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Processes and ThreadsPart II

Chapter 2

2.1 Processes2.2 Threads2.3 Interprocess communication2.4 Classical IPC problems2.5 Scheduling

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2.3 Interprocess Communication2.3.1 Race Conditions

Two processes want to access shared memory (variable in) at same time and they may put their files in the same slot

Race Condition: When more than one process shares the same memory and the final result depends on the order of execution

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2.3.2 Critical Regions (1)

Four conditions to provide mutual exclusion1. No two processes simultaneously in critical

region2. No assumptions made about speeds or

numbers of CPUs3. No process running outside its critical region

may block another process4. No process must wait forever to enter its

critical region

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2.3.2 Critical Regions (2)

Mutual exclusion using critical regions

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2.3.3 Mutual Exclusion with Busy Waiting (1)

• Disabling interrupts– Each process disables all interrupts just after

entering its critical region(CR) and enables all interrupts before leaving it

– Unsafe for user processes able to turn off all interrupts

• Lock Variables– Having a shared variable (lock), initially 0– Before entering its CR the process checks lock, if

lock is1, it waits; if lock is 0, it sets lock to 1 and enters its CR.

– Before leaving its CR, the process resets lock to 0 – It does not work, why?

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2.3.3 Mutual Exclusion with Busy Waiting (2)

Proposed solution to critical region problem(a) Process 0. (b) Process 1.Variable turn is shared by both processes

Strict Alternation

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2.3.3 Mutual Exclusion with Busy Waiting(3)Peterson's solution

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Mutual Exclusion with Busy Waiting (4)

TSL(Test and Set Lock) Machine instructionFormat: TSL RX, LOCK // LOCK is a shared variable, RX: a register

Function: RX LOCK

LOCK 1

Since it is a machine instruction, so it is always executed atomically.

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2.3.5 Semaphores (1)• Definition:

Class Semaphore {Private:

value: integer;list: list of processes

Public:void down() {

value = value –1;if value < 0 {

list.insert(this process)sleep();

}}

void up() {value = value + 1;if value <= 0 {

p = list.remove();wakeup(p);

}}

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Semaphore (2)

• Semaphores are system facilities

• The up() and down() operations are system calls.

• The operating system guarantees that the two operations are atomic operations. I.e., when an up() or down() is being executed, no other up() and down() would be executed on the same semaphore.

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Semaphore: application • Control execution sequence

– E.g., three processes: p1, p2 and p3, and p3’s stmnt3 must be executed after p1’stmnt1 and p2’s stmnt2 have completed.Semaphore seq = 0;P1 p2 p3… … …Stmnt1; stmnt2 Seq.down();Seq.up(); Seq.up(); Seq.down();… … stmnt3

• Mutual exclusionSemaphore mutex = 1;P1 p2Mutex.down(); mutex.down();Critical region; critical regionMutex.up(); mutex.up()

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2.3.5 Semaphores: application

The producer-consumer problem using semaphores

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2.3.5 Implementations of Semaphore (3)

• Disable all interrupts

• Use Peterson’s algorithm and treat the up() and down() operations as critical regions.

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2.3.6 Mutexes

Implementation of mutex_lock and mutex_unlock

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2.3.8 Message Passing (1)

• Processes can communicate with each other via message passing.

• The operating system provides two primitive functions:– Send(dest, &message);– Receive(source, &message);

• Design issues:– Sender waits for acknowledgement, no buffer needed– When sender does not wait, the OS must maintain a

buffer– Message scramble and loss

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2.3.8 Message Passing (2)

The producer-consumer problem with N messages

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2.4 Classical IPC Problems 2.4.0 The producer and consumer problem

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2.4.1 Dining Philosophers (1)

• Philosophers eat/think• Eating needs 2 forks• Pick one fork at a time

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2.4.1 Dining Philosophers (2)

A nonsolution to the dining philosophers problemDeadlock? How? How to prevent deadlock

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2.4.2 The Readers and Writers Problem (1)

• Multiple readers and writers need to access the database

• Readers only read the content• Writers modify the database• Multiple readers may read at the same time• Only one writer is allowed to write at a time• When a writer is writing, no reader is allowed

to read

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2.4.2 The Readers and Writers Problem (2)