Load Scheduler Threshold Control for Out-of-Order Hazard Detection
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Solution Overview
Problem
Existing load schedulers face challenges in issuing out-of-order load instructions efficiently while detecting and resolving hazards without incurring extensive resource overhead, particularly in systems with multiple processing units.
Innovation Solution
Implementing a load scheduler with a skipping load instruction tracking unit that uses max skipping and max skipped thresholds to limit the issuance of out-of-order load instructions, allowing for efficient detection and resolution of hazards by setting valid flags and address tags, and flushing instructions as necessary.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If unlimited out of order load instructions are issued, then processing performance is improved, but tracking overhead increases extensively
Solution Approach 1:
The patent applies partial action by limiting out-of-order load instruction issuance to a threshold number (e.g., one or two instructions) rather than allowing unlimited out-of-order execution. This partial approach captures most performance benefits while keeping tracking overhead manageable. The load scheduler monitors the number of outstanding out-of-order load instructions and stalls further issuance when the threshold is reached, balancing performance gain against complexity cost.
2Productivity
If out of order load instructions are issued, then instruction throughput is improved, but hazard detection complexity increases
Solution Approach 1:
The hazard detection mechanism is segmented into specific tracking fields within the load scheduler: a skip counter to track the number of out-of-order load instructions, and hazard detection logic that specifically checks for store-load hazards. This segmentation allows focused monitoring of critical hazard conditions without requiring comprehensive tracking of all possible instruction interactions, reducing overall detection complexity.
Solution Approach 2:
The system employs feedback mechanisms where the load scheduler continuously monitors the execution status of out-of-order load instructions and adjusts issuance accordingly. When a hazard is detected or the threshold is reached, feedback signals stall further out-of-order load instruction issuance until the hazard resolves or the instruction completes, dynamically controlling throughput based on current system state.
Data Source
AI summary
A load scheduler capable of limited issuing of out of order load instruction is disclosed. The load scheduler uses a max skipping threshold which limits the number of skipping load instructions and a max skipped threshold which limits the number of skipped load instructions. An address tag for a skipping instruction is stored in a skipping load instruction tracking unit when a skipping load instruction is issued. When a skipped load instruction issues, the address tag of the skipped load instruction is compared to the address tag of the skipping instruction to determine if a hazard from the out of order issuing of the skipping load instruction caused a hazard and must be flushed.