Cache Eviction Using Speculative Execution State Indicators
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Solution Overview
Problem
The limited size of caches in data processing apparatuses leads to the eviction of useful cache lines, resulting in the need for repeated fetching and reinsertion of data or instructions, which is undesirable and inefficient.
Innovation Solution
Incorporating cache circuitry with indicators to determine which items are least likely to be used based on the state of the processing circuitry at the time of insertion, allowing for targeted eviction of unnecessary items and optimizing cache usage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If cache size is limited to maintain fast access speed, then access speed is improved, but cache capacity deteriorates
Solution Approach 1:
The patent changes the parameter of cache line status by introducing a speculatively-executed indicator that can be in different states (set or cleared). This allows the cache to track and differentiate between cache lines that were loaded during speculative execution versus those loaded during normal execution, enabling more intelligent eviction decisions that resolve the contradiction between maintaining fast access with limited capacity.
2Quantity of substance
If cache lines are evicted to make room for new items, then cache capacity utilization is improved, but data reuse deteriorates
Solution Approach 1:
The patent implements feedback by having the detection circuitry monitor the execution state and set the indicator based on whether instructions are being executed speculatively. This feedback mechanism allows the eviction circuitry to make informed decisions about which cache lines to evict, prioritizing the retention of cache lines that are likely to be reused while safely evicting those from speculative execution that may not be needed.
3Productivity
If speculative execution is performed to improve processing throughput, then productivity is improved, but cache efficiency deteriorates
Solution Approach 1:
The patent segments cache lines into two categories based on how they were loaded: those loaded during speculative execution (indicator set) and those loaded during normal execution (indicator cleared). This segmentation allows the cache to manage these different types of cache lines differently during eviction, maintaining cache efficiency despite the benefits of speculative execution for processing throughput.
Data Source
AI summary
A data processing apparatus is provided. It includes cache circuitry to store a plurality of items, each having an associated indicator. Processing circuitry executes instructions using at least some of the plurality of items. Fill circuitry inserts a new item into the cache circuitry. Eviction circuitry determines which of the plurality of items is to be a victim item based on the indicator, and evicts the victim item from the cache circuitry. Detection circuitry detects a state of the processing circuitry at a time that the new item is inserted into the cache circuitry, and sets the indicator in dependence on the state.