memory_order

Concept: memory_order

In C++, memory_order is an enumeration used in conjunction with atomic operations provided by the C++ Standard Library to specify how memory operations are ordered across different threads. Understanding and correctly using memory_order is crucial in multi-threaded programming to ensure proper synchronization and avoid subtle concurrency bugs.

Background

In multi-threaded environments, atomic operations guarantee that a variable is modified or accessed by only one thread at a time, preventing race conditions. However, the ordering of these operations with respect to other memory operations is also critical. This is where memory_order comes into play. It allows you to specify how memory operations related to atomics are observed by other threads.

memory_order Enum Values

The memory_order enumeration has several values, each specifying a different level of ordering constraints:

• memory_order_relaxed:
  • No synchronization or ordering constraints. The operation is atomic, but no other guarantees are made.
  • Useful when you only care about the atomicity of operations but not their order with respect to other operations.
• memory_order_consume (deprecated in some compilers, often behaves like memory_order_acquire):
  • Ensures that subsequent operations that depend on the result of the atomic operation are not moved before it.
  • This is a weaker version of memory_order_acquire.
• memory_order_acquire:
  • Ensures that all subsequent memory operations (in the current thread) are not reordered before the atomic operation.
  • Used when reading from an atomic variable to ensure that all writes made before releasing that variable by another thread are visible.
• memory_order_release:
  • Ensures that all previous memory operations (in the current thread) are not reordered after the atomic operation.
  • Used when writing to an atomic variable to ensure that all previous writes are visible to any thread that acquires the same variable.
• memory_order_acq_rel:
  • Combines the effects of both memory_order_acquire and memory_order_release.
  • Ensures that operations before the atomic are not moved after it and operations after the atomic are not moved before it.
  • Typically used for read-modify-write operations.
• memory_order_seq_cst (Sequential Consistency):
  • Provides the strongest guarantees, ensuring a total global order of operations.
  • All operations with memory_order_seq_cst are seen in the same order by all threads.
  • This is the default and most restrictive memory order, but also the safest.

Example 1:Basic Usage

Let’s look at an example where we use memory_order with atomic variables to control the visibility and ordering of operations across threads.

#include <iostream>
#include <atomic>
#include <thread>

std::atomic<int> data(0);
std::atomic<bool> ready(false);

void producer() {
    data.store(42, std::memory_order_relaxed);  // Store data with relaxed ordering
    ready.store(true, std::memory_order_release);  // Store ready with release semantics
}

void consumer() {
    while (!ready.load(std::memory_order_acquire));  // Acquire ready to synchronize with producer
    std::cout << "Data: " << data.load(std::memory_order_relaxed) << std::endl;
}

int main() {
    std::thread t1(producer);
    std::thread t2(consumer);

    t1.join();
    t2.join();

    return 0;
}

Explanation

• Producer Thread:

  • data.store(42, std::memory_order_relaxed); writes 42 to data without any ordering constraints.
  • ready.store(true, std::memory_order_release); writes true to ready with release semantics, ensuring that the write to data happens before ready is set to true.

• Consumer Thread:

  • while (!ready.load(std::memory_order_acquire)); waits until ready becomes true. The acquire operation ensures that any subsequent reads (like data.load) see all writes that happened-before the release operation in the producer.
  • std::cout << "Data: " << data.load(std::memory_order_relaxed) << std::endl; reads data. Since we acquired ready, we are guaranteed to see the correct value (42) written by the producer.

Why Use Different memory_order?

• Performance: Weaker memory orders like memory_order_relaxed or memory_order_acquire/release can be more performant than memory_order_seq_cst because they allow the compiler and hardware more freedom to optimize.
• Correctness: Using stronger memory orders like memory_order_acquire, memory_order_release, and memory_order_seq_cst ensures proper synchronization between threads, which is critical for avoiding subtle bugs in multi-threaded programs.

Common Use Cases:

• memory_order_relaxed: Use when you only need atomicity, not ordering.
• memory_order_acquire/release: Use for synchronizing between threads, where one thread signals the completion of some work and another thread waits to start its own work after that.
• memory_order_seq_cst: Use when you need the strongest guarantees and want to ensure a consistent view of operations across all threads.

Summary

• memory_order controls the visibility and ordering of atomic operations across threads.
• Different memory_order values provide different levels of synchronization and performance.
• Choose the appropriate memory_order based on your synchronization needs: stronger orders provide more guarantees but can be slower, while weaker orders are faster but offer fewer guarantees.

Correctly using memory_order requires careful consideration of how different threads interact and the specific needs of your application’s synchronization.