sukzessiveOptimierungRelaxedSemantikEng

Ongoing Optimization: Relaxed Semantics with CppMem

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.

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
// 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?

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.

 

Rainer D 6 P2 500x500Modernes C++ Mentoring

  • "Fundamentals for C++ Professionals" (open)
  • "Design Patterns and Architectural Patterns with C++" (open)
  • "C++20: Get the Details" (open)
  • "Concurrency with Modern C++" (open)
  • "Generic Programming (Templates) with C++": October 2024
  • "Embedded Programming with Modern C++": October 2024
  • "Clean Code: Best Practices for Modern C++": March 2025
  • Do you want to stay informed: Subscribe.

     

    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. However 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.

    Thanks a lot to my Patreon Supporters: Matt Braun, Roman Postanciuc, Tobias Zindl, G Prvulovic, Reinhold Dröge, Abernitzke, Frank Grimm, Sakib, Broeserl, António Pina, Sergey Agafyin, Андрей Бурмистров, Jake, GS, Lawton Shoemake, Jozo Leko, John Breland, Venkat Nandam, Jose Francisco, Douglas Tinkham, Kuchlong Kuchlong, Robert Blanch, Truels Wissneth, Mario Luoni, Friedrich Huber, lennonli, Pramod Tikare Muralidhara, Peter Ware, Daniel Hufschläger, Alessandro Pezzato, Bob Perry, Satish Vangipuram, Andi Ireland, Richard Ohnemus, Michael Dunsky, Leo Goodstadt, John Wiederhirn, Yacob Cohen-Arazi, Florian Tischler, Robin Furness, Michael Young, Holger Detering, Bernd Mühlhaus, Stephen Kelley, Kyle Dean, Tusar Palauri, Juan Dent, George Liao, Daniel Ceperley, Jon T Hess, Stephen Totten, Wolfgang Fütterer, Matthias Grün, Phillip Diekmann, Ben Atakora, Ann Shatoff, Rob North, Bhavith C Achar, Marco Parri Empoli, Philipp Lenk, Charles-Jianye Chen, Keith Jeffery, Matt Godbolt, and Honey Sukesan.

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

    My special thanks to Embarcadero
    My special thanks to PVS-Studio
    My special thanks to Tipi.build 
    My special thanks to Take Up Code
    My special thanks to SHAVEDYAKS

    Modernes C++ GmbH

    Modernes C++ Mentoring (English)

    Do you want to stay informed about my mentoring programs? Subscribe Here

    Rainer Grimm
    Yalovastraße 20
    72108 Rottenburg

    Mobil: +49 176 5506 5086
    Mail: schulung@ModernesCpp.de
    Mentoring: www.ModernesCpp.org

    Modernes C++ Mentoring,

     

     

    0 replies

    Leave a Reply

    Want to join the discussion?
    Feel free to contribute!

    Leave a Reply

    Your email address will not be published. Required fields are marked *