Atomic Operations - TarisMajor/5143-OpSystems GitHub Wiki

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Atomic operations are fundamental synchronization primitives used in concurrent programming to ensure that specific operations on shared resources are completed without interruption. These operations are indivisible, meaning they either complete entirely or do not execute at all, thereby preventing race conditions and ensuring data consistency in multithreaded environments.

Key Characteristics of Atomic Operations

  1. Indivisibility: Atomic operations are executed as a single, uninterruptible unit, ensuring that no other operations can interleave during their execution.
  2. Consistency: By preventing partial updates to shared resources, atomic operations maintain data consistency and integrity.
  3. Hardware Support: Many modern processors provide hardware instructions that support atomic operations, such as Test-and-Set, Compare-and-Swap, and Fetch-and-Add.

Advantages of Atomic Operations

  1. Efficient Synchronization: Atomic operations provide a lightweight and efficient mechanism for synchronizing access to shared resources, often with less overhead than locks or other synchronization mechanisms.
  2. Simplified Coding: They simplify the implementation of concurrent algorithms by reducing the need for complex locking and unlocking logic.
  3. Reduced Contention: Because atomic operations are fast and executed without blocking, they reduce contention among threads and improve overall system performance.

Disadvantages of Atomic Operations

  1. Limited Scope: Atomic operations are typically limited to simple operations, such as increments, decrements, or comparisons. More complex synchronization tasks may still require higher-level synchronization primitives.
  2. Hardware Dependency: The availability and efficiency of atomic operations depend on the underlying hardware support, which can vary across different processors and architectures.
  3. Complexity for Large Tasks: Implementing larger or more complex atomic operations can be challenging and may require combining multiple atomic instructions, which can introduce complexity.

Common Atomic Operations

  1. Test-and-Set (TAS): Atomically tests a memory location and sets it to a new value if it matches an expected old value. This operation is commonly used to implement spinlocks.
  2. Compare-and-Swap (CAS): Compares the value at a memory location to an expected value and, if they match, swaps it with a new value. CAS is a versatile atomic operation used in various synchronization algorithms.
  3. Fetch-and-Add: Atomically increments a value at a memory location and returns the old value. This operation is useful for implementing counters and other shared data structures.

Use Cases for Atomic Operations

  1. Implementing Spinlocks: Atomic operations like Test-and-Set are commonly used to implement spinlocks, ensuring mutual exclusion in critical sections.
  2. Lock-Free Data Structures: Atomic operations enable the implementation of lock-free data structures, which improve concurrency and reduce contention in multithreaded environments.
  3. Reference Counting: Atomic increments and decrements are used for reference counting in memory management systems, ensuring accurate and thread-safe updates to reference counts.

Example of Atomic Operation

Consider an example of using the Compare-and-Swap (CAS) operation to implement a simple atomic increment function in C:

Pseudocode:

#include <stdatomic.h>

// Atomic increment function using Compare-and-Swap (CAS)
void atomic_increment(volatile int *ptr) {
    int old_value, new_value;
    do {
        old_value = *ptr;
        new_value = old_value + 1;
    } while (!atomic_compare_exchange_weak(ptr, &old_value, new_value));
}

In this example, the atomic_increment function uses the CAS operation to atomically increment the value at the memory location pointed to by ptr. The atomic_compare_exchange_weak function compares the current value of ptr with old_value and, if they match, sets ptr to new_value. This operation is repeated until the update is successful.

Sources for Further Reading