What Is a Cache Miss and How Can You Reduce Them?
A cache miss is a type of memory-fetching error that can result in increased system latency and reduced program performance.
One of the biggest issues with resolving cache miss issues is correctly identifying when one happens, and the type that’s occurred. However, it’s essential to understand what a cache miss is before exploring ways to reduce their occurrence.
In this article, KnownHost explains what cache misses are, the different types, and how best to stop them from slowing down your processor and network.
What Is Caching and How Does It Work?
Caching is a technique used to improve the performance and efficiency of computer systems. It does this by storing frequently accessed data in faster, more accessible storage locations.
This is done by creating a cache, which is a type of small, high-speed memory that acts as a middle ground between the system processor and the main memory.
When a computer system needs to access data, it first checks the cache. If the required data is found, it’s known as a ‘cache hit’. The data can then be retrieved quickly – avoiding the need to access the slower main memory.
Caches can be used at various levels of a computer system, including CPU caches, disk caches, and content delivery networks. By leveraging caching technology, systems can achieve faster site access times, reduce latency, and improve the overall efficiency of computing systems and the network as a whole.
So, what is a Cache Miss?
This results in the system having to search its higher-tier memory (for example, a CPU in a computer) to find the relevant information — which is often a slower memory source.
Cache misses result in increased latency and a decrease in system performance. Therefore, minimizing cache misses is an essential part of improving system performance.
What Happens During a Cache Miss?
It’s worth noting that the process during a cache miss can vary depending on the type of system architecture in place. However, during most cache misses, the following steps typically occur:
- Cache Miss Detection: The processor or cache controller will detect that data is not present in a cache. This can be determined by comparing the memory address of the requested data with the cache tags.
- Cache Miss Handler Activation: Upon detecting a cache miss, the cache handler routine is activated. This protocol is responsible for handling the cache miss and initiating the necessary recovery actions.
- Request to High-Level Memory: The cache handler sends a request to the next level of the memory hierarchy. This can include a higher-level cache or the main memory. This request is typically sent via the memory bus or through cache coherence protocols – depending on the system architecture.
- Memory Access and Data Transfer: The higher-level memory answers the request by providing the required data. That data is then transferred back to the cache.
- Cache Update: The cache handler updates the cache with the newly acquired data. This involves replacing or updating the existing cache line with the new data which ensures that future attempts to access the data will be successful.
- Resume Execution: Once the data is properly stored, the cache handler allows the system to resume execution. The processor can now access the required data from the cache.
What Are the Different Types of Cache Miss?
Cache misses come in three varieties. Understanding how each type of cache miss behaves is essential to understanding how to properly optimize cache performance:
- Compulsory Cache Miss: Also known as a ‘cold start’ or ‘first reference miss’. A compulsory cache miss occurs when a requested data item is accessed for the first time, but the required data isn’t present within the cache. Because the cache is empty, data must be requested from a higher-level memory — increasing latency.
- Capacity Cache Miss: A capacity cache miss occurs when the cache is too small to hold all the required data that needs to be accessed. As a result, some of the data that was previously fetched into the cache may be vacated to make room for new data. This leads to further cache misses when those vacated items are accessed.
- Conflict Cache Miss: Conflict cache misses occur due to limitations in the cache mapping or replacement policies. If multiple data items contend with the same cache location and the cache is unable to accommodate them simultaneously, then conflict cache misses occur.
How To Identify the Type of Cache Miss Encountered
The process of cache miss identification involves analyzing the cache behavior and system characteristics. Below are several methods that can be used to identify cache miss types:
- Profiling Tools: Profiling tools and performance analyzers can provide cache metrics and statistics. This allows a user to constantly monitor system performance, with cache errors automatically flagged when they occur. Many of these tools can also sort cache misses into their different types based on their characteristics — allowing the user to choose the best solution to the problem.
- Cache Simulations: Cache simulation techniques can construct detailed models of how a cache may perform within a system. By simulating cache accesses and analyzing patterns, it becomes easier to predict how a system will handle each cache miss type.
- Access Patterns: Studying the access patterns of a system can help provide clues about the type of cache misses occurring. For example, frequent misses on initial accesses may indicate compulsory misses.
- Cache Configuration: Review the cache configuration details. This might mean looking at cache size, associativity, and mapping scheme. Understanding the cache organization can help in identifying potential causes for different cache misses.
How To Reduce Cache Misses
The most effective ways to reduce cache misses and improve cache performance include:
- Optimize Data Locally: Accessing data from local caches allows for smoother, quicker access to the data than doing so from global caches. Localizing your data helps speed things up while minimizing cache misses.
- Enable Data Prefetching: CPU processors can predict which data you might need and fetch it from the higher memory storage and move it into a fast-access local cache before you need it — reducing cache misses caused by latency.
- Employ Loop Tiling: Data can be divided into smaller segments — known as blocks or tiles — that fit more easily within the cache. This makes it easier for the CPU to retrieve the data from the caches and reuse it, reducing the number of cache misses.
- Consider Cache Blocking: Users can leverage blocking or blocking-aware software to operate on smaller data blocks that fit within the cache. This reduces cache misses.
- Optimize Cache Size and Associativity: It’s possible to analyze workload characteristics and tailor cache size and associativity to fit the specific needs of the application.
- Optimize Memory Access Patterns: Be sure to align data structures, use proper data structures, and ensure efficient memory accesses to minimize cache misses.
- Utilize Compiler Optimization: Employ compiler optimizations like loop unrolling, software prefetching, and cache-aware optimizations to improve cache utilization and reduce misses.
- Profile and Analyse: Use profiling tools to identify cache miss patterns and performance bottlenecks — enabling targeted optimizations.
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Frequently Asked Questions (FAQs)
Q: How to Handle a Cache Miss?
A: When a cache miss occurs, the processor system will detect it and fix the error itself – so, you don’t actually need to do anything. The miss initiates a cache miss handler routine that fetches the required data from higher-level memory. The data is then transferred into the cache which replaces or updates the corresponding cache line. Once the data is in the cache, the processor resumes cache execution – accessing the data from the cache for future operations.
Q: Why Is a Cache Miss Expensive?
A: A cache miss can be a costly system fault because it introduces additional latency to the memory access process. When a cache miss occurs, the system must retrieve the requested data from a higher-level memory hierarchy or the main memory. This typically has higher access latency when compared with the cache and increases the overall time it takes to complete a memory operation – resulting in a slower program execution and reduced performance.
Q: Is a Cache Miss a Page Fault?
A: A cache miss is different from a page fault. A cache miss occurs when a system cannot find the requested data in the cache and needs to fetch it from a higher-level memory. A page fault refers to a situation where a requested memory page is not present in the main memory and needs to be loaded from a secondary storage – like a hard disk. A cache miss and page fault occur on different levels of the memory hierarchy.
Q: What Is a Dirty Cache Miss?
A: Also known as a ‘write miss’, a dirty cache miss refers to a situation where a processor attempts to write data to a cache line that’s marked as ‘dirty’ or modified. This means a cache with data that has been modified since it was last fetched from memory. When a dirty cache miss occurs, the cache controllers must first write the modified data back to the main memory before replacing the cache line with new data. This additional step increases the extreme latency in the cache operation.