Look-ahead Load Pre-fetch Mechanism for Processor Stall Reduction
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Solution Overview
Problem
High latency instructions cause significant stalls in processor instruction execution, leading to diminished performance due to the reliance on long latency accesses to main memory for load memory instructions.
Innovation Solution
Implementing look-ahead load pre-fetching by searching the instruction stream for load memory instructions during stalls and issuing pre-fetch operations to copy data into a cache, reducing latency by making data available ahead of time.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the processor waits for completion of previous instructions before executing subsequent instructions, then instruction dependencies are correctly maintained, but instruction execution stalls for long periods due to high latency memory accesses
Solution Approach 1:
The processor performs preliminary actions by issuing pre-fetch operations for load memory instructions found in the stalled instruction stream. These pre-fetch operations retrieve data from main memory into the cache before the original load instructions are executed, so that when the load instructions resume execution, the data is already available in the cache, eliminating the long latency stalls while maintaining correct instruction ordering through dependency checking
2Loss of time
If the processor issues pre-fetch operations for load memory instructions during stalls, then data availability is improved for subsequent instructions, but processor complexity increases due to additional pre-fetch logic
Solution Approach 1:
The pre-fetch mechanism serves itself by using the stalled time period productively. During instruction stalls, the processor automatically searches the stalled instruction stream for load memory instructions and issues pre-fetch operations without requiring external control signals or additional complex control logic. The existing stall condition triggers the pre-fetch operation, making the processor self-serve during otherwise wasted time
3Productivity
If the processor searches the instruction stream for load memory instructions during stalls, then pre-fetch opportunities are identified, but processing overhead increases during stall periods
Solution Approach 1:
The processor converts the harmful stall period into a beneficial pre-fetch opportunity. Instead of simply waiting idle during instruction stalls, the processor uses this time to search for load memory instructions and issue pre-fetch operations. The energy that would have been wasted during the stall is now productively used to perform pre-fetch operations that will eliminate future stalls, turning a harmful situation into a beneficial one
Data Source
AI summary
A method for look-ahead load pre-fetching that reduces the effects of instruction stalls caused by high latency instructions. Look-ahead load pre-fetching is accomplished by searching an instruction stream for load memory instructions while the instruction stream is stalled waiting for completion of a previous instruction in the instruction stream. A pre-fetch operation is issued for each load memory instruction found. The pre-fetch operations cause data for the corresponding load memory instructions to be copied to a cache, thereby avoiding long latencies in the subsequent execution of the load memory instructions.