Understanding Cache Coherence Protocols in Multicore Systems
Table of Contents
Cache Coherence Protocol Design
Cache coherence protocol design is an essential aspect of maintaining cache consistency in both snoop-based and directory-based systems. In snoop-based designs, multiple cores in a multicore system are connected via a common bus. Each core has a private cache, a shared cache, and a memory system. In order to implement a cache coherence protocol, separate tables are maintained for each private cache, providing state information for each cache block. When a core initiates a read or write request and finds a cache miss in its private cache, a transaction is placed on the bus. All other cache controllers snoop on the bus and respond accordingly, depending on the protocol.
Snoop-Based Design
In snoop-based cache coherence protocols, cores communicate through a common bus. When a core wants to update or read data, it places a transaction on the bus. Other cache controllers observe these transactions and take appropriate actions in their caches, such as supplying the data or invalidating/updating it. The state of each block in the private caches is maintained in tables. The cache controller updates block state based on processor and snoop events, ensuring cache coherence within the system. However, snoop-based systems become less feasible as the number of cores increases beyond a certain limit.
Directory-Based Design
To address scalability issues, directory-based cache coherence protocols are used in multicore systems with a large number of cores. In this design, an interconnection network connects the cores instead of a bus. Shared cache or memory is distributed across the cores, and a directory is maintained for each memory chunk. The directory provides state information for each block and tracks which cores are sharing the block. State transition diagrams are used for both the directory entries and the blocks in private caches, ensuring proper coherence.
Handling Read Miss in Snoop-Based Systems
When a core in a snoop-based system initiates a read request and encounters a cache miss, it places a read miss transaction on the bus. All cache controllers snoop the bus, searching for the requested data. If a cache has a matching block in the private or shared cache, it responds accordingly, either by supplying the data or invalidating/updating it. The requesting core receives the data from the appropriate cache.
Handling Write Miss in Snoop-Based Systems
In snoop-based systems, a write miss occurs when a core cannot find the requested data in its cache. This triggers a write miss transaction on the bus. If the cache replacement policy selects a victim block in an invalid state, the requesting core can directly write the data. If the victim block is exclusive or shared, the cache controller must perform additional operations, such as writing back the dirty block to the shared cache or invalidating it in case of a shared block.
Handling Read Miss in Directory-Based Systems
In directory-based cache coherence protocols, read miss transactions are sent as messages on the interconnection network. The home node receives the read miss message and checks the block's state in the directory. If the block is not cached, the home node supplies the data to the requesting core and updates the sharer's information in the directory. If the block is in the shared state, the home node still provides the data but retains the block's state and updates the sharer's information accordingly.
Handling Write Miss in Directory-Based Systems
When a write miss occurs in a directory-based system, the home node checks the block's state in the directory. If the block is shared, the home node invalidates the sharers' copies by sending invalidate signals. The requesting core gains exclusive permission for the block and obtains the data from the home node. If the block is in the modified state, the home node fetches the data from the owner node and invalidates the owner's copy. The requesting core then receives the data and updates the sharer's information.
Handling Read Miss to Modified Block
In directory-based systems, when a read miss occurs for a block in the modified state, the home node requests the data from the owner node using a fetch operation. The owner node sends the data to the home node, which then supplies it to the requesting core. The state transition involves changing the block's state to shared, updating the sharers' information, and adding the requesting core to the sharers' list.
Handling Write Back in Modified Block
When a modified block is evicted from a cache in a directory-based system, the cache controller performs a write back operation to update the home node with the modified data. The cache controller then invalidates its copy of the block. As a result, the block is not cached anywhere in the system, and the state in the home node is changed to uncached.
Conclusion
Cache coherence protocol design is crucial in maintaining cache consistency in multicore systems. Snoop-based and directory-based designs offer different approaches to achieving coherence. Snoop-based systems use a common bus to connect cores, while directory-based systems rely on an interconnection network. Both designs require careful handling of read and write misses to maintain consistent data across multiple cores. By considering the state transitions and transactions in each design, cache coherence protocols can be effectively implemented in multicore systems.
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FAQ
Q: What is cache coherence protocol design?
A: Cache coherence protocol design involves creating protocols to ensure that multiple cores in a multicore system have consistent data in their caches. It addresses issues such as cache misses, data sharing, and communication between cores.
Q: What are snoop-based cache coherence protocols?
A: Snoop-based protocols use a common bus to connect cores in a multicore system. When a core updates or reads data, it places a transaction on the bus. Other caches observe these transactions and take appropriate actions, such as supplying or invalidating the data.
Q: What are directory-based cache coherence protocols?
A: Directory-based protocols use an interconnection network instead of a bus to connect cores. Each memory chunk has a directory that tracks the state and sharers information for each block of data. Messages are sent on the network to maintain coherence.
Q: What is a read miss in cache coherence protocols?
A: A read miss occurs when a core requests data that is not present in its cache. Depending on the protocol, the core may need to request the data from another cache or the home node, ensuring that a consistent copy of the data is obtained.
Q: How are write misses handled in cache coherence protocols?
A: In a write miss, a core needs to update or write data that is not present in its cache. The protocol determines whether the block is in a shared or modified state and takes appropriate actions, such as sending invalidate signals to sharers or updating the block's state.
Q: What is the difference between snoop-based and directory-based cache coherence protocols?
A: Snoop-based protocols use a shared bus and observe transactions on the bus to maintain coherence. Directory-based protocols use an interconnection network and maintain a directory for each memory chunk, tracking the state and sharers information. The choice depends on the scalability requirements of the system.
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