Cache Way Prediction for Multi-Way Cache Power Reduction
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Solution Overview
Problem
In processor architectures, N-way caches lead to inefficiencies due to the need to read multiple cache ways on every access, resulting in unnecessary power consumption and complexity, especially when the desired value is not found, as the cache controller must read all values before determining which way to check first.
Innovation Solution
Implementing a way prediction mechanism that identifies the most likely cache way to hold the desired value, allowing the processor to read only that value initially and potentially reduce unnecessary reads by encoding the way in a register for subsequent memory access events.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the processor uses N-way cache to improve hit probability, then cache hit rate is improved, but the processor must read all cache ways which increases power consumption and complexity
Solution Approach 1:
The patent applies preliminary action by predicting which cache way is most likely to contain the desired value before actually reading from the cache. The way prediction mechanism uses historical access patterns and statistical information to pre-determine the probable cache way, allowing the processor to read only that predicted way rather than all N ways, thus reducing power consumption while maintaining high hit rate
2Reliability
If the processor reads all cache ways to ensure finding the value, then cache reliability is improved, but device complexity increases due to additional control logic
Solution Approach 1:
The control logic complexity is reduced by using preliminary way prediction. Instead of implementing complex control logic to manage reading all N cache ways and determining which way to check first, the system uses a prediction mechanism that pre-determines the most likely cache way based on historical data, simplifying the control logic while maintaining reliable cache access
Solution Approach 2:
The patent implements feedback by using historical cache access information to update way prediction statistics. The system monitors which cache ways are actually hit and uses this feedback to refine future predictions, creating a self-improving system that reduces complexity over time while maintaining high reliability
3Reliability
If the processor checks all cache ways sequentially, then complete search is ensured, but time is lost due to unnecessary reads
Solution Approach 1:
The patent eliminates time loss by performing preliminary way prediction before cache access. The system predicts which cache way is most likely to contain the desired value and reads only that way, avoiding the time waste of sequentially checking all N ways. The prediction accuracy is maintained through statistical analysis of historical access patterns
Data Source
AI summary
In an example, a system and method are provided for predicting in which way a requested memory address is most likely to be held in a multi-way cache, based on the last way accessed by the specified address register if available. If not available, then the system may determine that no best prediction is available. In that case, each way is read, and the superfluous values are disregarded, or a cache fill is performed as necessary. In certain embodiments, only a portion of the least significant bits of an add operation are used for way prediction in base-plus-offset addressing modes. This enables the decision to be made before the full-width add is complete, so that the clock cycle length is not unnecessarily lengthened by the prediction operation.