Modern C++ Concurrency: Get the most out of any machine/

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CppMem: Atomics with Sequential Consistency

CppMem: Atomics with Sequential Consistency

This lesson gives an overview of atomics with sequential consistency used in the context of CppMem.

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If you don’t specify the memory model, sequential consistency will be applied. Sequential consistency guarantees two properties: each thread executes its instructions in source code order, and all threads follow the same global order. Here is the optimized version of the program using atomics.

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// ongoingOptimisationSequentialConsistency.cpp
#include <atomic>
#include <iostream>
#include <thread>
std::atomic<int> x{0};
std::atomic<int> y{0};
void writing(){  
  x.store(2000);  
  y.store(11);
}
void reading(){  
  std::cout << y.load() << " ";  
  std::cout << x.load() << std::endl;
}
int main(){
  std::thread thread1(writing);
  std::thread thread2(reading);
  thread1.join();
  thread2.join();
};

Let’s analyze the program. The program is data race free because x and y are atomics. Therefore, only one question is left to answer: What values are possible for x and y? The question is easy to answer. Thanks to the sequential consistency, all threads have to follow the same global order.

It holds true:

  • x.store(2000); happens-before y.store(11);
  • std::cout << y.load() << " "; happens-before std::cout << x.load() << std::endl;

Hence ...

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