Directory Snoop Filter eCAM Multi-Pass Cache Eviction Processing
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Solution Overview
Problem
Existing cache coherency mechanisms in multiprocessor systems face challenges in efficiently handling cache evictions and maintaining data consistency, leading to performance overheads and complexity, particularly in systems with directory snoop filters and eviction content addressable memory (eCAM).
Innovation Solution
Implementing a multi-pass sequence for cache evictions in a directory snoop filter with eCAM, involving a miss and snoop queue (MSQ), eviction content addressable memory (eCAM), and pipeline logic to manage cache coherency by selecting cache lines for eviction, arbitrating entries, and mediating read requests, thereby reducing hardware requirements and improving resource utilization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional cache coherency mechanisms are used to handle cache evictions in multiprocessor systems, then data consistency is maintained, but hardware overhead and system complexity increase
Solution Approach 1:
The system segments the cache coherency management by separating the directory snoop filter (DSF) into multiple coherent elements arranged in an M×N mesh topology. Each element has its own directory snoop filter, distributing the coherency management workload across multiple independent units rather than requiring a centralized complex structure.
Solution Approach 2:
The patent introduces a multi-pass sequence as an intermediary mechanism between cache eviction requests and the directory snoop filter. This sequence includes a miss and snoop queue (MSQ) that buffers and manages eviction requests, allowing the system to handle cache evictions without directly increasing the complexity of the DSF itself.
2Productivity
If directory snoop filter capacity is increased to reduce evictions, then cache hit rate improves, but hardware resource requirements increase
Solution Approach 1:
Instead of increasing the capacity of a single directory snoop filter, the system segments the DSF across multiple coherent elements in an M×N mesh topology. Each element maintains a smaller, more manageable DSF, collectively providing the necessary caching capacity without requiring any single element to consume excessive hardware resources.
Solution Approach 2:
The patent transitions from a single-dimensional DSF capacity increase to a multi-dimensional mesh topology arrangement. By organizing coherent elements and their DSFs in an M×N grid, the system achieves increased overall caching capacity through spatial distribution rather than expanding individual filter size, thereby optimizing hardware resource utilization.
3Productivity
If multi-pass sequence is implemented for cache evictions, then resource utilization improves, but processing time increases
Solution Approach 1:
The multi-pass sequence performs preliminary actions by pre-managing eviction requests in the miss and snoop queue before they reach the directory snoop filter. The first pass identifies eviction candidates and prepares the MSQ, while subsequent passes execute the actual evictions, allowing the system to optimize resource utilization without creating bottlenecks during critical data access operations.
Solution Approach 2:
The patent implements periodic action through the multi-pass sequence that cycles through different eviction handling stages. The system periodically processes eviction requests in organized passes, with each pass completing specific sub-tasks. This periodic structure allows the system to maintain high resource utilization by keeping pipeline logic busy while managing the time cost through structured, predictable processing intervals.
Data Source
AI summary
Techniques for maintaining cache coherency while sharing data among multiple processors are disclosed. Multiple coherent elements are arranged in an M×N mesh topology. An element can include a compute coherency block (CCB), and a coherency ordering agent (COA). The COAs include a directory snoop filter (DSF), an eviction content addressable memory (eCAM), a miss and snoop queue (MSQ), and a pipeline logic. The CCB and COA include functions for interfacing with a hierarchical cache and directory snoop filter (DSF). A CCB from within one of the multiple coherent elements issues a read request. The corresponding directory snoop filter (DSF) is inspected to determine if there is a slot (way) available for storing information pertaining to the read request. In the event that no eligible vacancies are present in the DSF, a multi-pass process for handling a capacity limit in a DSF is performed.