What is a Cache?
A Cache is a small, high-speed storage layer that holds frequently accessed data. It helps computers, browsers, and applications retrieve information quickly, reducing the need to access slower main storage or databases. Think of cache like a notepad where you mention details you need to remember frequently. This saves time compared to flipping through a long book for the same information.
How a Cache Works
When a system needs data, it first checks the cache. If the data is there (a "cache hit"), the system retrieves it quickly. If the data isn’t there (a "cache miss"), the system fetches it from the main source, stores a copy in the cache, and uses it for future requests. This process speeds up data retrieval significantly.
Types of a Cache
CPU cache is a small, extremely fast memory located inside the processor. It stores frequently used instructions and data so the CPU can access them quickly. This minimizes the time needed to fetch information from the slower main memory (RAM). There are typically three levels of CPU cache:● L1 Cache: The smallest and fastest, located directly on the CPU core.● L2 Cache: Larger than L1 and slightly slower, but still faster than RAM.● L3 Cache: Shared between multiple cores, providing additional space for frequently accessed data.For instance, when a processor repeatedly performs calculations, the required instructions and data are stored in the CPU cache to speed up performance.
Browser cache stores website data such as images, HTML files, CSS styles, and scripts. This helps web pages load faster during future visits by reducing the need to download the same resources again. Browser cache also reduces data usage and server load. For example, if you visit an online store, your browser saves images of products and logos in the cache. The next time you visit, these images load quickly, making the website faster and smoother to use.
Memory cache uses system RAM to store data that applications frequently access. It speeds up performance by reducing the need to fetch data from slower storage devices like hard drives. Operating systems and software use RAM cache to optimize overall responsiveness.
When editing a large document in a word processor, parts of the document are stored in the RAM cache. This allows for faster editing and reduces delays when scrolling or making changes.Disk cache stores frequently accessed data on a computer’s storage device (HDD or SSD) for quicker retrieval. It improves performance by allowing faster read and write operations compared to accessing the main storage directly. The operating system may store a copy in the disk cache when you open a file frequently. It makes future access quicker than reading from the hard drive each time.
Application cache is used by software programs to store temporary data for faster processing. This can include user preferences, settings, or parts of files needed frequently.
A video editing app caches portions of a video project in memory, enabling smoother playback and editing without constant access to large video files.
Why Cache is Important
● Faster Performance: Accessing data from the cache is much quicker than from main storage.● Reduced Load: Less strain on databases or servers since frequently used data is cached.● Improved User Experience: Faster load times lead to smoother performance in websites and apps.
Examples of Cache in Action
● Web Browsing: Websites load faster because images and scripts are stored in the browser cache.● Mobile Apps: Apps load content quickly by storing commonly used data in a memory cache
Common Cache Terms
● Cache HitWhen data is found in the cache, allowing fast retrieval.● Cache MissWhen data isn’t in the cache, requiring access to the main source.● Cache EvictionRemoving old data from the cache to make room for new data.● Cache InvalidationUpdating or clearing the cache when data changes to avoid using outdated information.
Drawbacks of Cache
Cache Policies
Cache policies are rules that determine how data is stored, managed, and removed from the cache. These policies ensure that the cache operates efficiently and serves the most relevant data.
Cache Placement Policies
These policies decide where to store data within different levels of cache (e.g., CPU caches like L1, L2, or L3). Each level of cache varies in size and speed, with L1 being the fastest but smallest, and L3 being larger but slower.
Cache Replacement Policies
These policies determine which data to remove when the cache reaches its storage limit. Since cache has limited space, older or less useful data needs to be cleared to make room for new information.
Cache Invalidation Policies
These policies decide when to refresh or remove cached data to avoid using outdated information. Cache invalidation ensures that the cache stays up-to-date with the latest data from the main source.
Write Policies
Write policies determine how data changes are stored in main storage. In a write-through policy, data is written to both cache and main storage. In a write-back policy, data is first written to the cache and later to the main storage.