Prefetch Cache Invalidation for Microprocessor Data Access
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Solution Overview
Problem
The existing prefetching mechanisms in microprocessors place additional load on limited resources, such as the external bus and cache memories, due to the generation of prefetch requests, leading to inefficiencies and potential latency in data access.
Innovation Solution
A memory subsystem with a first-level cache, a second-level cache, and a prefetch cache that allows the same cache line to be simultaneously present in both the second-level and prefetch caches, with the prefetch cache invalidating its copy if a request hits in both, ensuring efficient resource utilization and reducing redundant bus transactions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If prefetch requests are generated to improve data access performance, then data access latency is reduced, but the load on external bus and cache resources increases
Solution Approach 1:
The prefetch cache speculatively prefetches cache lines from external memory into the cache hierarchy before they are actually needed by the processor. This preliminary action brings data into the cache system in advance, reducing future data access latency while the speculative nature allows for efficient resource management through validation mechanisms.
Solution Approach 2:
The prefetch cache acts as an intermediary component between the external memory and the main cache hierarchy. It receives prefetch requests, fetches data from external memory, and provides it to the second-level cache or directly to the first-level cache, thereby mediating the data flow and reducing the load on the external bus for subsequent accesses.
2Productivity
If the same cache line is allowed in both second-level cache and prefetch cache, then resource utilization is improved, but cache coherence management becomes more complex
Solution Approach 1:
The patent merges the functionality of the prefetch cache with the existing second-level cache by allowing them to share the same physical cache lines. This consolidation improves resource utilization by eliminating redundant storage of the same data in separate cache structures, while the unified management approach simplifies coherence protocols compared to completely separate cache systems.
Solution Approach 2:
The prefetch cache creates copies of cache lines from external memory that can simultaneously exist in both the prefetch cache and the second-level cache. This copying mechanism enables parallel access paths while the invalidation logic ensures that when a cache line is present in both caches, the second-level cache provides the authoritative copy to the first-level cache, maintaining coherence.
3Reliability
If prefetch cache invalidates its copy when hit in both caches, then cache coherence is maintained, but additional validation logic is required
Solution Approach 1:
The system implements feedback mechanisms where the prefetch cache monitors hits in both the prefetch cache and second-level cache. When a cache line is found in both caches, the prefetch cache receives feedback to invalidate its copy, ensuring that the second-level cache provides the authoritative data to the first-level cache. This feedback-driven invalidation logic maintains coherence with minimal additional complexity.
Data Source
AI summary
A memory subsystem in a microprocessor includes a first-level cache, a second-level cache, and a prefetch cache configured to speculatively prefetch cache lines from a memory external to the microprocessor. The second-level cache and the prefetch cache are configured to allow the same cache line to be simultaneously present in both. If a request by the first-level cache for a cache line hits in both the second-level cache and in the prefetch cache, the prefetch cache invalidates its copy of the cache line and the second-level cache provides the cache line to the first-level cache.