What is Cache Coherence?
In computer architecture, cache coherence ensures the consistency of shared resource data that resides in multiple local caches. When multiple clients in a system maintain caches of a shared memory resource, issues can arise due to inconsistent data. This is particularly relevant in multiprocessing systems where multiple CPUs are involved.
What is Memory Consistency?
Memory consistency defines the perceived order in which memory operations, regardless of the process, appear to execute in relation to one another. It establishes rules for how memory operations are observed and coordinated across different processes or threads.
Difference between Cache Coherence and Memory Consistency
| Sr. No. | Cache Coherence | Memory Consistency |
|---|---|---|
| 1. | Describes the behavior of reads and writes to the same memory location. | Describes the behavior of reads and writes in relation to other locations. |
| 2. | Required in cache-equipped systems. | Required in systems with or without caches. |
| 3. | Guarantees that caches will never affect the observable functionality of a program. | Specifies correctness for memory accesses, including guarantees and visibility. |
| 4. | Concerned with the ordering of writes to a single memory location. | Handles the ordering of reads and writes to all memory locations. |
Coherency Mechanisms: How We Keep Cache Data Consistent
Coherency mechanisms are essential when multiple processors or cores are involved, each with its own cache. If you’re working with a multi-core system like I am, it’s important to understand how these mechanisms maintain data consistency across caches. Without cache coherence, the system could end up using outdated or incorrect data, leading to errors in processing.
There are three main types of coherency mechanisms that we use:
- Directory-Based Coherence
- Snooping
- Snarfing
By using these coherency mechanisms, we can make sure all caches stay aligned with one another, reducing errors and improving performance in multi-core systems.