Note_Book_Multi-Core_CacheHierarchies-Ch1

Shared and Private Caches

• Shared LLC




• No duplicate in L2.
• May have Multiple copy in L1.
• Coherence must be maintained among L2 and L1.
• For bus, coherence is maintained with snooping protocol.
• For write back policy.
• Read
• A core put a request on the bus
• Other cores looks up their L1 upon receiving the message.
• If no core has the block in modified state.
• L2 cache should respond after the snoop of L1 finished.
• Write
• The block for writing is invalidated during the snooping.
• write-back and write-update protocols may lead to heavy traffic load on the bus.




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• For large-scale systems, interconnect and directory protocol are needed.
• Advantages of shared cache
• dynamically allocable storage
• Only need to maintain single copy of data
• better space ultilization
• better hit rates
• coherence misses of private caches can be resolved in shared cache
• Disadvantages of shared cache
• Miss rate will increase because cores may interfere each other, and leads to poor QoS 
• Unnecessary coherence for non-shared data 
• long access time
• The L2 cache here is inclusive




• Private LLC

• Both L1 and L2 are private
• Coherence is maintained among L2$ w/ directory-based protocol across scalable non-broadcast interconnect
• Private L2 is looked up before request on bus.
• Snooping takes quite of time
• The time is long for accessing large private L2
• Advantages of private LLC
• Threads within different cores won't affect each other
• Size of L2 is small
• Smaller access time without coherence.
• Non-shared data will be benefited.
• Disadvantages of private LLC
• Replication will occur for shared data, which leads to the waste of space
• Static allocation may confined the data block in one core
• Simple coherence on bus
• on L2 miss
• snoop to bus 
• if no response for snoop
• forward request to next level
• For coherence in directory-based protocol
• L2 miss -> send to directory(may be centralized or distributed directory)
• Directory keeps track of all private L2$s
• The directory needs #ofL2s * #ofWays comparisons of tag
• If block is found in another L2 ->Messages are exchanged between cores an directories
• With directory, the replication is not required for shared data 
• Workload Analysis
• More than half cache block are shared
• Shared must be better than private

Centralized and Distributed Shared Cache

• Centralized Cache