Cascaded Delayed Execution Pipeline for D-Cache Miss Prediction
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Solution Overview
Problem
Modern processors experience pipeline stalls due to cache misses, which lead to inefficiency and underutilization, as they cannot predict when data is needed until the instruction is executed, and relying on locality of reference assumptions can result in incorrect cache misses.
Innovation Solution
Implementing a cascaded delayed execution pipeline unit with multiple execution pipelines that execute instructions in a delayed manner relative to each other, allowing for cache miss prediction and scheduling, where instructions resulting in cache misses are executed in a pipeline with delayed execution, enabling early L2 cache access and reducing pipeline stalls.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the processor executes instructions in a pipelined manner to increase processing speed, then the number of instructions processed per unit time increases, but pipeline stalls occur when cache misses happen, leading to processor underutilization
Solution Approach 1:
The patent implements cache miss prediction that performs preliminary detection of potential cache misses before the actual data access occurs in the pipeline. By predicting cache misses in advance using historical cache behavior patterns, the system can proactively initiate data retrieval from lower cache levels or main memory, preventing pipeline stalls rather than reacting to them after they occur.
Solution Approach 2:
The patent dynamically adjusts pipeline execution by selectively stalling specific pipeline stages based on predicted cache miss outcomes. Rather than uniformly stalling the entire pipeline, the system dynamically identifies which pipeline stages will be affected by cache misses and applies targeted stalls, minimizing the overall impact on instruction throughput while maintaining pipeline efficiency.
2Reliability
If the processor waits for cache data before executing instructions, then data accuracy is ensured, but processor utilization decreases due to idle cycles
Solution Approach 1:
The cache miss prediction mechanism performs preliminary identification of instructions that will result in cache misses before execution. This allows the processor to prepare data retrieval operations in advance and overlap them with the execution of independent instructions, ensuring data accuracy is maintained while keeping the processor busy with other tasks during the retrieval process.
Solution Approach 2:
The patent maintains continuous processor utilization by redirecting execution flow to independent instructions or speculative executions while cache data is being retrieved. The prediction mechanism enables the processor to continue executing useful work rather than idling, ensuring that productive actions continue uninterrupted even when data dependency stalls occur.
3Productivity
If the processor uses locality of reference assumptions to pre-fetch cache data, then cache hit rate improves, but incorrect assumptions lead to wrong predictions and additional cache misses
Solution Approach 1:
The patent implements a feedback mechanism that continuously monitors actual cache access patterns and compares them with prediction outcomes. The prediction accuracy is dynamically evaluated by tracking whether predicted cache misses actually occur, and this feedback is used to adjust and refine prediction parameters, reducing the impact of incorrect locality assumptions over time and improving overall prediction reliability.
Data Source
AI summary
A method and apparatus for D-cache miss prediction and scheduling is provided. In one embodiment, execution of an instruction in a processor is scheduled. The processor may have at least one cascaded delayed execution pipeline unit having two or more execution pipelines that execute instructions in a common issue group in a delayed manner relative to each other. The method includes receiving an issue group of instructions, determining if a first instruction in the issue group resulted in a cache miss during a previous execution of the first instruction, and if so, scheduling the first instruction to be executed in a pipeline in which execution is delayed with respect to another pipeline in the cascaded delayed execution pipeline unit.