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What is the cache replacement policy?

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Cache replacement policy is a crucial factor that affects the performance and efficiency of your computer hardware. It determines how the cache memory, which is a small and fast storage area that holds frequently accessed data, handles the incoming and outgoing requests from the main memory or the disk. In this article, you will learn about the basic concepts and types of cache replacement policies, and how they influence the hit rate, miss rate, and latency of your cache.

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Amine MECIFI

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Carlos Heuser

Cyber Security & Threat Intelligence | Data & IA Engineer | Industry 4.0 | Privacy | Cloud & Security Engineer |…

1 Cache hit and miss

When the cache receives a request for a data item, it checks if it already has a copy of that item in its storage. If it does, it is called a cache hit, and the cache can quickly return the data to the requester. If it does not, it is called a cache miss, and the cache has to fetch the data from the lower level of the memory hierarchy, which is usually slower and more costly. The ratio of cache hits to cache misses is called the hit rate, and the ratio of cache misses to cache hits is called the miss rate. Ideally, you want to maximize the hit rate and minimize the miss rate, as they affect the overall speed and performance of your system.

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Carlos Heuser

Cyber Security & Threat Intelligence | Data & IA Engineer | Industry 4.0 | Privacy | Cloud & Security Engineer | DevSecOps
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Cache replacement policies are pivotal in managing system efficiency. A cache hit occurs when requested data is present, facilitating swift retrieval, while a cache miss necessitates fetching data from a slower memory level. Hit and miss rates gauge system effectiveness, with the goal of maximizing hits and minimizing misses for optimal performance. Various replacement policies, like LRU and FIFO, govern data selection for retention in cache lines. These policies balance efficient space utilization and minimizing the impact of cache misses. The choice of a policy depends on system architecture specifics, contributing to overall system speed and performance.

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2 Cache mapping

To store and retrieve data items in the cache, the cache uses a mapping scheme that assigns each data item a specific location or a set of possible locations in the cache storage. There are three main types of cache mapping schemes: direct mapping, associative mapping, and set-associative mapping. Direct mapping assigns each data item to exactly one location in the cache, based on its address. Associative mapping allows each data item to be stored in any location in the cache, based on its availability. Set-associative mapping divides the cache into several sets, and each data item can be stored in any location within its assigned set, based on its address and availability.

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3 Cache replacement

When the cache is full and needs to make room for a new data item, it has to evict or replace an existing data item from its storage. This is where the cache replacement policy comes in. The cache replacement policy is a rule or an algorithm that decides which data item to remove from the cache, based on some criteria or factors. The goal of the cache replacement policy is to minimize the impact of cache eviction on the hit rate and the miss rate, by choosing the data item that is least likely to be requested again in the near future.

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Amine MECIFI

Public Speaker - Author - Quiet Quitting Advocate - Fighting Corporate Toxicity
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In most algorithms, data that is no longer needed is not evicted from memory. Why should this be done? What would be the benefit of removing data from memory and writing zeros in its place? If anything, we are just adding unnecessary operations. In most cases, the data that is no longer needed is marked as "suitable for overwriting" but is still there. It not deleted from the cache unless its space is needed. This leads to the concept of hard and soft cache hits. In a hard hit, the data was found in the cache. In a soft hit, the data has been marked for overwriting but is now needed before it is overwritten. There would be no soft hit if the cache had actually been overwritten and filled with zeros.

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4 Cache replacement types

There are many types of cache replacement policies, such as FIFO (First In First Out), LRU (Least Recently Used), LFU (Least Frequently Used), and Random. Each policy has its own advantages and disadvantages, depending on the workload and data item characteristics. For example, FIFO is simple to implement but can lead to cache thrashing, while LRU is more adaptive but requires more hardware overhead. LFU works well with data items with stable frequencies, but can cause cache pollution. Random is fair but can be inefficient. Ultimately, the best choice depends on the specific situation.

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Cache replacement policies are like rules for deciding which stuff to kick out of the computer's memory when it's full. There are different ways to do it: Least Recently Used (LRU): Get rid of the stuff you haven't used in a while. FIFO (First In, First Out): Chuck out the old stuff you put in first. Random Replacement: Just pick something to get rid of randomly. Least Frequently Used (LFU): Toss out the stuff you hardly ever use. Most Recently Used (MRU): Ditch the stuff you just used. People pick these rules based on what the computer needs, and each rule has its own good and bad points.

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5 Cache replacement trade-offs

Choosing the right cache replacement policy for your system is not a straightforward task. It depends on many factors, such as the cache size, the cache mapping scheme, the data access pattern, the data locality, the data size, the data update frequency, and the performance metrics. You have to balance the trade-offs between the hit rate, the miss rate, and the latency of your cache, as well as the complexity and the cost of your cache replacement policy. You may also have to consider other aspects, such as the power consumption, the reliability, the security, and the scalability of your cache. There is no one-size-fits-all solution for cache replacement policy. You have to experiment and evaluate different options and scenarios, and find the best fit for your specific needs and goals.

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6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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Amine MECIFI

Public Speaker - Author - Quiet Quitting Advocate - Fighting Corporate Toxicity
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Caching is about guessing. Just guessing what information is needed and keeping it close at hand. It's the same principle when you are working on your car. Depending on the task, you have the right tools nearby. But you are still exposed to a new situation that would require you to go back to your garage and get another tool that is now needed. No caching algorithm is perfect. They were all developed under certain assumptions. Their performance is maximised when the workload adheres to these assumptions. If this is not the case, they will be stall and latency spikes may occur.

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What is the cache replacement policy?

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2

3

4

5

6

1 Cache hit and miss

2 Cache mapping

3 Cache replacement

4 Cache replacement types

5 Cache replacement trade-offs

6 Here’s what else to consider

Computer Hardware

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What is the cache replacement policy?

1

2

3

4

5

6

1 Cache hit and miss

2 Cache mapping

3 Cache replacement

4 Cache replacement types

5 Cache replacement trade-offs

6 Here’s what else to consider

Computer Hardware

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