Primary and Overflow Cache with Hash Indexing for TLB Utilization
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Solution Overview
Problem
Conventional L1 TLB configurations in microprocessors suffer from inefficiencies, leading to underutilization of cache sets and increased latency due to their singular indexing schemes, which result in a larger cache size without significant performance improvement.
Innovation Solution
Implementing a cache memory system with a primary cache and an overflow cache that use different indexing schemes, where the overflow cache serves as both an extension and eviction array for the primary cache, utilizing a hash function to distribute evicted entries and improve cache utilization, thereby reducing the overall size of the L1 TLB while maintaining performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a conventional L1 TLB configuration with a single indexing scheme is used, then the cache structure is simple, but the cache sets are underutilized and performance is limited
Solution Approach 1:
The L1 TLB is segmented into a primary cache portion and an overflow cache portion, each using different indexing schemes. The primary cache uses traditional virtual address bits for indexing, while the overflow cache uses a hash function applied to virtual address bits. This segmentation allows each portion to be optimized for different access patterns, improving overall cache utilization without requiring a completely complex new indexing system.
Solution Approach 2:
Different portions of the cache use different indexing strategies tailored to their specific functions. The primary cache uses direct virtual address indexing for commonly accessed translations, while the overflow cache uses hash-based indexing for evicted entries. This local optimization ensures that each cache portion operates at maximum efficiency for its intended purpose.
2Productivity
If the L1 TLB size is increased to improve performance, then more cache entries are available, but the cache size increases and underutilization problems persist
Solution Approach 1:
The overflow cache adds a new dimension to the cache structure by introducing hash-based indexing as a separate access path. Instead of simply expanding the primary cache, the system creates a parallel cache structure that can be accessed through a different indexing method. This allows the system to effectively increase cache capacity without proportionally increasing the physical size, as the overflow cache can be more compact due to its hash-based organization.
3Productivity
If a single indexing scheme is used for the L1 TLB, then the structure is simple, but evicted entries are not effectively retained and performance degrades
Solution Approach 1:
The L1 TLB structure serves multiple functions through its dual-cache design. The primary cache handles frequently accessed virtual address translations using traditional indexing, while the overflow cache captures and retains evicted entries using hash-based indexing. This multi-functional design allows the system to both quickly service common translations and effectively retain evicted entries that may be reaccessed, improving overall cache hit rate without excessive complexity.
Data Source
AI summary
A cache memory system including a primary cache and an overflow cache that are searched together using a search address. The overflow cache operates as an eviction array for the primary cache. The primary cache is addressed using bits of the search address, and the overflow cache is addressed by a hash index generated by a hash function applied to bits of the search address. The hash function operates to distribute victims evicted from the primary cache to different sets of the overflow cache to improve overall cache utilization. A hash generator may be included to perform the hash function. A hash table may be included to store hash indexes of valid entries in the primary cache. The cache memory system may be used to implement a translation lookaside buffer for a microprocessor.