What are the types of coherence protocols?

What are the different types of coherence protocols used in distributed systems?

Coherence protocols ensure that multiple copies of data remain consistent. The main types include: 1. Directory-based protocols: Use a central directory to track the state of each cache line. 2. Bus-based protocols: Rely on a shared bus to communicate changes. 3. Snooping protocols: Allow caches to monitor (or snoop) the bus for changes. 4. Token-based protocols: Use tokens to control access to shared data.

Can you explain directory-based protocols in more detail?

Sure! Directory-based protocols maintain a directory that keeps track of which caches have copies of a particular data block. When a cache wants to read or write data, it checks the directory to see if it needs to update other caches. This reduces the amount of traffic on the bus and helps maintain coherence efficiently.

What are the advantages of using snooping protocols?

Snooping protocols are advantageous because they allow caches to quickly detect changes made by other caches, leading to faster coherence. They are also simpler to implement in smaller systems. However, they can become less efficient as the number of caches increases due to bus traffic.

Are there any disadvantages to token-based protocols?

Yes, token-based protocols can introduce latency since a cache must wait for a token to access shared data. Additionally, if a token is lost or not properly managed, it can lead to inconsistencies and require complex recovery mechanisms.