Cache coherence

Cache coherence is a hardware phenomenon that occurs when two or more processors share a common memory. When one processor writes to a memory location, the other processors see the change. This phenomenon is called coherence.

Why cache coherence is required? Cache coherence is required because it ensures that all copies of a given piece of data are kept up-to-date. Without cache coherence, it would be possible for two different processors to have different copies of the same data, which would lead to inconsistency and potential errors. Cache coherence ensures that all processors have the same view of the data, avoiding these potential errors.

What are the cache coherence protocols?

Cache coherence protocols are designed to ensure that data in the cache is consistent with data in the main memory. There are two main types of cache coherence protocols: write-through and write-back.

Write-through protocols write data to the cache and main memory at the same time. This ensures that the data in the cache is always consistent with the data in main memory. However, write-through protocols can be slower than write-back protocols because they involve two writes (one to the cache and one to main memory).

Write-back protocols write data to the cache and main memory at different times. This allows the data in the cache to be different from the data in main memory for a short period of time. However, write-back protocols can be faster than write-through protocols because they involve only one write (to the cache).

What is cache coherency and how is it eliminated?

Cache coherency is the need to keep multiple copies of a piece of data in sync. When data is updated in one location, all other locations need to be updated as well. Cache coherency is typically achieved by using a central repository (such as a database) to store the data, and then requiring all clients to read and write data through the repository.

Cache coherency can be eliminated by using a distributed system, where each client has its own copy of the data. The data is then synchronized between clients as needed.

What is the difference between cache coherence and memory consistency?

Cache coherence is the property that ensures that data stored in different caches is consistent with each other. This means that if one cache stores a piece of data, all the other caches will be updated with that data as well.

Memory consistency is a related but different concept. It is the property that ensures that the order in which data is stored in memory is the same as the order in which it is accessed. So, if data is stored in memory in a certain order, it will be accessed in that same order.

What is data coherence?

Data coherence is a condition that exists when all copies of a piece of data are identical. This is typically achieved by ensuring that all writes to a piece of data are completed before any subsequent reads of that data. Data coherence is important in shared memory systems, where multiple processors can read and write to the same data. Incoherent data can lead to incorrect results.