Cache coherency refers to the problem of maintaining consistency between multiple copies of the same data in different caches in a computer system. When multiple processors or cores access the same data stored in memory, they may each have their own copy of the data in their local cache. If one processor updates its copy of the data, the other processors may not be aware of this update and may continue to use their own copies of the data, leading to inconsistencies and errors in the system
There are many ways to prevent cache coherency issues,
- Snooping: is one way to maintain cache coherency. In this method, each cache controller monitors the bus for any read or write requests to the memory location it is caching. If a cache controller detects a write request, it invalidates its own copy of the data to ensure that subsequent reads from that cache will fetch the latest data from memory.
- Directory-based coherence: In this method, a directory is maintained that keeps track of which caches are holding a copy of a particular memory location. When a processor writes to that location, the directory is updated to reflect the new value, and any caches holding copies are invalidated.
- Bus snooping with write-back: In this method, the caches snoop on the bus for read and write requests and maintain a dirty bit for each block of memory they hold. When a cache controller detects a write request to a dirty block, it writes the updated value back to memory and invalidates its own copy of the block.
- MESI protocol: The MESI protocol is a widely used cache coherence protocol that uses four states to maintain cache coherency: Modified, Exclusive, Shared, and Invalid. When a processor requests a memory location, the cache controller checks the MESI state of its copy of the data and responds accordingly. If a processor modifies a memory location, its cache controller sets the state to Modified and broadcasts an invalidation message to other caches holding the same data.