Speculative Lookup in N-Way Set Associative Cache
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Solution Overview
Problem
N-way set associative caches face high power consumption during sequential and non-sequential access requests, especially when accessing data across cache line boundaries, as they require extensive lookups in both TAG and data memories, leading to inefficient power usage.
Innovation Solution
Implementing a speculative lookup mechanism in the cache memory to determine if the TAG value associated with the next cache line matches the expected TAG value, allowing for reduced memory access when a match is found, thereby minimizing power consumption by avoiding unnecessary lookups in TAG and data memories.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the cache performs lookups in all TAG memories and data memories for non-sequential accesses, then the data value can be found, but power consumption increases significantly
Solution Approach 1:
The cache performs a speculative lookup in the TAG memory of the next way before actually needing to access that way. This preliminary action predicts which way will be needed next and prepares the TAG value in advance, allowing the cache to avoid full lookups in all TAG and data memories when the prediction is correct, thereby reducing power consumption while maintaining data access accuracy
Solution Approach 2:
The cache dynamically adjusts its lookup behavior based on the sequence of access requests. For sequential accesses, it uses a simplified path with minimal lookups; for non-sequential accesses, it performs full lookups in all TAG and data memories. This dynamic adaptation allows the cache to optimize power consumption according to the actual access pattern while ensuring reliable data access
2Reliability
If the cache treats sequential accesses crossing cache line boundaries as non-sequential, then data access is correct, but power consumption increases due to accessing n TAG memories and n data memories
Solution Approach 1:
When a sequential access request is detected, the cache performs a speculative lookup in the TAG memory of the next way to predict whether the next cache line will be in the same way. This preliminary action allows the cache to prepare for the next access without performing full lookups, reducing power consumption while maintaining data access correctness even when crossing cache line boundaries
Solution Approach 2:
The cache copies the TAG value from the current way's TAG memory to a prediction register or buffer before actually needing it. This copying mechanism allows the cache to have the TAG value ready in advance, avoiding the need to perform full lookups in all TAG memories when the prediction is correct, thereby reducing power consumption while ensuring correct data access
Data Source
AI summary
The present invention provides a data processing apparatus and method for handling cache accesses. The data processing apparatus comprises a processing unit operable to issue a series of access requests, each access request having associated therewith an address of a data value to be accessed. Further, the data processing apparatus has an n-way set associative cache memory operable to store data values for access by the processing unit, each way of the cache memory comprising a plurality of cache lines, and each cache line being operable to store a plurality of data values. The cache memory further comprises for each way a TAG storage for storing, for each cache line of that way, a corresponding TAG value. The cache memory is operable, when the processing unit is issuing access requests specifying data values held sequentially in a cache line of a current way of the cache memory, to perform a speculative lookup in at least one TAG storage to determine whether the TAG value associated with the next cache line in one way associated with the at least one TAG storage equals an expected tag value. If that TAG value does equal the expected tag value, and following an access request identifying a last data value in the cache line of the current way, a further access request is issued identifying the next cache line, then the cache memory is operable, without further reference to any TAG storage of the cache memory, to access from that next cache line of the one way the data value the subject of the further access request. This provides significant power savings when handling accesses to a cache memory.