Cache line prefetching is a technique used in computer processors to improve memory access performance. It involves fetching multiple contiguous cache lines from memory into the processor’s cache in advance, anticipating that they will be needed in the near future.
The cache is a small but fast memory located on the processor chip that stores recently accessed data and instructions. It acts as a buffer between the much slower main memory (RAM) and the processor. When the processor needs to access data, it first checks if it is present in the cache. If it is, it can retrieve it much faster than if it had to access it directly from RAM.
Cache line prefetching takes advantage of the principle of spatial locality, which suggests that if a program accesses a particular memory location, it is likely to access nearby locations in the near future. By fetching contiguous cache lines, which are typically 64 bytes or larger, the processor can reduce the latency of subsequent memory accesses.
There are different techniques for cache line prefetching, such as:
1. Hardware prefetching: Modern processors often have dedicated hardware units that monitor memory access patterns and automatically issue prefetch instructions. These units analyze the memory access patterns and predict which cache lines will be needed next, fetching them in advance.
2. Software prefetching: Programmers can manually insert prefetch instructions in their code to explicitly request the processor to fetch specific memory locations into the cache before they are accessed. This technique requires careful analysis of memory access patterns and an understanding of the underlying hardware.
3. Compiler-assisted prefetching: Some compilers can automatically analyze code and insert prefetch instructions based on the detected memory access patterns. This technique reduces the burden on the programmer, as the compiler takes care of prefetching optimizations.
4.Coherence Miss – It is also known as Invalidation. These misses occur when other external processors, i.e., I/O updates memory.
Cache line prefetching can improve performance by reducing memory latency and minimizing stalls in the processor pipeline caused by memory access delays. However, its effectiveness depends on the predictability of memory access patterns and the efficiency of the prefetching mechanism employed by the processor.
Properties of these Cache misses : These are various properties of Cache misses for same data set and various types of caches:
- Compulsory misses occur same in all types of direct mapped, set associative and associative caches.
- Coherence misses occur same in all types of direct mapped, set associative and associative caches.
- Conflict misses occur high in direct mapped cache, medium in set associative cache, and zero in associative mapped cache.
- Capacity misses occur low in direct mapped cache, medium in set associative cache, and high in associative mapped cache.
Generally, we use Random block replacement, LRU, or FIFO page replacement technique to bring a miss page in cache from main memory.