C++11 Atomic Types and Memory Model E81 CSE 532S: Advanced Multi-Paradigm Software Development C++11 Atomic Types and Memory Model Chris Gill Department of Computer Science and Engineering Washington University in St. Louis cdgill@cs.wustl.edu Title slide

Atomic Types Many atomic types in C++11 STL, some are lock-free Always lock-free: std::atomic_flag If it matters, must test others with is_lock_free() Also can specialize std::atomic<> class template This is already done for many standard non-atomic type Can also do this for your own types that implement a trivial copy-assignment operator, are bitwise equality comparable Watch out for semantic details E.g., bitwise evaluation of float, double, etc. representations Equivalence may differ under atomic operations

Reasoning about Concurrency Operations on atomic types semantically well defined Synchronizes-with relationship ensures that (unambiguously) operation X happens before or after operation Y Can leverage this so eventually Xi happens-before Yj Transitivity then lets you build various happens-before cases, including inter-thread happens-before relationships Other variations on this theme are also useful Dependeny-ordered-before and carries-a-dependency-to are used to reason about cases involving data dependencies I.e., the result of one atomic operation is used in another

Memory Models and Design Trading off stricter ordering vs. higher overhead Sequential consistency is easiest to think about (implement) because it imposes a consistent global total order on threads Acquire-release consistency relaxes that to a pair-wise partial order that admits more concurrency Relaxed ordering is least expensive, but should be applied selectively where available to optimize performance Even these basic constucts allow sophisticated design Memory fences to synchronize otherwise relaxed segments Release chains offer a similar idea for data dependencies Mixing atomic operations in with non-atomic ones to reduce overhead, but still enforce correct semantics (Chapter 7)