Ongoing Optimization: Acquire-Release Semantics with CppMem

With the acquire-release semantics, we break the sequential consistency. In the acquire-release semantics, synchronization occurs between atomic operations on the same atomic and not between threads.

Acquire-release semantic

The acquire-release semantic is more lightweight and, therefore, faster than the sequential consistency because the synchronization only occurs between atomic operations. But although the intellectual challenge increases.

// ongoingOptimizationAcquireRelease.cpp

#include <atomic>
#include <iostream>
#include <thread>

std::atomic<int> x{0};
std::atomic<int> y{0};

void writing(){  
  x.store(2000,std::memory_order_relaxed);  
  y.store(11,std::memory_order_release);
}

void reading(){  
  std::cout << y.load(std::memory_order_acquire) << " ";  
  std::cout << x.load(std::memory_order_relaxed) << std::endl;
}

int main(){
  std::thread thread1(writing);
  std::thread thread2(reading);
  thread1.join();
  thread2.join();
};

At first glance, you will notice that all operations are atomic. So the program is well-defined. But the second glance shows more. The atomic operations on y are attached with the flags std::memory_order_release (line 12) and std::memory_order_acquire (line 16). In contrast, the atomic operations on x are annotated with std::memory_order_relaxed. So there are no synchronization and ordering constraints for x. The key for the possible values for x and y can only be answered by y.

It holds:

y.store(11,std::memory_order_release) synchronizes-with y.load(std::memory_order_acquire)
x.store(2000,std::memory_order_relaxed is visible before y.store(11,std::memory_order_release)
y.load(std::memory_order_acquire) is visible before x.load(std::memory_order_relaxed)

I will elaborate a little bit more on these three statements. The key idea is that the store of y in line 10 synchronizes with the load of y in line 16. The reason is that the operations occur on the same atomic and follow the acquire-release semantic. So y uses std::memory_order_release in line 12 and std::memory_order_acquire in line 16. But the pairwise operations on y have another very interesting property. They establish a kind of barrier relative to y. So x.store(2000,std::memory_order_relaxed) can not be executed after y.store(std::memory_order_release), so x.load() can not be executed before y.load().

The reasoning was in the case of the acquire-release semantic more sophisticated than in the case of the sequential consistency. But the possible values for x and y are the same. Only the combination y == 11 and x == 0 is no possible.

Three different interleavings of the threads are possible, producing the three different combinations of x and y.

thread1 will be executed before thread2.
thread2 will be executed before thread1.
thread1 executes x.store(2000), before thread2 will be exectued.

At the end the table.

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CppMem

At first, the program once more with CppMem.

int main(){
  atomic_int x= 0; 
  atomic_int y= 0;
  {{{ { 
      x.store(2000,memory_order_relaxed);
      y.store(11,memory_order_release);
      }
  ||| {
      y.load(memory_order_acquire);
      x.load(memory_order_relaxed);
      } 
  }}}
}

We already know all results except of (y=11, x=0) are possible.

Possible executions

Have a look at the three graphs with consistent execution. The graphs show an acquire-release semantics between the store-release of y and the load-acquire from y. It makes no difference if the reading of y (rf) occurs in the main thread or a separate thread. The graphs show the synchronizes-with relation with an sw arrow.

Execution for (y=0, x= 0)

Execution for (y= 0, x= 2000)

Execution for (y=11, x= 2000)

What’s next?

But we can do better. Why should x be atomic? There is no reason. That was my first but incorrect assumption. Why? You will read it in the next post.

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