1. 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
Title slide

2. 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

3. 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

4. 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)