C++: We make a std::shared_mutex 10 times faster

Introduction: 

High performance of lock-based data structures. 

In this article, we will detail the atomic operations and C++11 memory barriers and the assembler instructions generated by it on x86_64 CPUs.

Next, we’ll show how to speed up the work of contfree_safe_ptr<std::map> up to the level of complex and optimized lock-free data structures that are similar to std::map<> in terms of their functionality, for example: SkipListMap and BronsonAVLTreeMap from libCDS library (Concurrent Data Structures library): https://github.com/khizmax/libcds

And we can get such multi-threaded performance for any of your initially non-thread-safe T-class used as contfree_safe_ptr<T>  –  it is  safe_ptr<T, contention_free_shared_mutex>  class, where contention_free_shared_mutex  is own optimized shared-mutex.

Namely, we will show how to realize your own high-performance contention-free shared-mutex, which almost does not conflict on readings. We implement our own active locks - spinlock and recursive-spinlock - to lock the rows in the update operations. We will create RAII-blocking pointers to avoid the cost of multiple locking. Here are the results of performance tests.

  • For 0% of modifications – our shared-mutex (as part of «contfree_safe_ptr<map>») shows performance 34.60 Mops, that is 14 times faster than standard std::shared_mutex (as part of «safe_ptr<map, std::shared_mutex>») that shows only 2.44 Mops.                                                                                               
  • For 15% of modifications – our shared-mutex (as part of «contfree_safe_ptr<map> ») shows performance 5.16 Mops, that is 7 times faster than standard std::shared_mutex (as part of «safe_ptr<map, std::shared_mutex>») that shows only 0.74 Mops.

Compares: std::mutex, std::shared_mutex, contention_free_shared_mutex<>



  1. We make any object thread-safe
  2. We make a std::shared_mutex 10 times faster - this article
  3. Thread-safe std::map with the speed of lock-free map

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