Using Execution Policies
Explore how to use C++ execution policies like sequential, parallel, and parallel unsequenced to optimize algorithm performance. Understand task partitioning, vectorization, and safety considerations to write thread-safe parallel code using the Standard Library's <execution> header.
In the previous lesson, we saw that modern hardware achieves performance primarily through parallel execution. However, writing low-level multi-threaded code directly is complex, error-prone, and difficult to maintain.
C++ offers a higher-level alternative through parallel execution policies. Rather than managing threads explicitly, we can instruct the Standard Library to execute familiar algorithms in parallel. In many cases, enabling parallelism requires changing only a single argument, allowing us to leverage multi-core processors and vectorized execution without increasing code complexity.
In this lesson, we will explore in detail how to apply execution policies to standard algorithms to achieve cleaner code and improved performance.
The <execution> header and standard policies
To enable parallel algorithms, we must include the <execution> header. This header defines execution policies, which instruct the Standard Library how an algorithm is allowed to execute its work.
The C++ standard defines three primary execution policies:
std::execution::seq(sequential): This is the default behavior. The algorithm executes entirely on the calling thread, just like traditional C++ algorithms. This policy is useful when parallelism is undesirable; for example, during debugging or when the dataset is too small to justify the overhead of parallel execution.std::execution::par(parallel): This policy permits the algorithm to divide its work across multiple threads. The runtime system, ...