Selective Speculative Prefetch for Last-Level Cache
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Solution Overview
Problem
Cache misses in a last-level cache (LLC) result in significant lag times and reduced processing efficiency due to the need to retrieve data from memory rather than a cache.
Innovation Solution
A processor generates speculative prefetch requests for data likely to cause a cache miss in the LLC, bypassing the LLC check and issuing requests directly to a memory-side cache, thereby reducing processing times.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If speculative prefetch requests are issued for all potential cache misses, then processing efficiency is improved, but memory-side cache capacity is consumed and eviction of other prefetch fills occurs
Solution Approach 1:
The system performs preliminary actions by issuing speculative prefetch requests for data that is likely to be needed soon, retrieving it from memory and storing it in the memory-side cache before it is actually requested. This preliminary retrieval and storage prevents future cache misses and reduces processing delays.
Solution Approach 2:
The system changes the parameter of prefetch request confidence by analyzing historical access patterns and system state to determine which speculative prefetch requests have confidence above a threshold. Only requests meeting this parameter criterion are issued, optimizing the balance between improving processing efficiency and preserving memory-side cache capacity.
2Quantity of substance
If speculative prefetch requests are selectively issued based on confidence threshold, then memory-side cache capacity is preserved, but processing efficiency improvement is reduced compared to issuing all prefetch requests
Solution Approach 1:
The system dynamically adjusts the confidence threshold parameter based on system conditions, memory-side cache occupancy, and observed access patterns. This parameter adjustment optimizes the selection of speculative prefetch requests, ensuring that only high-confidence requests are issued, thereby preserving memory-side cache capacity while still achieving significant processing efficiency improvements.
Data Source
AI summary
In response to generating one or more speculative prefetch requests for a last-level cache, a processor determines prefetch analytics for the generated speculative prefetch requests and compares the determined prefetch analytics of the speculative prefetch requests to selection thresholds. In response to a speculative prefetch request meeting or exceeding a selection threshold, the processor selects the speculative prefetch request for issuance to a memory-side cache controller. When one or more system conditions meet one or more condition thresholds, the processor issues the selected speculative prefetch request to the memory-side cache controller. The memory-side cache controller then retrieves the data indicated in the selected speculative prefetch request from a memory and stores it in a memory-side cache in the data fabric coupled to the last-level cache.