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Lecture 5

InterprocessCommunication

7 September, 2010

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the

producer-consumer

problem

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producer consumer

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while (1)

if (buffer is empty)

sleepconsume

if (buffer was full)

wake up producer

while (1)

if (buffer is full)

sleep

produce

if (buffer was empty)

wake up consumer

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how to remember

sleep / wake

message?

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the

semaphore

abstraction

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3

semaphore

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down( )

if value is 0 sleep(put in wait list)

value = value - 1

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up( )

value = value + 1 if value is 1

wake someone

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new

ready running

blocked exit

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up( ) and down( )

are atomic

can use enter( ) and leave( ) fromlast lecture to ensure mutual exclusion

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while (1)

if (buffer is empty)

sleepconsume

if (buffer was full)

wake up producer

while (1)

if (buffer is full)

sleep

produce

if (buffer was empty)

wake up consumer

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Process 2::

up(S)

Process 1::

down(S)

semaphore S = 0

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Process 2

: down(S)

:

up(S)

:

semaphore S = 1

Process 1

: down(S)

:

up(S)

:

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semaphore used for:

1. synchronization2. mutual exclusion

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while (1)down(free_slots)produce

up(used_slots)

while (1)

down(used_slots)consume

up(free_slots)

semaphore free_slots = Nsemaphore used_slots = 0

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while (1)down(free_slots)down(mutex)

produceup(mutex)up(used_slots)

while (1)down(used_slots)down(mutex)

consumeup(mutex)up(free_slots)

semaphore free_slots = Nsemaphore used_slots = 0semaphore mutex = 1

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pitfalls ofsemaphore

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Process 2

:down(T)

down(S)

up(S)

up(T)

semaphore S = T = 1

Process 1

:down(S)

down(T)

up(T)

up(S)

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:

down(T)

down(S)

:down(S)

down(T)

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deadlock

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while (1)down(mutex)down(free_slots)

produceup(mutex)up(used_slots)

while (1)down(mutex)down(used_slots)

consumeup(mutex)up(free_slots)

semaphore free_slots = Nsemaphore used_slots = 0semaphore mutex = 1

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while (1)down(mutex)

down(free_slots)

produceup(mutex)

up(used_slots)

while (1)

down(mutex)down(used_slots)consumeup(mutex)

up(free_slots)

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Flickr photo Some rights reserved by fazen

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http://www.flickr.com/photos/fazen/http://www.flickr.com/photos/fazen/http://creativecommons.org/licenses/by-nd/2.0/http://creativecommons.org/licenses/by-nd/2.0/

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while (1)

thinkpick left chopstick

pick right chopstick eat

put down left chopstickput down right chopstick

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while (1)

thinkwait till left chopstick is available

pick left chopstick

wait till right chopstick is available

pick right chopstick

eatput down left chopstick

put down right chopstick28

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starvation

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while (1)

thinkenter( )

pick left chopstick

pick right chopstickeat

put down left chopstickput down right chopstick

leave( )31

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eat think

hungry

(may block)

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while (1)

think

if aneighbor is eating

wait for chopsticks

eat if a neighbor is waiting and is

ready to eatwake up neighbor

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while (1)

think

state[ i ] = HUNGRYif aneighbor is eating

wait for chopsticks

state[ i ] = EATeat

state[ i ] = THINK

if a neighbor is waitingwake up neighbor

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while (1)

think

state[ i ] = HUNGRY if state[ L ] == EAT || state[ R ] == EAT

down(semaphore[ i ])

state[ i ] = EATeat

state[ i ] = THINK

if state[L] == HUNGRY && state[LL] != EATup(semaphore[ L ])

if state[R] == HUNGRY && state[RR] != EAT

up(semaphore[ R ])

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while (1)thinkstate[ i ] = HUNGRY

if state[ i ] == HUNGRY&&state[ L ] != EAT && state[ R ] != EATup(semaphore[ i ])state[ i ] = EAT

down(semaphore[ i ])

eatstate[ i ] = THINK if state[L] == HUNGRY && state[LL] != EAT && state[ LR ] != EAT

up(semaphore[ L ])

state[ L ] = EATif state[R] == HUNGRY && state[RL] != EAT && state[RR] != EATup(semaphore[ R ])state[ L ] = EAT

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while (1)thinkstate[ i ] = HUNGRY

test( i )

down(semaphore[ i ])eatstate[ i ] = THINK

test( L )

test( R )

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while (1)think

down(mutex)state[ i ] = HUNGRYtest( i )up(mutex)

down(semaphore[ i ])eatdown(mutex)state[ i ] = THINK

test( L )test( R )up(mutex)

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Flickr photo Some rights reserved by Unhindered by Talent

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http://www.flickr.com/photos/nicmcphee/http://www.flickr.com/photos/nicmcphee/http://creativecommons.org/licenses/by-sa/2.0/http://creativecommons.org/licenses/by-sa/2.0/

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the

mutex

abstraction

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1/0

mutex

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the

condition variable

abstraction

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conditionvariable

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POSIX threads in C

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#include

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gcca.c-lpthread

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pthread_create(..)

pthread_exit(..)pthread_join(..)

pthread_yield(..)

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pthread_mutex_init(..)pthread_mutex_lock(..)

pthread_mutex_unlock(..)pthread_mutex_trylock(..)

pthread_mutex_destroy(..)

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pthread_cond_init(..)pthread_cond_wait(..)

pthread_cond_signal(..)pthread_cond_broadcast(..)

pthread_cond_destroy(..)