Speculative Cache Requests for Multi-Level Memory Hierarchy Miss Delays
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Solution Overview
Problem
Current caching techniques in processors lead to substantial cache miss delays due to sequential misses across multiple levels of the memory hierarchy, as they sequentially handle requests without efficiently addressing potential misses in lower levels.
Innovation Solution
The system employs speculative cache requests by determining the likelihood of a cache miss before completing a full lookup operation, sending preemptive requests to lower levels of the memory hierarchy, using tracking data and counters to identify miss-prone instructions and send parallel requests to reduce latency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If sequential cache lookup operations are performed across multiple levels of the memory hierarchy, then cache access accuracy is improved, but cache miss delay increases substantially
Solution Approach 1:
The system performs preliminary actions by sending speculative cache requests to lower levels of the memory hierarchy before the full cache lookup is complete. When a cache miss is detected at L1, the L2 cache is already being accessed in parallel, and similarly for L3 cache. This preliminary action eliminates the sequential waiting time that previously occurred between cache level checks.
Solution Approach 2:
The system maintains continuity of useful action by ensuring that cache lookup operations continue without interruption across multiple cache levels. Instead of stopping at L1 miss to wait for L2, the lookup operation continues seamlessly in parallel across L2 and L3 caches, keeping the processor pipeline filled and eliminating idle waiting periods.
2Loss of time
If speculative cache requests are sent to lower levels of the memory hierarchy, then cache miss delay is reduced, but device complexity increases
Solution Approach 1:
The cache control logic is enhanced with preliminary action capabilities that detect cache misses at each level and automatically initiate parallel lookup operations at lower levels before the current lookup completes. This requires additional control logic to manage speculative requests, validate results, and handle hit/miss scenarios across multiple cache levels simultaneously.
Solution Approach 2:
The system introduces intermediary mechanisms including validation logic that acts as a mediator between speculative cache requests and the processor. This intermediary validates whether speculative requests are appropriate, manages the parallel lookup coordination, and ensures correct result selection, thereby controlling the complexity increase through structured intermediary components.
3Productivity
If parallel speculative requests are sent to multiple cache levels, then instruction throughput is improved, but energy consumption increases
Solution Approach 1:
The system uses preliminary action by sending speculative requests only when there is a high probability of cache misses, based on detection mechanisms that identify likely miss scenarios. This selective approach avoids the energy cost of parallel requests for every memory access,而是 only when beneficial, thereby improving throughput while controlling energy consumption.
Solution Approach 2:
The system dynamically changes operational parameters by adjusting the aggressiveness of speculative request sending based on observed cache behavior patterns. When miss rates are high, more speculative requests are sent; when miss rates are low, speculative requests are reduced or eliminated. This parameter adaptation optimizes the balance between throughput improvement and energy consumption.
Data Source
AI summary
The disclosed embodiments provide a system that uses speculative cache requests to reduce cache miss delays for a cache in a multi-level memory hierarchy. During operation, the system receives a memory reference which is directed to a cache line in the cache. Next, while determining whether the cache line is available in the cache, the system determines whether the memory reference is likely to miss in the cache, and if so, simultaneously sends a speculative request for the cache line to a lower level of the multi-level memory hierarchy.