Concurrency: Mutual Exclusion and Synchronization Techniques

Mutual Exclusion

The ability to exclude all other processes from a course of action while one process is granted that ability.

Dekker's Algorithm

A software approach to achieve mutual exclusion.

Peterson's Algorithm

A software approach to achieve mutual exclusion.

Race Condition

A situation where two or more processes access shared data and they try to change it at the same time.

Operating System Concerns

Issues related to the management of processes and threads in an operating system.

Process Interaction

The way processes communicate and synchronize with each other.

Requirements for Mutual Exclusion

Conditions that must be met to ensure that mutual exclusion is achieved.

Interrupt Disabling

A hardware support mechanism that prevents interrupts to enforce mutual exclusion.

Special Machine Instructions

Hardware instructions designed to support mutual exclusion.

Semaphores

A synchronization mechanism that can be used to control access to a common resource in concurrent programming.

Producer/Consumer Problem

A classic concurrency problem that illustrates the use of semaphores.

Monitors

A synchronization construct that allows safe access to shared resources.

Monitor with Signal

A type of monitor that uses signals to manage process synchronization.

Alternate Model of Monitors with Notify and Broadcast

A model of monitors that includes notify and broadcast mechanisms for process synchronization.

Message Passing

A method of communication between processes that can be used for synchronization.

Queueing Discipline

The order in which messages are processed in a message-passing system.

Readers/Writers Problem

A classic synchronization problem that involves managing access to a shared resource by multiple readers and writers.

Readers Have Priority

A solution to the readers/writers problem where readers are given preference over writers.

Writers Have Priority

A solution to the readers/writers problem where writers are given preference over readers.

Multiprogramming

The management of multiple processes within a uniprocessor system.

Multiprocessing

The management of multiple processes within a multiprocessor system.

Distributed Processing

The management of multiple processes executing on multiple, distributed computer systems.

Concurrency

The ability of the operating system to manage multiple processes and threads simultaneously.

Busy Waiting

A technique used in software solutions to achieve mutual exclusion where a process repeatedly checks for a condition.

Atomic operation

A function or action implemented as a sequence of one or more instructions that appears to be indivisible; that is, no other process can see an intermediate state or interrupt the operation. The sequence of instruction is guaranteed to execute as a group, or not execute at all, having no visible effect on system state. Atomicity guarantees isolation from concurrent processes.

Critical section

A section of code within a process that requires access to shared resources, and that must not be executed while another process is in a corresponding section of code.

Deadlock

A situation in which two or more processes are unable to proceed because each is waiting for one of the others to do something.

Livelock

A situation in which two or more processes continuously change their states in response to changes in the other process(es) without doing any useful work.

Mutual exclusion

The requirement that when one process is in a critical section that accesses shared resources, no other process may be in a critical section that accesses any of those shared resources.

Race condition

A situation in which multiple threads or processes read and write a shared data item, and the final result depends on the relative timing of their execution.

Starvation

A situation in which a runnable process is overlooked indefinitely by the scheduler; although it is able to proceed, it is never chosen.

Dekker's Algorithm

An algorithm for mutual exclusion for two processes, designed by the Dutch mathematician Dekker.

Busy waiting

A procedure where a process repeatedly reads the value of a variable until it is allowed to enter its critical section, consuming processor time while waiting.

turn

A shared global variable that indicates which process can enter its critical section.

mutual exclusion property

A property that ensures that only one process can access its critical section at a time.

coroutine

A programming construct that allows multiple routines to cooperate by passing control back and forth.

first attempt

An initial solution where processes strictly alternate their use of the critical section, leading to potential blocking.

second attempt

An improvement where each process has its own flag to indicate its status, allowing for better handling of failures.

flag

A Boolean vector used to indicate whether a process is in its critical section, with flag[0] for P0 and flag[1] for P1.

critical section

A part of the program where shared resources are accessed and modified.

Boolean vector

An array that holds Boolean values, used to track the state of each process regarding access to the critical section.

P0

The first process in the mutual exclusion algorithm.

P1

The second process in the mutual exclusion algorithm.

flag[0]

The flag corresponding to process P0, indicating whether it is in its critical section.

flag[1]

The flag corresponding to process P1, indicating whether it is in its critical section.

failure outside critical section

When a process fails while not in its critical section, allowing the other process to continue execution.

failure inside critical section

When a process fails while in its critical section, causing the other process to be permanently blocked.

execution pace

The rate at which a process executes its critical section, which can be dictated by the slower process.

first process execution rate

If P0 uses its critical section only once per hour.

second process execution rate

If P1 would like to use its critical section at a rate of 1,000 times per hour.

blocking

A situation where one process cannot proceed because it is waiting for another process to release a resource.

checking process

The process that periodically checks the other process's flag to determine if it can enter its critical section.

setting flag to true

The action taken by a process to indicate it is about to enter its critical section.

setting flag to false

The action taken by a process to indicate it has exited its critical section.

algorithm

A step-by-step procedure for solving a problem, in this case, managing access to critical sections.

permanent blocking

A condition where one process is indefinitely unable to proceed due to the failure of another process.

Mutual Exclusion

A condition where two or more processes cannot access a critical section simultaneously.

Flag

A boolean variable used to indicate the intention of a process to enter its critical section.

Deadlock

A situation where two or more processes cannot proceed because each is waiting for the other to release resources.

Livelock

A condition where processes continuously change states in response to each other without making progress.

Dekker's Algorithm

A solution for mutual exclusion that uses flags and a turn variable to manage access to critical sections.

Execution Speed

The rate at which a process executes instructions, which can affect mutual exclusion.

Flag Setting

The action of a process setting its flag to true to indicate its desire to enter the critical section.

Process Blocking

A state where a process cannot proceed because it is waiting for another process to release a resource.

State Observation

The ability to check the status of other processes to make decisions about entering critical sections.

Turn Variable

A variable that indicates which process has the right to enter its critical section.

Interchange of Statements

A method used to modify the order of operations in a program to resolve issues like deadlock.

Flag Resetting

The action of a process resetting its flag to defer to another process's desire to enter the critical section.

Process P0

The first process in the mutual exclusion scenario being analyzed.

Process P1

The second process in the mutual exclusion scenario being analyzed.

While Statement

A control structure used to repeatedly check a condition, often used in mutual exclusion algorithms.

Flag[0]

The flag variable associated with process P0.

Flag[1]

The flag variable associated with process P1.

Execution Sequence

The order in which processes execute their instructions, which can affect mutual exclusion.

Mutual Courtesy

A situation where processes defer to each other, potentially leading to livelock.

Proposed Solution

An approach to resolving issues in mutual exclusion algorithms, which may not always be effective.

Process State Change

An alteration in the status of a process that can affect mutual exclusion.

Indefinite Extension

A scenario where processes can continue to execute without making progress due to mutual exclusion issues.

Incorrect Program Behavior

A situation where the program does not function as intended due to flaws in mutual exclusion implementation.

Dekker's Algorithm

A solution for mutual exclusion where processes set flags and check turns to enter critical sections.

flag

A boolean array indicating whether a process is interested in entering its critical section.

turn

An integer variable that indicates whose turn it is to enter the critical section.

P0

A process that attempts to enter its critical section by setting its flag and checking the other process's flag.

P1

Another process that also attempts to enter its critical section using a similar mechanism as P0.

critical section

A section of code that must be executed by only one process at a time to prevent data inconsistency.

parbegin

A construct that initiates concurrent execution of multiple procedures.

mutual exclusion

A property that ensures that only one process can access a critical section at a time.

Peterson's Algorithm

A simpler solution for mutual exclusion that uses flags and a turn variable to manage access to critical sections.

mutual blocking

A situation where two processes are blocked from entering their critical sections due to each other's flags.

while loop

A control flow statement that allows code to be executed repeatedly based on a condition.

boolean

A data type that can hold one of two values: true or false.

int

A data type used to represent integer values.

P0's flag setting

P0 sets flag[0] to true to indicate its interest in entering the critical section.

P1's flag setting

P1 sets flag[1] to true to indicate its interest in entering the critical section.

turn = 1

Indicates that it is P1's turn to enter the critical section.

turn = 0

Indicates that it is P0's turn to enter the critical section.

flag[0] = false

Indicates that P0 is no longer interested in entering its critical section.

flag[1] = false

Indicates that P1 is no longer interested in entering its critical section.

while (turn == 1)

A condition that causes P0 to wait if it is not its turn.

while (turn == 0)

A condition that causes P1 to wait if it is not its turn.