Cache Coherence Protocols What is Cache Coherence When
- Slides: 18
Cache Coherence Protocols:
What is Cache Coherence? • When one Core writes to its own cache the other core gets to see it, when they read it out of its own cache. • Provides underlying guarantees for the programmer with respect to data validation. • Even one large L 1 Cache per core will not be able to update itself fast enough to processor requests. Less throughput
Cache Coherence: Do we need it?
Coherence Property - I: • Read R from Address X on Core C 0 returns the value written by the most recent write W on X on C 0, if no other core has written to X between W and R
Coherence Property - II • If C 0 writes to X and C 1 reads after a sufficient time and there are no other writes in between, then C 1’s read returns the value from C 0’s write.
Coherence Property –III: • Writes to the same location are serialized: Any 2 OR multiple writes to X must be seen to occur in the same order on all Cores.
How to get Cache Coherence? • No Caches (Bad Performance) • All Cores share the same L 1 Cache (Bad Performance) • Force Read in One Cache to see Write made in another: Broadcasts writes to update other caches (Write Update Coherence)
Write Update Snooping Coherence(The initial issue):
Write Update Snooping (Issue Resolved)– II • Snooping: Cache 0 monitors the write of 1 in A through the bus • Update: When Write is seen, the value is updated in relevant Core’s Cache having Memory block A
Multiple Writes maintains synchronized:
Write Update Enhanced Version (Avoid Memory Writes): • In previous write update protocol every Processor needs to broadcast it on the bus and the Memory(Write Through Caches) • Add a dirty bit to each Cache. It would allow us delay a Memory Write until replaced from Cache.
Core 0 Block Refreshed and Read with A (value? ) • Dirty Bit : Memory needs to be updated(WB to RAM), Dirty Bit Cache only has the updated value
Multi – Writes and Dirty Block Replacement: • Memory won’t be updated until Dirty block is replaced
Writing from a different Cache:
Dirty Bit Benefits: • Write to Memory only when Dirty Block replaced • Read from Memory only if no block in a dirty state, else all reads from the Dirty Bit Cache • Significantly reduces Read and Write transactions to Memory
Write Update Bus Enhancement:
Write to same Memory Location (S = 1)
Broadcast only when shared among cores:
- Cache coherence protocols
- Cache coherence example
- Cache bellek
- Cache coherence tutorial
- Chained cache coherence protocol
- Gpu cache coherence
- Local government service protocols
- Application layer protocols
- Categories of attacks
- Reliability concepts and measures in distributed database
- Routing and switching protocols
- Real world protocols
- Communication protocols for rpc
- Ssv ems protocols
- Wireless lan protocols
- Data link control
- Accountable talk protocols
- Vaeai protocols
- Remsa protocols