multiprocessing -interprocessing communication and process sunchronization,semaphore

INTERPROCESSOR COMMUNICATION AND

PROCESS SYNCHRONIZATION

Presented ByNeena R Krishna

S8,BTECH CSESNGIST

CONTENTS

• INTRODUCTION• INTERPROCESSOR COMMUNICATION• PROCESS SYNCHRONIZATION AND

SEMAPHORE

INTRODUCTION

• Independent process cannot affect or be affected by the execution of another process

• Cooperating process can affect or be affected by the execution of another process

• Advantages of process cooperation – Information sharing – Modularity – Convenience

Interprocessor Communication• Exchange of data between two or more processes.– Os provide facilities for IPC. • IPC facility provides two operations:

send(message)and receive(message) (message size fixed or

variable). • If P and Q wish to communicate, they need to:–establish a communication link between them

exchange messages via send/receive

Contd..

• Multiprocessor and multiple memory modules are connected together via some interconnection network.

• They fall on two categories:-1)Shared memory2)Message passing

INTERPROCESSOR COMMUNICATION THROUGH ADDRESS SPACE SHARING

INTERPROCESSOR COMMUNICATION VIA KERNAL SPACE

SHARED MEMORY

• Processors exchange information through their central shared memory system.

• Inter co-ordination through a global memory shared by all processor.

• Server system that communicate through a bus and cache memory controller.

• Access to shared memory is balanced – Symmetric multiprocessor

Contd..

• Each processor has equal opportunities to read/write to memory including equal access speed.

PROCESSORs

Registers

Local memory banks (additional memory resource

Cache

Buffers

Contd..

• Basic issues in the design of shared memory system have to be taken in consideration :-1)Access Control2)Synchronization3)Protection and Security

ACCESS CONTROL

• Determines which process access are possible to which resource.

• The latter contain flag that determine the legality of each access attempt.

• If there are access attempt to resources then until the desired access is completed, all disallowed access are attempt and illegal process are blocked.

SYNCHRONIZATION

• Constraints limit the time of access from sharing processes to shared resource.

• Appropriate synchronization ensures that the information flows properly and ensure system functionality.

PROTECTION AND SECURITY

• It is a system feature that prevents processes from making arbitrary access to resource belonging to other processes.

• Sharing and protection are incompatible:-– Sharing allow access & protection restricts it.

FEATURES

• All communications are done using implicit loads and stores to a global address space.

• Synchronization and communication are distinct.

MESSAGE PASSING

• Combine the local memory and processor at each node of the interconnection network.

• There is no global memory.– Move data from one local memory to another by

means of message passing.– Done by a send/reciever pair of commands and

dealing consistency issues.– Eg: c.1990 Ncube.

Contd..

• A node in message passing system of processor and its local memory.

• They are able to store message buffers and able to perform send/receive operation at the same time as processing.

• Processor do not share a global memory and each processor has access to its own address space.

• Basic advantage :-– Scalable.

SYNCHRONIZATION

• Required when one process must wait for another to complete some operation before proceeding.– Eg:-one process (called a writer) may be writing

data to a certain main memory area, while another process (a reader) may be reading data from that area and sending it to the printer. The reader and writer must be synchronized so that the writer does not overwrite.

IMPORTANCE OF SYNCHRONIZATION

• Prevention and elimination of race conditions, deadlocks and starvation.

• Data integrity/consistency.• Collaboration.• Efficiency.

SYNCHRONIZATION PROBLEMS

• Race conditions– the exact timing in the execution of concurrent

processes is critical• Critical sections– part of a program that must be protected from

interference– usually resolved via mutual exclusion

• Mutual exclusion– the usage of resources by processes is restricted– usually: only one process may use one instance of a

resource

RACE CONDITION

TYPES OF SYNCHRONIZATION

• Barrier • Lock• Semaphore

BARRIER

• Usually implies that all tasks are involved• Each task performs its work until it reaches

the barrier. It then stops, or "blocks".• When the last task reaches the barrier, all

tasks are synchronized.• What happens from here varies. Often, a serial

section of work must be done. In other cases, the tasks are automatically released to continue their work.

LOCKSuppose there are 2 processes – Write-process 1 and Read-process 2

PROCESS 1

WRITE

MEMORY LOCKED

AFTER WRITING

UNLOCK MEMORY

PROCESS 2

READ

Contd..• The LOCK(x) operation may be implemented

as follows:Var x:shared integer;LOCK (x):beginvar y: integer;y x;

While y =1 do yx;//wait until gate is open //x1 //set gate to unavailable status //end

• The UNLOCK(x) operation may be implemented as

UNLOCK(x): x 0;

SEMAPHORE

• It is a resource that contains an integer value and allows process to synchronize by testing and setting this value on a single atomic operations.– Process that test the value of a semaphore and

sets it to a different value,is guaranteed no other process will interfere with the operation in the middle.

Contd..

• Two types of operations :-– Wait– Signal.

FEATURES OF SEMAPHORE• Semaphores are used to synchronize operations

when processes access a common, limited, and possibly non-shareable resource.

• Each time a process wants to obtain the resource, the associated semaphore is tested. A positive, non-zero semaphore value indicates the resource is available.

• Semaphore system calls will, by default, cause the invoking process to block if the semaphore value indicates the resource is not available

TYPES OF SEMPAHORE

• COUNTING SEMAPHORE– Semaphores controlling access to N (multiple)

resources, thus assuming a range of non-negative values, are frequently.

• BINARY SEMAPHORE– Semaphores that control access to a single

resource, taking the value of 0 (resource is in use) or 1 (resource is available).

SEMAPHORE IMPLEMENTATION

var s: semaphoreP(s): MUTEXBEGIN (s) s s-1; If s < 0 then begin Block the process executing the P(s) and

put it in a FIFO queue associated with the semaphore s;

end MUTEXEND

V(s): MUTEXBEGIN (s) SS + 1; If s < 0 then begin if an inactive process associated with

semaphore s exists, then wake up the highest priority blocked process associated with s and

put it in a ready list. end MUTEXEND

EXAMPLE

PRODUCER CONSUMER PROBLEMProducerWhile(true){

while(((in+1) % Buffer_Size)==out);buffer[in]=item;in=(in+1)%Buffer_Size;

}

Contd..

ConsumerWhile(true){

while(in==out);item=buffer[out];out=(out+1)%Buffer_Size;return item;

}

Contd..

Solution with Semaphore

Empty=n,full=0.mutex=1ProducerDo{

//Produce itemwait(empty);wait(mutex);//add item to the buffersignal(mutex);signal(full);

}while(true);

Contd..

ConsumerDo{

wait(full);wait(mutex);//Remove an item from buffersignal(mutex);signal(empty);//consume the remove item

}while(true);