Instruction Cache Prefetch Logic for Low Power
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Solution Overview
Problem
Conventional instruction cache prefetching techniques lead to increased memory access bandwidth usage and power consumption due to unnecessary prefetching of instructions, especially when non-sequential operations occur, causing cache misses and processor stalls.
Innovation Solution
A method where the prefetch decision logic determines attributes of the missed fetch address, such as sequential or non-sequential access, and position within the cache line, to intelligently decide whether to prefetch instructions from the next cache line, reducing unnecessary prefetching and optimizing memory access.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the next cache line is always prefetched on a miss, then cache miss rates are reduced, but memory access bandwidth is wasted and power consumption increases
Solution Approach 1:
The patent applies local quality by making the prefetch behavior location-dependent. Instead of uniformly prefetching on all misses, the system determines whether the miss is sequential or non-sequential and applies prefetching only in the appropriate case. This localized approach ensures prefetching occurs only where beneficial (sequential accesses) and avoids it where harmful (non-sequential accesses), thereby reducing unnecessary power consumption while maintaining low cache miss rates.
Solution Approach 2:
The patent implements dynamics by making the prefetch decision adaptive rather than static. The prefetch logic dynamically adjusts its behavior based on the type of access (sequential vs. non-sequential) detected at the time of the miss. This dynamic adaptation allows the system to optimize between cache hit rate and power consumption in real-time based on actual access patterns.
2Reliability
If the next cache line is always prefetched on a miss, then cache miss rates are reduced, but memory access bandwidth usage increases
Solution Approach 1:
The patent applies local quality by making the prefetch behavior location-dependent. Instead of uniformly prefetching on all misses, the system determines whether the miss is sequential or non-sequential and applies prefetching only in the appropriate case. This localized approach ensures prefetching occurs only where beneficial (sequential accesses) and avoids it where harmful (non-sequential accesses), thereby reducing unnecessary memory bandwidth usage while maintaining low cache miss rates.
Solution Approach 2:
The patent applies partial action by performing prefetching only partially - specifically, only for sequential misses rather than all misses. This partial application of the prefetching action is sufficient to capture the majority of beneficial cases while avoiding the excessive bandwidth consumption that would result from universal prefetching.
3Reliability
If prefetching is performed on non-sequential accesses, then cache miss rates are reduced, but processor performance decreases due to unnecessary memory accesses
Solution Approach 1:
The patent applies local quality by making the prefetch behavior location-dependent. Instead of uniformly prefetching on all misses, the system determines whether the miss is sequential or non-sequential and applies prefetching only in the appropriate case. This localized approach ensures prefetching occurs only where beneficial (sequential accesses) and avoids it where harmful (non-sequential accesses), thereby reducing unnecessary memory accesses and improving processor performance while maintaining low cache miss rates.
Solution Approach 2:
The patent converts the potentially harmful effect of unnecessary prefetching on non-sequential accesses into a benefit by implementing logic that detects and prevents such prefetching. The harm of reduced processor performance from unnecessary memory accesses is converted into a benefit by making the prefetching logic intelligent enough to recognize when prefetching should not occur, thereby eliminating the performance penalty while retaining the benefits of prefetching where applicable.
Data Source
AI summary
When misses occur in an instruction cache, prefetching techniques are used that minimize miss rates, memory access bandwidth, and power use. One of the prefetching techniques operates when a miss occurs. A notification that a fetch address missed in an instruction cache is received. The fetch address that caused the miss is analyzed to determine an attribute of the fetch address and based on the attribute a line of instructions is prefetched. The attribute may indicate that the fetch address is a target address of a non-sequential operation. Another attribute may indicate that the fetch address is a target address of a non-sequential operation and the target address is more than X % into a cache line. A further attribute may indicate that the fetch address is an even address in the instruction cache. Such attributes may be combined to determine whether to prefetch.