Boost::concurrent_flat_map: A High-Performance Associative Container for Parallel Scenarios

Boost::concurrent_flat_map, introduced in Boost 1.83, is a powerful associative container designed specifically for high-load parallel scenarios. This container leverages the work done for boost::unordered_flat_map but introduces innovative features in low-contention operation and API design.

In the world of C++ concurrent hashmaps, there are various competing techniques, ranging from traditional lock-based structures to specialized approaches like hazard pointers and Read-Copy-Update (RCU). Boost::concurrent_flat_map stands out as a general-purpose container, offering a reference interface that is flexible and efficient.

One notable design decision is the absence of iterators. Instead, boost::concurrent_flat_map provides an access API based on internal visitation. This approach eliminates the potential risks of unsafe or blocking iterators, reducing contention and the possibility of deadlocks.

Under the hood, boost::concurrent_flat_map adopts the open-addressing layout used by boost::unordered_flat_map. The bucket array is split into 2n groups, each with 15 slots and an associated 16-byte metadata word for SIMD-based reduced-hash matching and insertion overflow control. This layout ensures efficient and scalable performance.

To maintain thread safety, boost::concurrent_flat_map incorporates two levels of synchronization. The core algorithms are variations of those found in boost::unordered_flat_map, with minimal modifications to prevent data races and minimize group-level contention.

The lookup algorithm of boost::concurrent_flat_map is predominantly lock-free, with only a few steps executed within the scope of a group lock. SIMD matching plays a crucial role in identifying potential candidates, while double-checking for slot occupancy and element comparison are performed within the group lock.

Boost::concurrent_flat_map is a valuable addition to the Boost library, providing developers with a high-performance concurrent hashmap that meets the demands of parallel scenarios. Its efficient design and flexible API make it a go-to choice for developers working on parallel applications.

#include <boost/concurrent_flat_map.hpp>

int main() {
  boost::concurrent_flat_map<int, std::string> map;

  // Insert key-value pairs
  map.emplace(1, "One");
  map.emplace(2, "Two");
  map.emplace(3, "Three");

  // Access values
  std::cout << map[1] << std::endl;  // Output: "One"
  std::cout << map[2] << std::endl;  // Output: "Two"
  std::cout << map[3] << std::endl;  // Output: "Three"

  return 0;
}

Boost::concurrent_flat_map empowers developers to handle parallel scenarios efficiently while maintaining thread safety. With its robust design and intuitive API, this container is a valuable tool for modern C++ developers. Stay tuned for more updates and news on Boost and other programming languages and frameworks in Dev Radar magazine.