atomics

ABA - A is not the same as A

A common problem in concurrency is the so-called ABA problem. That means you read a value twice and each time it returns the same value A. Therefore you conclude that nothing changed in between. But you forgot the B.

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Tags: atomics
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Blocking and Non-Blocking Algorithms

Blocking, non-blocking, lock-free and wait-free. Each of these terms describes a key characteristic of an algorithm when executed in a concurrent environment. So, reasoning about the runtime behaviour of your program often means to put your algorithm in the right bucket. Therefore, this post is about buckets.

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My Conclusion: Summation of a Vector in three Variants

After I've calculated in three different ways the sum of a std::vector I want to draw my conclusions.

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Multithreaded: Summation with Minimal Synchronization

Until now I've used two strategies for the summation of a std::vector. First I did the whole math in one thread (Single Threaded: Summation of a vector); second multiple threads shared the same variable for the result (Multithreaded: Summation of a vector). In particular the second strategy was extremely naive. In this post I will apply my knowledge of both posts. My goal is it that the thread will perform their summation as independent form each other as possible and therefore reduce the synchronization overhead. 

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Single Threaded: Summation of a Vector

What is the fastest way to add the elements of a std::vector?. A question which I will pursue in the next posts. I use the single threaded addition as reference number. In further posts I discuss atomics, locks, and thread local data.

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Tags: atomics, lock
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Thread-Safe Initialization of a Singleton

There are a lot of issues with the singleton pattern. I'm totally aware of that. But the singleton pattern is an ideal use case for a variable, which has only to be initialized in a thread safe way. From that point on you can use it without synchronization. So in this post I discuss different ways to initialize a singleton in a multithreading environment. You get the performance numbers and can reason about your uses cases for the thread safe initialization of a variable.

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Ongoing Optimization: Relaxed Semantic with CppMem

With the relaxed semantic, we have no synchronisations and ordering constraints on atomic operations.

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Ongoing Optimization: A Data Race with CppMem

But we can do better and further improve the acquire-release semantic of the last post. Why should x be an atomic? There is no reason. That was my first, but incorrect assumption. See why?

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Ongoing Optimization: Acquire-Release Semantic with CppMem

With the acquire-releae semantic, we break the sequential consistency. In the acquire-release semantic the synchronization takes place between atomic operations on the same atomic and not between threads.

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Ongoing Optimization: Sequential Consistency with CppMem

With atomic data types you can tailor your program to your needs and therefore optimize it. But now we are in the domain of the multithreading experts.

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