Shadow Tag Memory Relaxed Cache Probe Invalidation
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Solution Overview
Problem
Conventional caching techniques lead to excessive cache probe traffic due to immediate propagation of write operations, resulting in inefficient cache coherence and performance impacts.
Innovation Solution
Implementing a shadow tag memory to maintain state information for cachelines across multiple private caches, delaying cache probe transmission until a synchronization event occurs, such as a scope acquire operation, and using bit vectors to manage stale data indicators.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If immediate cache probe transmission is implemented for write operations, then cache coherence is maintained, but excessive cache probe traffic occurs and performance is impacted
Solution Approach 1:
The patent sets stale indicator bits in advance when write operations occur, but delays actual cache probe transmission until synchronization events (such as scope acquire operations) are detected. This preliminary marking without immediate propagation reduces unnecessary probe traffic while maintaining coherence through deferred validation.
Solution Approach 2:
The system dynamically adjusts cache probe transmission timing based on synchronization events. Cache probes are transmitted conditionally - only when a scope acquire operation is detected - rather than following a fixed immediate transmission protocol. This dynamic approach optimizes the balance between coherence maintenance and performance.
2Reliability
If frequent cache invalidations are performed to maintain coherence, then data freshness is ensured, but power usage and processor cycles increase
Solution Approach 1:
Instead of continuous or immediate cache invalidation, the system uses periodic validation triggered by synchronization events. Cache probes are transmitted at specific intervals tied to scope acquire operations rather than continuously, reducing the frequency of invalidation operations and associated power consumption.
Solution Approach 2:
The system uses stale indicator bits that automatically mark cachelines as potentially stale without requiring immediate validation. This self-marking mechanism reduces the need for active coherence management, allowing the system to defer validation until naturally occurring synchronization points, thereby reducing power usage.
3Reliability
If cache probes are transmitted immediately for each write operation, then coherence is maintained, but unnecessary cache invalidations occur
Solution Approach 1:
The system performs preliminary marking of cachelines with stale indicator bits without immediately transmitting cache probes. This allows cachelines to remain valid longer in the cache hierarchy, extending their useful life while still preparing for potential invalidation at appropriate synchronization points.
Solution Approach 2:
The system uses feedback from synchronization events (scope acquire operations) to determine when cache probe transmission is actually necessary. This feedback mechanism prevents unnecessary invalidations by only triggering probe transmission when a scope acquire operation indicates that other cores may be accessing the data.
Data Source
AI summary
Methods, systems, and devices maintain state information in a shadow tag memory for a plurality of cachelines in each of a plurality of private caches, with each of the private caches being associated with a corresponding one of multiple processing cores. One or more cache probes are generated based on a write operation associated with one or more cachelines of the plurality of cachelines, such that each of the cache probes is associated with cachelines of a particular private cache of the multiple private caches, the particular private cache being associated with an indicated processing core. Transmission of the cache probes to the particular private cache is prevented until, responsive to a scope acquire operation from the indicated processing core, the cache probes are released for transmission to the respectively associated cachelines in the particular private cache.