Multi-Level Cache Prefetch Pollution Control
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Solution Overview
Problem
Modern prefetching mechanisms in cache memory systems often result in 'wasted' prefetches, leading to performance hits due to incorrect predictions, consumption of system and memory bandwidth, and cache pollution, as they prefetch instructions that are not consistently accessed by the processor.
Innovation Solution
A multi-level cache system that identifies cache entries as 'untouched' or 'accessed' and employs a preselected cache line replacement algorithm to prevent or store these entries in a second-level cache, ensuring that only accessed entries are stored in the second cache, and 'untouched' entries are either discarded or marked as the least recently used, thereby reducing cache pollution and optimizing storage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If aggressive prefetching is used to keep the processor busy, then processor speed is improved, but cache pollution increases and performance hits occur due to wasted prefetches
Solution Approach 1:
The patent applies preliminary action by implementing a prefetching mechanism that proactively loads data into the cache before the processor actually needs it. The L2 cache controller monitors processor access patterns and preemptively fetches data from main memory into the L2 cache, ensuring that frequently accessed data is already available when needed, thus maintaining high processor speed without excessive cache pollution.
Solution Approach 2:
The patent implements feedback through the L2 cache controller that continuously monitors which prefetched data is actually accessed by the processor. The controller tracks access patterns and adjusts prefetching behavior based on this feedback - reducing prefetching for data that isn't accessed and maintaining or increasing prefetching for data that is frequently used, thereby optimizing the balance between processor speed and cache pollution.
2Productivity
If prefetching algorithms are used to predict instruction access, then processor continuity is improved, but incorrect predictions cause wasted memory bandwidth consumption
Solution Approach 1:
The patent applies dynamics by making the prefetching behavior adaptive rather than static. The L2 cache controller dynamically adjusts prefetching intensity and targets based on real-time monitoring of processor access patterns. When predictions are accurate and processor continuity is maintained, prefetching continues; when incorrect predictions are detected through monitoring, the controller reduces or adjusts prefetching to minimize wasted memory bandwidth consumption.
3Speed
If prefetched instructions are stored in cache, then processor access speed is improved, but cache space is consumed by unused instructions
Solution Approach 1:
The patent applies the taking out principle by selectively extracting only the useful prefetched data from the cache. The L2 cache controller monitors which prefetched instructions are actually accessed by the processor and keeps those in the cache, while allowing unused prefetched instructions to be evicted. This ensures that cache space is utilized efficiently for data that provides actual speed benefits rather than being filled with unused prefetched instructions.
Data Source
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AI summary
A method and apparatus for controlling a first and second cache is provided. A cache entry is received in the first cache, and the entry is identified as having an untouched status. Thereafter, the status of the cache entry is updated to accessed in response to receiving a request for at least a portion of the cache entry, and the cache entry is subsequently cast out according to a preselected cache line replacement algorithm. The cast out cache entry is stored in the second cache according to the status of the cast out cache entry.