L2 Cache Latency Prediction for Replay-Efficient Instruction Scheduling
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Solution Overview
Problem
Existing microprocessors face inefficiencies due to unpredictable latency times in level-2 data cache accesses, leading to increased replay of instructions and resource utilization, particularly with load instructions, which affect performance and power consumption.
Innovation Solution
A processor with a time counter and static scheduling algorithm that predicts and adjusts for level-2 cache latency times, allowing instructions to be dispatched at optimal times based on resource availability and data dependencies, using a time counter to increment periodically and adjust latency times based on actual cache responses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the processor uses a fixed latency time for level-2 cache accesses, then the instruction scheduling is simple, but the performance deteriorates due to unpredictable actual cache access times causing instruction replays
Solution Approach 1:
The processor predicts the latency time of level-2 cache accesses before actual access occurs. The prediction mechanism estimates cache access time in advance, allowing the instruction scheduler to plan instruction execution and data dependency resolution ahead of time, reducing the need for instruction replays and improving overall execution efficiency
Solution Approach 2:
The latency prediction mechanism incorporates feedback from actual cache access outcomes. When the predicted latency differs from the actual latency, the system adjusts future predictions based on this feedback, enabling dynamic adaptation to varying cache access patterns and improving prediction accuracy over time
2Reliability
If the processor replays instructions on cache misses, then correctness is maintained, but power consumption increases due to repeated instruction execution
Solution Approach 1:
The processor uses the predicted latency time to pre-schedule dependent instructions before cache access completion is known. By planning instruction execution ahead of time based on predictions, the system reduces the frequency of instruction replays, thereby maintaining correctness while reducing the energy waste associated with repeated execution
Solution Approach 2:
The instruction scheduling mechanism dynamically adjusts execution plans based on predicted latency times. When predictions indicate potential cache misses, the scheduler proactively repositions or delays dependent instructions, adapting the execution flow to minimize replays and reduce power consumption while maintaining correctness
3Productivity
If the processor speculatively schedules load dependent instructions, then throughput is improved, but the complexity of handling replays increases
Solution Approach 1:
The processor speculatively schedules load dependent instructions in advance based on predicted latency times. By making scheduling decisions before cache access completion, the system achieves higher instruction throughput. The prediction mechanism provides a basis for these speculative decisions, reducing the need for complex replay management compared to traditional approaches
4Adaptability or versatility
If the processor uses traditional dynamic scheduling, then adaptability to varying cache latency is achieved, but the complexity of instruction management increases significantly
Solution Approach 1:
The processor performs preliminary latency prediction for level-2 cache accesses before instruction scheduling. This advance prediction provides the scheduler with estimated latency information, enabling it to make informed scheduling decisions without requiring complex real-time adaptation mechanisms. The prediction layer simplifies the scheduling complexity while maintaining adaptability to varying cache latency conditions
Data Source
AI summary
A processor includes a time counter and issuing instruction and executing instruction at a future time which is based on the time counter. The execution times are based on fixed latency times of instructions with exception of the load instruction which is based on the data cache hit latency time. A data cache miss causes the load instruction to fetch data from the level 2 cache wherein a time tracker unit adjusts the level 2 cache latency time based on a counter.