Process synchronization (hệ điều HÀNH NÂNG CAO SLIDE)

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Chapter 5: Process

Synchronization

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Chapter 5: Process Synchronization

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 Processes can execute concurrently

 May be interrupted at any time, partially completing execution

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Producer

while (true) {

/* produce an item in next produced */

while (counter == BUFFER_SIZE) ;

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Race Condition

 counter++ could be implemented as

register1 = counter register1 = register1 + 1 counter = register1

 counter could be implemented as

register2 = counter register2 = register2 - 1 counter = register2

 Consider this execution interleaving with “count = 5” initially:

S0: producer execute register1 = counter {register1 = 5}

S1: producer execute register1 = register1 + 1 {register1 = 6}

S2: consumer execute register2 = counter {register2 = 5}

S3: consumer execute register2 = register2 – 1 {register2 = 4}

S4: producer execute counter = register1 {counter = 6 } S5: consumer execute counter = register2 {counter = 4}

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Critical Section Problem

 Consider system of n processes {p 0 , p 1 , … p n1}

 Each process has critical section segment of code

 Process may be changing common variables, updating table, writing file, etc

 When one process in critical section, no other may be in its critical section

 Critical section problem is to design protocol to solve this

 Each process must ask permission to enter critical section in

entry section, may follow critical section with exit section, then remainder section

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Critical Section

 General structure of process P i

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Algorithm for Process Pi

do {

while (turn == j);

critical section turn = j;

remainder section } while (true);

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Solution to Critical-Section Problem

1. Mutual Exclusion  If process P i is executing in its critical

section, then no other processes can be executing in their critical sections

2. Progress If no process is executing in its critical section and

there exist some processes that wish to enter their critical section, then the selection of the processes that will enter the critical section next cannot be postponed indefinitely

3. Bounded Waiting A bound must exist on the number of

times that other processes are allowed to enter their critical sections after a process has made a request to enter its critical section and before that request is granted

Assume that each process executes at a nonzero speed

No assumption concerning relative speed of the n

processes

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Essentially free of race conditions in kernel mode

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 The flag array is used to indicate if a process is ready to enter

the critical section. flag[i] = true implies that process Pi is ready!

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Algorithm for Process Pi

remainder section } while (true);

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Peterson’s Solution (Cont.)

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Solution to Critical-section Problem Using Locks

do {

acquire lock

critical section release lock

remainder section } while (TRUE);

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Solution using test_and_set()

 Shared Boolean variable lock, initialized to FALSE

 Solution:

do {

while (test_and_set(&lock)) ; /* do nothing */

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compare_and_swap Instruction

Definition:

int compare _and_swap(int *value, int expected, int new_value) {

int temp = *value;

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Solution using compare_and_swap

 Shared integer “lock” initialized to 0;

 Solution:

do {

while (compare_and_swap(&lock, 0, 1) != 0) ; /* do nothing */

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Bounded-waiting Mutual Exclusion with test_and_set

do { waiting[i] = true;

else waiting[j] = false;

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 Simplest is mutex lock

 Protect a critical section by first acquire() a lock then

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acquire() and release()

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Semaphore

 Synchronization tool that provides more sophisticated ways (than Mutex locks) for process to synchronize their activities.

}

 Definition of the signal() operation

signal(S) { S++;

}

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wait(synch);

S 2 ;

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Semaphore Implementation with no Busy waiting

 wakeup – remove one of processes in the waiting queue and place it in the ready queue

 typedef struct{

int value;

struct process *list;

} semaphore;

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Implementation with no Busy waiting (Cont.)

wait(semaphore *S) { S->value ;

if (S->value < 0) { add this process to S->list;

block();

} }

signal(semaphore *S) { S->value++;

if (S->value <= 0) { remove a process P from S->list;

wakeup(P);

} }

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Deadlock and Starvation

 Priority Inversion – Scheduling problem when lowerpriority process

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Classical Problems of Synchronization

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Bounded-Buffer Problem

 n buffers, each can hold one item

 Semaphore mutex initialized to the value 1

 Semaphore full initialized to the value 0

 Semaphore empty initialized to the value n

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Bounded Buffer Problem (Cont.)

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Bounded Buffer Problem (Cont.)

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 Semaphore rw_mutex initialized to 1

 Semaphore mutex initialized to 1

 Integer read_count initialized to 0

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Readers-Writers Problem (Cont.)

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Readers-Writers Problem (Cont.)

signal(mutex);

} while (true);

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Readers-Writers Problem Variations

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Dining-Philosophers Problem Algorithm

 The structure of Philosopher i:

do { wait (chopstick[i] );

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Dining-Philosophers Problem Algorithm (Cont.)

 Deadlock handling

 Allow at most 4 philosophers to be sitting simultaneously at the table

 Allow a philosopher to pick up the forks only if both are available (picking must be done in a critical

section

 Use an asymmetric solution an oddnumbered philosopher picks up first the left chopstick and then the right chopstick. Evennumbered philosopher picks

up first the right chopstick and then the left chopstick.

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Problems with Semaphores

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// shared variable declarations procedure P1 (…) { … }

procedure Pn (…) {……}

Initialization code (…) { … } }

}

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Schematic view of a Monitor

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Condition Variables

 condition x, y;

 Two operations are allowed on a condition variable:

 x.wait() – a process that invokes the operation is suspended until x.signal()

 x.signal() – resumes one of processes (if any) that

invoked x.wait()

 If no x.wait() on the variable, then it has no effect on the variable

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Monitor with Condition Variables

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Condition Variables Choices

 Both Q and P cannot execute in paralel. If Q is resumed, then P must wait

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Monitor Solution to Dining Philosophers

monitor DiningPhilosophers {

enum { THINKING; HUNGRY, EATING) state [5] ; condition self [5];

void pickup (int i) { state[i] = HUNGRY;

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Solution to Dining Philosophers (Cont.)

void test (int i) {

if ((state[(i + 4) % 5] != EATING) &&

(state[i] == HUNGRY) &&

(state[(i + 1) % 5] != EATING) ) { state[i] = EATING ;

self[i].signal () ; }

}

initialization_code() { for (int i = 0; i < 5; i++) state[i] = THINKING;

} }

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 Each philosopher i invokes the operations pickup() and

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Monitor Implementation Using Semaphores

 Variables

semaphore mutex; // (initially = 1) semaphore next; // (initially = 0) int next_count = 0;

 Each procedure F will be replaced by

wait(mutex);

… body of F;

…

if (next_count > 0) signal(next)

else signal(mutex);

 Mutual exclusion within a monitor is ensured

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Monitor Implementation – Condition Variables

else signal(mutex);

wait(x_sem);

x_count ;

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Monitor Implementation (Cont.)

 The operation x.signal can be implemented as:

if (x_count > 0) { next_count++;

signal(x_sem);

wait(next);

next_count ;

}

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Resuming Processes within a Monitor

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 Allocate a single resource among competing processes using

priority numbers that specify the maximum time a process plans to use the resource

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A Monitor to Allocate Single Resource

monitor ResourceAllocator {

boolean busy;

condition x;

void acquire(int time) {

if (busy) x.wait(time);

busy = TRUE;

} void release() { busy = FALSE;

x.signal();

} initialization code() { busy = FALSE;

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 Dispatcher objects either signaledstate (object available)

or nonsignaled state (thread will block)

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Linux Synchronization

 Linux:

 Prior to kernel Version 2.6, disables interrupts to implement short critical sections

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Alternative Approaches

 Transactional Memory

 OpenMP

 Functional Programming Languages

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}

The code contained within the #pragma omp critical directive

is treated as a critical section and performed atomically

OpenMP

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End of Chapter 5

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