Processor Way Predictor Suppression for Replay Reduction
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Solution Overview
Problem
Existing processor schedulers have limited visibility into data cache speculation, leading to unnecessary waking up of dependent operations when data is not in the cache, resulting in pipeline stalls and performance degradation.
Innovation Solution
A method and processor configuration that utilize a way predictor to determine if data is in the cache, generating a suppress wakeup signal if it's not, to prevent dependent operations from waking up and reducing the number of operations that need to be replayed.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the scheduler speculatively executes instructions assuming data is in cache, then instruction processing speed is improved, but pipeline stalls occur when data is not in cache requiring replay
Solution Approach 1:
The way predictor performs preliminary detection of data cache presence before the scheduler commits to speculative execution. By checking the way predictor early in the pipeline, the system can determine cache availability in advance, allowing the scheduler to make informed decisions about which instructions to speculatively execute, thereby maintaining high speed while avoiding pipeline stalls from misprediction
Solution Approach 2:
The system implements feedback from the way predictor to the scheduler, where the predictor's determination of cache presence or absence feeds back to control the scheduler's speculative execution decisions. This feedback mechanism ensures that speculative execution only proceeds when the way predictor indicates data is likely in cache, improving reliability without sacrificing speed
2Productivity
If the scheduler wakes up dependent operations based on limited cache speculation visibility, then instruction throughput is improved, but power is wasted and bandwidth is consumed unnecessarily
Solution Approach 1:
The way predictor serves as an intermediary between the cache system and the scheduler, providing enhanced visibility into actual cache contents. This intermediary structure gives the scheduler more accurate information about data availability, allowing it to wake up dependent operations only when truly necessary, thus maintaining high throughput while reducing wasted power and bandwidth on operations that wouldn't execute anyway
3Productivity
If the scheduler wakes up dependent operations without accurate cache visibility, then instruction throughput is maintained, but the number of replayed operations increases
Solution Approach 1:
The way predictor performs preliminary assessment of cache contents before the scheduler wakes up dependent operations. This early detection allows the scheduler to avoid waking up operations that would inevitably fail due to cache misses, thereby maintaining instruction throughput while significantly reducing the number of replayed operations and the associated time loss
Data Source
AI summary
A method for reducing a number of operations replayed in a processor includes decoding an operation to determine a memory address and a command in the operation. If data is not in a way predictor based on the memory address, a suppress wakeup signal is sent to an operation scheduler, and the operation scheduler suppresses waking up other operations that are dependent on the data.