computer science

Least frequently used

Least frequently used 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 Least frequently used rather than just read about it. In short: Least Frequently Used (LFU) is a type of cache replacement policy used to manage memory within a computer. The standard characteristics of this method involve the system keeping track of the number of times a block is referenced in memory.

Key takeaways

Least frequently used 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 Least frequently used to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Least frequently used from memory before moving on to harder problems.

Reference excerpt

Least Frequently Used (LFU) is a type of cache replacement policy used to manage memory within a computer. The standard characteristics of this method involve the system keeping track of the number of times a block is referenced in memory. When the cache is full and requires more room the system will purge the item with the lowest reference frequency. LFU is sometimes combined with a Least Recently Used algorithm and called LRFU.

Implementation The simplest method to employ an LFU algorithm is to assign a counter to every block that is loaded into the cache. Each time a reference is made to that block the counter is increased by one. When the cache reaches capacity and has a new block waiting to be inserted the system will search for the block with the lowest counter and remove it from the cache, in case of a tie (i.e., two or more keys with the same frequency), the least recently used key would be invalidated.

Ideal LFU: there is a counter for each item in the catalogue Practical LFU: there is a counter for the items stored in cache. The counter is forgotten if the item is evicted.

Problems While the LFU method may seem like an intuitive approach to memory management it is not without faults. Consider an item in memory which is referenced repeatedly for a short period of time and is not accessed again for an extended period of time. Due to how rapidly it was just accessed its counter has increased drastically even though it will not be used again for a decent amount of time. This leaves other blocks which may actually be used more frequently susceptible to purging simply because they were accessed through a different method. Moreover, new items that just entered the cache are subject to being removed very soon again, because they start with a low counter, even though they might be used very frequently after that. Due to major issues like these, an explicit LFU system is fairly uncommon; instead, there are hybrids that utilize LFU concepts.

See also Cache replacement policies Memory paging Page replacement algorithm § Not frequently used (NFU)

References

External links An O(1) algorithm for implementing the LFU cache eviction scheme, 16 August 2010, by Ketan Shah, Anirban Mitra and Dhruv Matani

Worked examples

Example 1 — a first encounter with Least frequently used

Start with the simplest possible case. Write down what Least frequently used 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 Least frequently used 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 Least frequently used 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 Least frequently used

In research: Least frequently used 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 Least frequently used 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: Least frequently used is common in secondary-school and first-year university syllabi. It links to neighbouring topics Memory management algorithms, Online algorithms, Virtual memory, so understanding it makes those chapters shorter.
In everyday life: Look for Least frequently used 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 Least frequently used in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Least frequently used 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 Least frequently used out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Least frequently used in simple terms?

Why does Least frequently used 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 Least frequently used?

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 Least frequently used.