Latches and Barriers: Synchronizing Concurrent Operations
In concurrent programming, coordinating the execution of multiple threads is crucial to avoid race conditions and ensure correct program behavior. Latches and Barriers are synchronization primitives that help manage the timing and dependencies between threads, allowing them to wait for each other before proceeding.
Understanding Latches
A latch is a synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes. Think of it as a gate that remains closed until a specific number of 'signals' are received. Once the required number of signals is received, the latch opens, and all waiting threads are released.
Latches are used to wait for a specific number of events to occur.
A latch starts with a count. Threads can wait on the latch. Other threads decrement the count. When the count reaches zero, all waiting threads are released.
In C++, std::latch (introduced in C++20) is a synchronization primitive that allows threads to wait until a specified number of events have occurred. It is initialized with a count. Threads can call wait() to block until the latch's count reaches zero. Other threads can signal the latch by calling count_down(), which decrements the count. When the count reaches zero, all threads waiting on the latch are unblocked. A latch can only be used once; after its count reaches zero, it remains open.
std::latch?To allow one or more threads to wait until a specified number of events (signaled by count_down()) have occurred.
Understanding Barriers
Barriers are similar to latches in that they synchronize threads, but they are designed for scenarios where multiple threads need to reach a common point in their execution before any of them can proceed. Unlike latches, barriers can be reset and reused.
Barriers synchronize threads at a common execution point.
Threads arrive at a barrier and wait. Once all threads arrive, they are all released simultaneously. The barrier then resets for the next phase.
A barrier, often represented by std::barrier in C++20, is a synchronization primitive that allows a set of threads to wait for each other to reach a common point in their execution. When a thread arrives at the barrier using arrive_and_wait(), it blocks until all participating threads have also arrived. Once the last thread arrives, all waiting threads are released. Barriers are typically used in iterative algorithms where each iteration must complete before the next can begin for all threads.
| Feature | Latch | Barrier |
|---|---|---|
| Primary Use | Waiting for a specific number of events | Synchronizing threads at a common execution point |
| Reusability | Single-use | Reusable (resets automatically) |
| Signaling | Decrementing a count (count_down()) | All threads arriving at the barrier (arrive_and_wait()) |
| C++ Standard | C++20 | C++20 |
Practical Applications and Considerations
Latches are useful for scenarios like initializing resources that multiple threads depend on, or for signaling the completion of a specific phase of work. Barriers are ideal for parallel algorithms that proceed in stages, such as parallel sorting or matrix operations, where each stage must be fully completed by all threads before the next stage can commence.
When choosing between a latch and a barrier, consider if your synchronization point needs to be reused. If so, a barrier is the appropriate choice. If it's a one-time synchronization event, a latch is simpler and more efficient.
Imagine a group of runners starting a relay race. The starting pistol (latch) signals the first runner to go. The baton exchange at each stage is like a barrier; all runners in that stage must complete their leg before the next runner can start. The entire race is a sequence of such synchronized events.
Text-based content
Library pages focus on text content
C++20 Implementation Details
Both
std::latch
std::barrier
std::barrier?The <barrier> header.
Learning Resources
Official documentation for `std::latch`, detailing its members, usage, and behavior in C++20.
Official documentation for `std::barrier`, explaining its functionality, phases, and how to use it for thread synchronization.
A blog post explaining the concepts of latches and barriers with practical C++20 examples and code snippets.
A video tutorial that visually explains and demonstrates the usage of `std::latch` and `std::barrier` in C++20.
An article discussing various C++20 concurrency features, including a section dedicated to latches and barriers.
An excerpt from a well-regarded book on C++ concurrency, covering fundamental synchronization mechanisms.
A discussion on effective use of synchronization primitives in C++, offering best practices and common pitfalls.
A live code example demonstrating `std::barrier` in action, allowing experimentation and observation.
A live code example showcasing `std::latch` usage, providing a practical demonstration of its functionality.
A comprehensive overview of various concurrency control mechanisms in C++, including latches and barriers within the broader context.