computer science

Memory model (programming)

Memory model (programming) is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Memory model (programming) rather than just read about it. In short: In computing, a memory model describes the interactions of threads through memory and their shared use of the data. History and significance A memory model allows a compiler to perform many important optimizations.

Key takeaways

Memory model (programming) belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Memory model (programming) to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Memory model (programming) from memory before moving on to harder problems.

Reference excerpt

In computing, a memory model describes the interactions of threads through memory and their shared use of the data.

History and significance A memory model allows a compiler to perform many important optimizations. Compiler optimizations like loop fusion move statements in the program, which can influence the order of read and write operations of potentially shared variables. Changes in the ordering of reads and writes can cause race conditions. Without a memory model, a compiler may not apply such optimizations to multi-threaded programs at all, or it may apply optimizations that are incompatible with multi-threading, leading to bugs. Modern programming languages like Java therefore implement a memory model. The memory model specifies synchronization barriers that are established via special, well-defined synchronization operations such as acquiring a lock by entering a synchronized block or method. The memory model stipulates that changes to the values of shared variables only need to be made visible to other threads when such a synchronization barrier is reached. Moreover, the entire notion of a race condition is defined over the order of operations with respect to these memory barriers. These semantics then give optimizing compilers a higher degree of freedom when applying optimizations: the compiler needs to make sure only that the values of (potentially shared) variables at synchronization barriers are guaranteed to be the same in both the optimized and unoptimized code. In particular, reordering statements in a block of code that contains no synchronization barrier is assumed to be safe by the compiler. Most research in the area of memory models revolves around:

Designing a memory model that allows a maximal degree of freedom for compiler optimizations while still giving sufficient guarantees about race-free and (perhaps more importantly) race-containing programs. Proving program optimizations that are correct with respect to such a memory model. The Java memory model was the first attempt to provide a comprehensive threading memory model for a popular programming language. After it was established that threads could not be implemented safely as a library without placing certain restrictions on the implementation and, in particular, that the C and C++ standards (C99 and C++03) lacked necessary restrictions, the C++ threading subcommittee set to work on suitable memory model; in 2005, they submitted C working document n1131 to get the C Committee on board with their efforts. The final revision of the proposed memory model, C++ n2429, was accepted into the C++ draft standard at the October 2007 meeting in Kona. The memory model was then included in the next C++ and C standards, C++11 and C11. The Rust programming language inherited most of C/C++'s memory model.

See also Memory ordering Memory barrier Consistency model Shared memory (interprocess communication)

References

Worked examples

Example 1 — a first encounter with Memory model (programming)

Start with the simplest possible case. Write down what Memory model (programming) claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Memory model (programming) before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Memory model (programming) ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Memory model (programming)

In research: Memory model (programming) appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Memory model (programming) in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Memory model (programming) is common in secondary-school and first-year university syllabi. It links to neighbouring topics Compiler construction, Computer memory, Computer programming stubs, so understanding it makes those chapters shorter.
In everyday life: Look for Memory model (programming) outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Memory model (programming) in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Memory model (programming) means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Memory model (programming) out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Memory model (programming) in simple terms?

In computing, a memory model describes the interactions of threads through memory and their shared use of the data. History and significance A memory model allows a compiler to perform many important optimizations.

Why does Memory model (programming) matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Memory model (programming)?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Memory model (programming).