Process Synchonization Chapter 6. Outline Process Synchronization The Critical-Section Problem...

Process Synchonization

Chapter 6

Outline Process Synchronization The Critical-Section Problem Peterson’s Solution Synchronization Hardware Semaphores

Implementation Deadlocks and Starvation

Classic Problems of Synchronization The Bounded-Buffer Problem The Readers-Writers Problem The Dining-Philosophers problem

Process Synchronization Cooperating Processes

That can affect or be affected by other processes executing in the system.

Concurrent access to shared data may result in data inconsistency.

Maintaining data consistency requires mechanisms to ensure that cooperating processes access shared data sequentially.

Bounded-Buffer Problem

Shared data#define BUFFER_SIZE 10typedef struct {

. . .} item;

item buffer[BUFFER_SIZE];int in = 0, out = 0;int counter = 0;

Producer processitem nextProduced;…while (1) { while (counter == BUFFER_SIZE) ; buffer[in] = nextProduced; in = (in + 1) % BUFFER_SIZE; counter++;}


item nextConsumed;while (1) { while (counter == 0) ; nextConsumed = buffer[out]; out = (out + 1) % BUFFER_SIZE; counter--;}

Bounded-Buffer ProblemConsumer process

“counter++” in assembly language

MOV R1, counter

INC R1 MOV counter, R1

“counter--” in assembly languageMOV R2, counter

DEC R2 MOV counter, R2


If both the producer and consumer attempt to update the buffer concurrently, the machine language statements may get interleaved.

Interleaving depends upon how the producer and consumer processes are scheduled.


Assume counter is initially 5. One interleaving of statements is:

producer: MOV R1, counter (R1 = 5) INC R1 (R1 = 6)consumer: MOV R2, counter (R2 = 5) DEC R2 (R2 = 4)producer: MOV counter, R1 (counter = 6)consumer: MOV counter, R2 (counter = 4)

The value of count may be either 4 or 6, where the correct result should be 5.


Race Condition: The situation where several processes access and manipulate shared data concurrently, the final value of the data depends on which process finishes last.

Process Synchronization

Critical Section: A piece of code in a cooperating process in which the process may updates shared data (variable, file, database, etc.).

Critical Section Problem: Serialize executions of critical sections in cooperating processes


More Examples Bank transactionsAirline reservation


Bank Transactions

Balance

Deposit WithdrawalMOV A, Balance

ADD A, Deposited

MOV Balance, A

MOV B, Balance

SUB B, Withdrawn

MOV Balance, B

D W

Bank Transactions Current balance = Rs. 50,000Check deposited = Rs. 10,000ATM withdrawn = Rs. 5,000

Bank Transactions

Bank TransactionsCheck Deposit:

MOV A, Balance // A = 50,000ADD A, Deposit ed // A = 60,000

ATM Withdrawal:MOV B, Balance // B = 50,000

SUB B, Withdrawn // B = 45,000

Check Deposit:MOV Balance, A // Balance =

60,000

ATM Withdrawal:MOV Balance, B // Balance =

45,000

Assembly Language usage in C++ compiler

#include <iostream.h>main(){

int c = 5;__asm {

mov eax,cmov ebx,10add eax,ebxmov c,eax

}cout << c << endl;

}

#include <iostream.h>main(){

int c = 0;__asm {

mov eax,cadd eax,1mov c,eax

}cout << c << endl;

}

Software based solutions Hardware based solutions Operating system based

solution

Solution of the Critical Problem

do {

critical section

reminder section

} while (1);

entry section

exit section

Structure of Solution

Solution to Critical-Section Problem

2-Process Critical Section Problem N-Process Critical Section Problem Conditions for a good solution:

1. Mutual Exclusion: If a process 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 some processes wish to enter their critical sections, then only those processes that are not executing in their remainder sections can decide which process will enter its critical section next, and this decision 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.


AssumptionsAssume that each process executes at a nonzero speed

No assumption can be made regarding the relative speeds of the N processes.

Possible Solutions Only 2 processes, P0 and P1 Processes may share some common

variables to synchronize their actions. General structure of process Pi

do {

critical section

remainder section} while (1);

entry section

exit section

Algorithm 1

Shared variables: int turn;initially turn = 0

turn = i Pi can enter its critical section

Algorithm 1 Process Pi

do {

critical section

remainder section

} while (1); Does not satisfy the progress condition

while (turn != i) ;

turn = j;

Only two processes (P0 and P1) executing concurrently

P0 executes first, enters the critical region… and leaves it, leaving turn = 1.

Turn = 1 means P1 can enter the critical region, and finishes the critical section quickly, now turn = 0.

P0 enters the loop and finishes its critical region quickly. Giving turn to P1 (turn = 1)

At this point as we assumed that there is no process speed considerations say process P0 gets ready to run again while P1 (Turn=1) is still in its remainder section.

P0 cannot enters it critical region because Turn = 1. Violation of Condition (2. Progress)

Process P1Process P1do {

while (turn != 1) ;critical sectionturn = 0;reminder section

} while (1);

Process P0 Process P0 do {

while (turn != 0) ;critical sectionturn = 1;reminder section

} while (1);

Algorithm 2

Shared variablesboolean flag[2]; // Set to false flag [i] = true Pi ready to

enter its critical section


do {

critical section

remainder section

} while (1); Does not satisfy the progress condition

flag[i] = true;

while (flag[j]) ;

flag[i] = false;

Problem!!!!! Problem!!!!!

Tx: P0 sets flag[0] true to enter its regionTy: P1 sets its flag i.e. flag[1] to true to enter

its regionA Deadlock!!!!!Both will wait forever to enter their regions.

Process P0do {

flag[0] := true;while (flag[1]) ;critical sectionflag [0] = false;remainder section

} while (1);

Process P1do {

flag[1] := true;while (flag[0]) ;critical sectionflag [1] = false;remainder section

} while (1);

Algorithm 3

Combined shared variables of algorithms 1 and 2.

boolean flag[2]; // Set to false int turn=0;


do {

critical section

remainder section

} while (1);

flag[i] = true;turn = j;while (flag[j] && turn == j) ;

flag[i] = false;

Algorithm 3Meets all three requirements:

Mutual Exclusion: ‘turn’ can have one value at a given time (0 or 1)

Bounded-waiting: At most one entry by a process and then the second process enters into its CS

Algorithm 3

Progress: Exiting process sets its ‘flag’ to false … comes back quickly and set it to true again … but sets turn to the number of the other process

Meets all the requirementsSolves critical section problem of the two

processes

Process Pido {

flag [0]:= true;turn = 1;while (flag [1] and turn = 1) ;critical sectionflag [0] = false;remainder section

} while (1);

Process Pido {

flag [1]:= true;turn = 0;while (flag [0] and turn = 0) ;critical sectionflag [1] = false;remainder section

} while (1);

n-Process Critical Section Problem

Consider a system of n processes (P0, P1 ... Pn-1).

Each process has a segment of code called a critical section in which the process may change shared data.

n-Process Critical Section ProblemWhen one process is executing

its critical section, no other process is allowed to execute in its critical section.

The critical section problem is to design a protocol to serialize executions of critical sections.

Bakery Algorithm

By Leslie Lamport Before entering its critical section,

process receives a ticket number. Holder of the smallest ticket number enters the critical section.

If processes Pi and Pj receive the same number, if i < j, then Pi is served first; else Pj is served first.

Bakery Algorithm

The ticket numbering scheme always generates numbers in the increasing order of enumeration; i.e., 1, 2, 3, 4, 5 ...

Bakery Algorithm

Notations

(ticket #, process id #)

(a,b) < (c,d) if a < c or if a == c and b <

dmax (a0,…, an-1) is a number, k,

such that k ai for i = 0, …, n–1

Bakery Algorithm

Data Structuresboolean choosing[n]; int number[n];

These data structures are initialized to false and 0, respectively

Bakery Algorithm

Structure of Pi

do { choosing[i] = true;number[i] = max(number[0],

number[1], …, number [n – 1]) + 1;

choosing[i] = false;

Bakery Algorithm

Critical Section

for (j = 0; j < n; j++) { while (choosing[j]) ; while ( (number[j] != 0) &&

((number[j], j) < (number[i], i)) ) ;}

Bakery Algorithm

remainder section

} while (1);

number[i] = 0;

Bakery Algorithm do {

choosing[i] = true;number[i] = max(number[0], number[1], …, number [n – 1])+1;choosing[i] = false;for (j = 0; j < n; j++) {while (choosing[j]) ; while ((number[j] != 0) && (number[j,j] < number[i,i])) ;}critical sectionnumber[i] = 0;remainder section

} while (1);

Bakery Algorithm

Process NumberP0 3P1 0P2 7P3 4P4 8

Bakery Algorithm

P0 P2 P3 P4(3,0) < (3,0) (3,0) < (7,2) (3,0) < (4,3) (3,0) < (8,4)

Number[1] = 0 Number[1] = 0 Number[1] = 0 Number[1] = 0

(7,2) < (3,0) (7,2) < (7,2) (7,2) < (4,3) (7,2) < (8,4)

(4,3) < (3,0) (4,3) < (7,2) (4,3) < (4,3) (4,3) < (8,4)

(8,4) < (3,0) (8,4) < (7,2) (8,4) < (4,3) (8,4) < (8,4)

P1 not interested to get into its critical section number[1] is 0

P2, P3, and P4 wait for P0

P0 gets into its CS, get out, and sets its number to 0

P3 get into its CS and P2 and P4 wait for it to get out of its CS

Bakery Algorithm

P2 gets into its CS and P4 waits for it to get out

P4 gets into its CS

Sequence of execution of processes:

<P0, P3, P2, P4>

Bakery Algorithm

Meets all three requirements:Mutual Exclusion: (number[j], j) < (number[i], i) cannot be

true for both Pi and Pj

Bounded-waiting: At most one entry by each process (n-1 processes) and then a requesting process enters its critical section (First-Come-First-Serve)

Bakery Algorithm

Progress: Decision takes complete

execution of the ‘for loop’ by one process

No process in its ‘Remainder Section’ (with its number set to 0) participates in the decision making

Bakery Algorithm

Synchronization HardwareSynchronization Hardware

Bakery algorithm is very difficult to implement. Next attempt was to solve issues with a little help of Hardware

Two methodsDisable InterruptsSpecial Hardware Instructions

Normally, access to a memory location excludes other accesses to that same location.

Extension: designers have proposed machine instructions that perform two operations atomically (indivisibly) on the same memory location (e.g., reading and writing).

Synchronization Hardware

The execution of such an instruction is also mutually exclusive (even on Multiprocessors).

They can be used to provide mutual exclusion but other mechanisms are needed to satisfy the other two requirements of a good solution to the CS problem.

Synchronization Hardware

2023年4月21日 © Copyright Virtual University of Pakistan

Test and modify a word atomically.

boolean TestAndSet(boolean &target)

{

boolean rv = target;

target = true;

return rv;

}

Test-And-Set (TSL) Instruction


Structure for Pi (‘lock’ is set to false)

while ( TestAndSet(lock) ) ;

lock = false;

do {

Critical Section

Remainder Section}

Solution with TSL

Is the TSL-based solution good?

No

Mutual Exclusion: SatisfiedProgress: SatisfiedBounded Waiting: Not satisfied

Solution with TSL


Swaps two variables atomically

void swap (boolean &a, boolean &b)

{

boolean temp = a;

a = b;

b = temp;

}

Swap Instruction


Structure for Pi ‘key’ is local and set to false

key = true;

while (key == true) swap(lock,key);

lock = false;

do {

Critical Section

Remainder Section}

Solution with Swap

Is the swap-based solution good?

No

Mutual Exclusion: SatisfiedProgress: SatisfiedBounded Waiting: Not satisfied

Solution with swap


waiting[i] = true;key = true;while (waiting[i] && key) key = TestAndSet(lock);waiting[i] = false;

do {

Critical Section

A Good Solution ‘key’ local; ‘lock’ and ‘waiting’ global All variables set to false


j = (i+1) % n;while ( (j != i) && !waiting[j] )

j = (j+1) % n;if (j == i)

lock = false;else

waiting[j] = false;

Remainder Section

A Good Solution

}

Is the given solution good?

Yes

Mutual Exclusion: SatisfiedProgress: SatisfiedBounded Waiting: Satisfied

Solution with Test-And-Set

Outline Process Synchronization The Critical-Section Problem Peterson’s Solution Synchronization Hardware Semaphores

Implementation Deadlocks and Starvation

Classic Problems of Synchronization The Bounded-Buffer Problem The Readers-Writers Problem The Dining-Philosophers problem

Semaphores

SemaphoresSynchronization tool

Available in operating systems

Semaphore S – integer variable that can only be accessed via two indivisible (atomic) operations, called wait and signal

Semaphores

Synchronization tool.Semaphore S – integer variableCan only be accessed via two indivisible (atomic) operations.

wait (S):

while S 0 do no-op;S--;

signal (S):

S++;Single CPU system disable all interrupts when executing Wait and

SignalMultiple CPU systems protect semaphore by TSL ..

Two Types of Semaphores

Counting semaphore – integer value (S) can range over an unrestricted domain.

Binary semaphore – integer value can range only between 0 and 1; can be simpler to implement.

Can implement a counting semaphore S as a binary semaphore.

Semaphoreswait(S){ while S 0

; //no-opS--;

}

signal(S){ S++;

}

n-Processes CS Problem Shared data:

semaphore mutex = 1;do { critical section


wait(mutex);

signal(mutex);

Remember wait and signal are atomic

P0

do { wait(mutex);

critical section signal(mutex);

remainder section

} while (1);

P1

do { wait(mutex);

critical section signal(mutex);


Wait executes as one indivisible operation that once executing cannot be preemptedSame holds for SignalIf both Processes execute Wait(mutex) at same time then only one would get access As Race condition would apply (wait and signal are atomic)

Is it a Good Solution?

Mutual Exclusion: YesProgress: YesBounded Waiting: No

SemaphoresMutexes

Mutex variable can have only two states.. Locked and Unlocked

Good when only mutual exclusion is requiredIf lock is 0 any process may set it to 1 using TSL

instruction and then READ or WRITE Shared dataIf lock is 1 then process may set it to 1 using TSL

(note no effect on lock state) and then Compare if it was 0 or not. It was non-zero (1) in this case hence process loops to check the state of the lock.

Mutex Code in Assembly

Enter_region:TSL Rx, Mutex //get lock value in Rx and Set lock to 1

CMP RX, 0 //Compare old value of Lock with 0

JZE ok //It was 0 hence enter critical region

CALL Thread_Yield //If already 1 then loop to enter_region

JMP enter_region

OKRET //If Lock was 0 then return and enter critical region

Leave_region:MOV MUTEX, 0 //Set lock to 0 so other waiting or looping process may enter

RET

Busy Waiting

Processes wait by executing CPU instructions

Problem? Wasted CPU cycles

Solution? Modify the definition of semaphore

Semaphore Implementation Define a semaphore as a record

typedef struct {

int value; struct process *L;} semaphore;

Semaphore ImplementationAssume two simple operations:

block() suspends the process that invokes it.

wakeup(P) resumes the execution of a blocked process P.

Semaphore ImplementationThe negative value of S.value

indicates the number of processes waiting for the semaphore

A pointer in the PCB needed to maintain a queue of processes waiting for a semaphore

Semaphore wait()Semaphore operations now

defined as void wait(semaphore S){

S.value--;if (S.value < 0) {

add this process to S.L;block();

}}

Semaphore signal()void signal(semaphore S){ S.value++; if (S.value <= 0) { remove process P from S.L; wakeup(P); }}

There are numerous Win32 API’s Block and Wakeup equivalent in win32 are ResumeThread(), SuspendThread,

WaitForSingleObject(), WaitForMultipleOjects() etc.. To create a semaphore variable

CSemaphore Object, CreateSemaphore() returns handle

Two Implementations

Busy-waiting version is better when critical sections are small and queue-waiting version is better for long critical sections (when waiting is for longer periods of time).


Use semaphore S initialized to 0 Pi Pj

A wait(S)signal(S) B

Execute statement B in Pj only after statement A has been executed in Pi

Process Synchronization Give a semaphore based solution

for the following problem:

Statement S1 in P1 executes only after statement S2 in P2 has executed, and statement S2 in P2 should execute only after statement S3 in P3 has executed.


P1 P2 P3 S1 S2 S3

Solution

Semaphore A=0, B=0;

P1 P2 P3 wait(A) wait(B) S3 S1 S2 signal(B)

signal(A)

Problems with Semaphores

Semaphores provide a powerful tool for enforcing mutual exclusion and coordinating processes.

But wait(S) and signal(S) are scattered among several processes. Hence, difficult to understand their effects.

Problems with Semaphores

Usage must be correct in all the processes.

One bad (or malicious) process can fail the entire collection of processes.

Deadlocks and Starvation A set of processes are said to be in

a deadlock state if every process is waiting for an event that can be caused only by another process in the set. Traffic deadlocks One-way bridge-crossing

Starvation (infinite blocking) due to unavailability of resources

Deadlock

P0 P1wait(S); wait(Q);wait(Q); wait(S);

signal(S); signal(Q);signal(Q); signal(S);

Deadlock

P0 P1

signal(S);

signal(Q);

Starvation (Infinite Blocking)

P0 P1wait(S); wait(S);

wait(S); signal(S);

Violation of Mutual Exclusion

P0 P1signal(S); wait(S);

wait(S); signal(S);

Main Cause of Problem and Solution

Cause: Programming errors due to the tandem use of wait() and signal() operations

Solution: Higher-level language constructs such as critical region (region statement) and monitor.

Producer consumer using Semaphores

#define N 100typedef int Semaphore;//Any critical instruction be executed by a semaphoreSemaphore mutex =1, empty = N, full = 0;void producer(void){

int item;while (TRUE){

item = produce_item();down(&empty);down(&mutex);insert_item(item);up(&mutex);up(&full);

}}

Producer consumer using Semaphores …

void customer(void){

int item;while(TRUE){

down(&full);down(&mutex);item = remove_item();up(&mutex);up(&empty);consume_item(item);

}}

Classical Problems of Synchronization

Bounded-Buffer Problem Do it yourself !

Readers and Writers ProblemDo it yourself

Dining-Philosophers ProblemSee Next Slides: But do it yourself

Readers-Writers Problem

Shared data

semaphore mutex, wrt;

Initially

mutex = 1, wrt = 1, readcount = 0

Readers-Writers Problem Writer Process

wait(wrt); …

writing is performed …

signal(wrt);

Readers-Writers Problem Reader Process

wait(mutex);readcount++;if (readcount == 1)

wait(wrt);signal(mutex);

…reading is performed

…wait(mutex);readcount--;if (readcount == 0)

signal(wrt);signal(mutex):

Dining-Philosophers Problem

Shared data

semaphore chopstick[5];

Initially all values are 1

Dining-Philosophers Problem Philosopher i:

do {wait(chopstick[i])wait(chopstick[(i+1) % 5])

…eat() …

signal(chopstick[i]);signal(chopstick[(i+1) % 5]);

…think …

} while (1);

Deadlock can occur if all the philosophers become hungry at the same time

Semaphores in Win32

CreateSemahore()WaitForSingleObject()ReleaseSemahore()

How to use SemaphoresHANDLE hs

hs= CreateSemaphore

( NULL, // no security attributes

InitialValue, // initial count .. For binary cMax =1

MaxValue, // maximum count .. For binary cMax =1

NULL); // pointer to semaphore-object name

WaitForSingleObject(hs,INFINITE);

count++; //simulation of critical region

ReleaseSemaphore(hs,1,NULL);

Monitors

Continuation of idea of ``object'' and object-oriented programming, abstract data types, data encapsulation, software engineering

Monitor contains programmer-defined operations that are provided mutual exclusion within the monitorOnly one process can be active inside a monitorProgrammer doesn’t need to code synchronization

explicitly

Integral part of the programming language so the compiler can generate the correct code to implement the monitor using the programming language run-time support

Monitors monitor monitor-name

{shared variable declarationsprocedure body P1 (…) {

. . .}procedure body P2 (…) {

. . .} procedure body Pn (…) {

. . .} {

initialization code}

}

Monitors… What if programmer wants explicit Synchronization

To allow a process to wait within the monitor, a condition variable must be declared, as

condition x, y;Condition variable can only be used with the operations wait

and signal. The operation

x.wait();means that the process invoking this operation is suspended until another process invokes

x.signal(); The x.signal operation resumes exactly one suspended process.

If no process is suspended, then the signal operation has no effect.

Normally x.signal resumes the process which first issued x.wait()

Dining Philosophers Example

monitor dp {

enum {thinking, hungry, eating} state[5];condition self[5];void pickup(int i) // following slidesvoid putdown(int i) // following slidesvoid test(int i) // following slidesvoid init()

{for (int i = 0; i < 5; i++)

state[i] = thinking;}

}

Dining Philosophers

void pickup(int i) {

state[i] = hungry;test[i];if (state[i] != eating)

self[i].wait();}

void putdown(int i) {

state[i] = thinking;// test left and right neighborstest((i+4) % 5);test((i+1) % 5);

}

Philosopher issues pickup() whenIt wants to eat. In this code it alsoChecks if chopsticks are available

Putdown is used after eating

Dining Philosophers

void test(int i) {

if ( (state[(i + 4) % 5] != eating) && (state[i] == hungry) && (state[(i + 1) % 5] != eating))

{state[i] = eating;self[i].signal();

}}

Check the states of neighboring philosophers. They must not be eatingi.e. must not be in critical region.

Dining Philosophers Example

Code is now simple. Dinning philospher picks up the chop sticks and if they are not available it waits

Dp.pickup(i)

Eat

Do.putdown(i)

Interesting Reading… Sec 2.3 Tanenbaum

Producer_Consumer Problem with monitors.

Base StructureMonitor ProducerConsumer

{Function Consumer

Function Producer

Function InsertItem

Function RemoveItem

}

Monitor With Condition Variables

Windows 2000 Synchronization

Uses interrupt masks to protect access to global resources on uniprocessor systems.

Uses spinlocks on multiprocessor systems.

Dispatcher objects may also provide events. An event acts much like a condition variable.

END OF CHAPTERNext: Memory Management

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