When dealing with concurrent programming in C++, one frequently encounters the dilemma of choosing between std::atomic variables and ordinary non-atomic variables for maintaining shared state between multiple threads. Both have their distinct applications, and understanding their performance implications is crucial. This article delves into the differences in performance between these two types of variables, providing technical explanations and examples.

Understanding std::atomic

std::atomic is a template class in the C++ Standard Library that provides atomic operations on a single variable. Atomic operations are operations that complete in a single step relative to other threads, essentially making them indivisible. When a thread performs an atomic operation, it's guaranteed that no other thread will be able to see the operation half-complete.

Key Features of std::atomic

1. Atomicity: Ensures that operations on variables are completed in a single step.
2. Memory Ordering: Provides memory order options to control the visibility and ordering of operations.
3. Lock-free Programming: Often implemented using lock-free constructs, providing efficiency advantages.

Non-Atomic Variables

Non-atomic variables (regular variables) do not have the atomicity guarantees that std::atomic provides. In a multithreaded environment, accessing or modifying these variables without proper synchronization can lead to race conditions, where the behavior depends on the sequence or timing of the threads' execution.

Performance Comparison

The performance difference between std::atomic and non-atomic variables primarily boils down to the overhead of ensuring atomicity and synchronization versus the simplicity of unsynchronized access.

Example Scenario

Consider the following simplified scenario where a counter variable is accessed by multiple threads.

• Execution Time: Atomic operations are generally slower than non-atomic operations due to the overhead of ensuring atomicity. This involves complex processor instructions and memory fencing, which can slow down execution.
• Thread Synchronization: std::atomic naturally provides a means to prevent data races, whereas non-atomic variables would require explicit locking mechanisms such as mutexes, which could introduce even more overhead if not used correctly.
• std::memory_order_relaxed: No synchronization, only atomicity.
• std::memory_order_acquire/release: Ensures that memory operations before acquire are visible after release.
• std::memory_order_seq_cst: The strictest, providing a total order.