

*eneral rule o% t#umb7se a searate sema#ore %or eac# constraint Semaphore not_empty; // consumers constraint

Semaphore not_!ull; // producers constraint Semaphore mute"; // mutual e"clusion


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Producer does #(not_!ull)$ %(not_empty)

Consumer does #(not_empty)$ %(not_!ull)

?s order o% P@s imortant> esB Can cause deadloc'

?s order o% @s imortant>

9o! ecet t#at it mig#t a%%ect sc#eduling e%%icienc(

#at i% &e #a-e 2 roducers or 2 consumers> o &e need to c#ange an(t#ing>


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e-ie& Moti-ation %or Monitors and

Condition ariables Sema#ores are a #uge ste u! but

+#e( are con%using because t#e( are dual urose 3ot# mutual eclusion and sc#eduling constraints

Damle t#e %act t#at %liing o% P@s in bounded bu%%er gi-es deadloc' isnot immediatel( ob-ious

Cleaner idea 7se locks%or mutual eclusion and condition variables%or sc#eduling constraints

e%inition Monitor a loc' and ,ero or more condition-ariables %or managing concurrent access to s#ared data 7se o% Monitors is a rogramming aradigm

Some languages li'e Ea-a ro-ide monitors in t#e language +#e loc' ro-ides mutual eclusion to s#ared data

Al&a(s ac;uire be%ore accessing s#ared data structure

Al&a(s release a%ter %inis#ing &it# s#ared data

Foc' initiall( %ree


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Condition ariables "o& do &e c#ange t#e get6 routine to &ait until somet#ing is

in bu%%er> Could do t#is b( 'eeing a count o% t#e number o% t#ings on t#e;ueue &it# sema#ores6! but error rone

Condition ariable a ;ueue o% t#reads &aiting %or somet#inginsidea critical section

Ge( idea allo& sleeing inside critical section b( atomicall( releasingloc' at time &e go to slee

Contrast to sema#ores Can@t &ait inside critical section

:erations &ait('lock) Atomicall( release loc' and go to slee. e/ac;uire

loc' later! be%ore returning. Signal() a'e u one &aiter! i% an( roadcast() a'e u all &aiters

ule Must #old loc' en doing condition -ariable osB


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Producer Consumer using Monitors

-onitorProducer_Consumer& any_t buf[N; int n = 0! tail = 0! hea" = 0; con"ition not_empty! not_full;

#oi" put$char ch% &.hile$n == N%

.ait$not_full%;buf[hea"(N = ch;hea"));n));

si,nal$not_empty%; +

char ,et$% &.hile$n == 0% .ait$not_empty%;ch = buf[tail(N;tail));n;si,nal$not_full%;return ch;

+


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eminders Subtle asects

9otice t#at en a t#read calls &ait6! i% it bloc's it also

automaticall( releases t#e monitor@s mutual eclusion

loc'

+#is is an elegant solution to an issue seen &it#

sema#ores

Caller #as mutual eclusion and &ants to call PnotHemt(6I

but t#is call mig#t bloc'

?% &e just do t#e call! t#e solution deadloc'sI

3ut i% &e first call mute6! &e get a race conditionB


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e-ie& Mesa -s. "oare monitors

9eed to be care%ul about recise de%inition o% signal and &ait.

Consider a iece o% our de;ueue codewhile (n*) {

wait(not_empty); // +! nothing$ sleep}ch ,u!-tail0; // 1et ne"t item

#( didn@t &e do t#is>

i! (n*) {wait(not_empty); // +! nothing$ sleep

}ch ,u!-tail0;// 1et ne"t item

Ans&er deends on t#e t(e o% sc#eduling

"oare/st(le most tetboo's6 Signaler gi-es loc'! CP7 to &aiter= &aiter runs immediatel( aiter gi-es u loc'! rocessor bac' to signaler en it eits critical

section or i% it &aits again

Mesa/st(le Ea-a! most real oerating s(stems6 Signaler 'ees loc' and rocessor

aiter laced on read( ;ueue &it# no secial riorit( Practicall(! need to c#ec' condition again a%ter &ait


13/31

1)

e-ie& Can &e construct Monitors

%rom Sema#ores> Foc'ing asect is eas( Eust use a mute Can &e imlement condition -ariables t#is &a(>

&ait() { #("_sem); }

Signal() { %("_sem); }

oesn@t &or' ait6 ma( slee &it# loc' #eld

oes t#is &or' better>&ait() { %(mute"); // 2elease mute" lock #("_sem);

#(mute"); // 3c4uire mute" lock}Signal() { %("_sem); }

9o Condition -ars #a-e no #istor(! sema#ores #a-e #istor( #at i% t#read signals and no one is &aiting> 9:/:P #at i% t#read later &aits> +#read aits #at i% t#read @s and noone is &aiting> ?ncrement #at i% t#read later does P> ecrement and continue


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Construction o% Monitors %rom

Sema#ores con@t6 Problem &it# re-ious tr( P and are commutati-e result is t#e same no matter at order

t#e( occur Condition -ariables are 9:+ commutati-e

oes t#is %i t#e roblem>&ait(5ock lock) { %(mute"); // 2elease mute" lock #("_sem);

#(mute"); // 3c4uire mute" lock}Signal() { i! semaphore 4ueue is not empty %("_sem);

} 9ot legal to loo' at contents o% sema#ore ;ueue +#ere is a race condition signaler can sli in a%ter loc' release and

be%ore &aiter eecutes sema#ore.P6

?t is actuall( ossible to do t#is correctl( Comle solution %or "oare sc#eduling in boo'

Can (ou come u &it# simler Mesa/sc#eduled solution>


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15

Construction o% HoareMonitors

using Sema#ores

or each proce"ure :

'$mutex%;

/* bo"y of */

if$next_count 0%$next%;

else

$mutex%;

Wait(){

x_count++;

if(next_count > 0)

V(next);

else

V(mutex); P(x_sem);

x_count--;

Signal(){

If(x_count > 0) { next_count++;

V(x_sem);

P(next);

next_count--;

}


16/31

1 :r suose t#at a &riter is &aiting and an endless o% stream o%

readers 'ees s#o&ing u. ?s it %air %or t#em to become acti-e>

e@ll %a-or a 'ind o% bac'/and/%ort# %orm o% %airness :nce a reader is &aiting! readers &ill get in net. ?% a &riter is &aiting! one &riter &ill get in net.


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1$

eaders and riters

-onitorReadersNWriters& int 2aitin,2riters! 2aitin,3ea"ers!N3ea"ers! N2riters; 4on"ition 4an3ea"! 4an2rite;

oi" 5e,in2rite$% & if$N2riters == 1 66 N3ea"ers 0%

& ))2aitin,2riters; .ait$4an2rite%; 2aitin,2riters; + N2riters = 1;

+ oi" 7n"2rite$% & N2riters = 0; if$2aitin,3ea"ers% Si,nal$4an3ea"%; else

Si,nal$4an2rite%; +

oid 3eginead6

N

i%9riters 1 OO aitingriters 06

N QQaitingeaders=

aitCanead6=

//aitingeaders=

R

QQ9eaders=

SignalCanead6=

R

oid Dndead6

N

i%//9eaders 06

SignalCanrite6=

R


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eaders and riters





Si,nal$4an2rite%; +

oid 3eginead6

N


N QQaitingeaders=

aitCanead6=

//aitingeaders=

R

QQ9eaders=

SignalCanead6=

R

oid Dndead6

N

i%//9eaders 06

SignalCanrite6=

R


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eaders and riters





Si,nal$4an2rite%; +

oid 3eginead6

N


N QQaitingeaders=

aitCanead6=

//aitingeaders=

R

QQ9eaders=

SignalCanead6=

R

oid Dndead6

N

i%//9eaders 06

SignalCanrite6=

R


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eaders and riters





Si,nal$4an2rite%; +

oid 3eginead6

N


N QQaitingeaders=

aitCanead6=

//aitingeaders=

R

QQ9eaders=

SignalCanead6=

R

oid Dndead6

N

i%//9eaders 06

SignalCanrite6=

R


23/31

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7nderstanding t#e Solution

A &riter can enter i% t#ere are no ot#er acti-e &riters and

no readers are &aiting


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eaders and riters



& ))2aitin,2riters; .ait$4an2rite%; 2aitin,2riters; + N2riters = 1; + oi" 7n"2rite$% & N2riters = 0; if$2aitin,3ea"ers% Si,nal$4an3ea"%; else

Si,nal$4an2rite%; +

oid 3eginead6

N


N QQaitingeaders=

aitCanead6=

//aitingeaders=

R

QQ9eaders=

SignalCanead6=

R

oid Dndead6

N

i%//9eaders 06

SignalCanrite6=

R


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7nderstanding t#e Solution

A reader can enter i%

+#ere are no &riters acti-e or &aiting

So &e can #a-e man( readers acti-e all at once

:t#er&ise! a reader &aits ma(be man( do6


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