21.2.5 Cache Coherence

TL;DR

Cache coherence protocols are necessary to ensure consistent communication and sharing of data in multicore systems.

Transcript

The problem with our simple multicore system is that there is no communication when the value of a shared variable is changed. The fix is to provide the necessary communications over a shared bus that's watched by all the caches. A cache can then "snoop" on what's happening in other caches and then update its local state to be consistent. The requi... Read More

Key Insights

• ❓ Cache coherence protocols enable consistent sharing of data in multicore systems.
• 🫥 The MESI protocol provides four states (INVALID, EXCLUSIVE, MODIFIED, and SHARED) to manage cache line states.
• 🫠 Snoopy buses and communication between caches are used to ensure data consistency during read and write operations.
• 👨‍💼 The CPU and snoopy bus have separate request streams, and modifications are made to cache hardware to support cache coherence.

Questions & Answers

Q: Why is a cache coherence protocol necessary in a multicore system?

A cache coherence protocol is needed to ensure all caches have consistent and up-to-date copies of shared variables. Without it, changes made in one cache may not be reflected in other caches, leading to incorrect results.

Q: What are the different states in the MESI cache coherence protocol?

The four states in the MESI protocol are INVALID, EXCLUSIVE, MODIFIED, and SHARED. Each state represents the status of a cache line and determines how it interacts with other caches when reading or writing data.

Q: How does the cache coherence protocol handle cache misses?

When a cache misses a requested data, it can check other caches through the snoopy bus. If another cache has the requested data in EXCLUSIVE or SHARED state, it supplies the data. Otherwise, the cache retrieves the data from main memory.

Q: How does the cache coherence protocol handle write operations?

If a cache line is in the SHARED state during a write operation, the cache sends an INVALIDATE message through the snoopy bus to other caches, ensuring exclusive access. If the cache line is already in the EXCLUSIVE state, no communication is needed, and the cache line can be modified and marked as MODIFIED.

Summary & Key Takeaways

• Cache coherence protocols provide necessary communication channels for shared variables in a multicore system.

• The protocol ensures the different cache lines are in consistent states, such as INVALID, EXCLUSIVE, MODIFIED, or SHARED.

• Caches can snoop on other caches and update their local state based on the observed changes.