Ongoing Optimization: Relaxed Semantic with CppMem

21 August 2016
Contents[Show]

With the relaxed semantics, we have no synchronizations and ordering constraints on atomic operations.

 

Relaxed Semantics

With the relaxed semantics, only the atomicity of the operations on atomics is left.

 

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// ongoingOptimizationRelaxedSemantic.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_relaxed);
}

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

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

 

Now, the questions are very easy to answer. Does the program have well-defined behavior? Which values for x and y are possible? On the one hand, all operations on x and y are atomic. So the program is well-defined. On the other hand, there are no restrictions on the interleaving of the threads. In the end, thread two can see the operations on thread 1 in a different order. So this is the first time in our process of ongoing optimizations that thread two can display x == 0 and y == 1. All combinations of x and y are possible.

sukzessiveOptimierungRelaxedSemantikEng

 I'm curious, what the graph of CppMem will look like for x == 0 and y == 1?

 

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CppMem

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

 

That was the CppMem program. Now to the graph.

Execution for (y=0,x=2000)

The graph shows crystal clear unintuitive behavior.

relaxed

x reads the value 2000 from the writing thread, but y reads 0 from the main thread. What happens when the reading of y is sequenced before the reading of x? Sequenced before exact means that the operation e:Rrix sb is sequenced-before the operation f:Rrix.

What's next?

This was the last post in my mini-series about ongoing optimization. So, what's next? There are a lot of issues with the singleton pattern. I'm aware of that. But the singleton pattern is an ideal use case for a variable, which has to be initialized in a thread-safe way. From that point on, you can use it without synchronization.
So in the next post, I discuss different ways to initialize a singleton in a multithreading environment. You get the performance numbers and can reason about your use cases for the thread-safe initialization of a variable.

 

 

 

 

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Thanks, in particular, to Jon Hess, Lakshman, Christian Wittenhorst, Sherhy Pyton, Dendi Suhubdy, Sudhakar Belagurusamy, Richard Sargeant, Rusty Fleming, John Nebel, Mipko, Alicja Kaminska, and Slavko Radman.

 

 

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Tags: Ongoing Optimization, CppMem, Atomics, Relaxed Semantics

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