Hierarchical Core Valid Tracking for Precise Cache Coherency
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Solution Overview
Problem
Conventional cache coherency tracking mechanisms in multi-core processors suffer from increased complexity and inefficiency due to coarse-grained tracking, leading to excessive cache/snoop traffic and performance degradation, particularly in systems with complex sharing patterns and large core counts.
Innovation Solution
Implementing hierarchical core valid (CV) tracking technology that utilizes a hierarchical data structure to precisely track core ownership, independent of physical topology, allowing for fine-grained tracking of core associations and reducing false aliasing through a bit vector format that includes cluster, sub-cluster, and relative logical ID fields.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If coarse-grained tracking is used in conventional cache coherency mechanisms, then device complexity is reduced, but cache/snoop traffic increases and performance degrades
Solution Approach 1:
The patent segments the core tracking mechanism into hierarchical levels: directory-level tracking for coarse-grained groups and snoop filter-level tracking for fine-grained individual core identification. This segmentation allows the system to maintain low complexity at the directory level while achieving high performance through precise tracking at the snoop filter level, resolving the contradiction between simplicity and effectiveness.
2Device complexity
If coarse-grained tracking is used, then device complexity is reduced, but fabric bandwidth loss increases
Solution Approach 1:
The hierarchical tracking structure segments bandwidth consumption by allowing directory operations to use compact group identifiers while snoop operations use precise core-level identification. This reduces unnecessary fabric traffic by avoiding broad broadcasts, thereby reducing fabric bandwidth loss without significantly increasing device complexity.
3Measurement precision
If fine-grained tracking is implemented, then cache coherency precision is improved, but device complexity increases
Solution Approach 1:
The patent introduces a hierarchical dimension to the tracking mechanism, organizing core identification into multiple levels (directory level and snoop filter level). This dimensional organization allows precise fine-grained tracking at the snoop filter level while maintaining simpler structures at higher levels, thereby achieving high precision without proportionally increasing overall device complexity.
4Productivity
If fine-grained tracking is implemented, then performance is improved, but false aliasing occurs with coarse-grained methods
Solution Approach 1:
The patent segments the tracking function between directory and snoop filter, with the snoop filter performing fine-grained tracking to eliminate false aliasing. This segmentation ensures high reliability in core ownership identification, which directly improves application and workload performance by preventing incorrect coherency decisions.
Data Source
AI summary
Techniques and mechanisms to provide hierarchical core valid tracking. In an embodiment, an apparatus comprises a cache to store information accessible by two or more cores, and circuitry coupled to the cache to maintain coherence of the information stored in the cache. The circuitry is further to hierarchically track respective associations of the information stored in the cache with the two or more cores. In another embodiment, a lowest hierarchical level of the hierarchically tracked associations is to indicate a logical core identifier of a particular core of the two or more cores.