Miss Lookahead Processor Power Reduction via Register Invalidation
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Solution Overview
Problem
The existing miss-lookahead technique for improving microprocessor performance is costly in terms of hardware and power consumption, especially for processors with many architectural registers, and can slow down other strands sharing the same resources.
Innovation Solution
The system eliminates the need for checkpointing architectural registers by marking them as invalid during lookahead mode, limits the number of strands in lookahead mode, and terminates lookahead mode when unlikely to uncover additional cache misses, reducing hardware costs and power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If checkpointing architectural registers is implemented to support miss-lookahead mode, then processor performance is improved through effective prefetching, but hardware costs and device complexity increase significantly
Solution Approach 1:
The patent extracts the checkpointing operation from the miss-lookahead mode support mechanism. Instead of checkpointing architectural registers, the system only checkpoints the program counter and a small set of control flags, thereby removing the expensive register checkpointing component while retaining the performance benefits of miss-lookahead
Solution Approach 2:
The patent uses lightweight, temporary markers (invalidation bits) instead of expensive persistent checkpoint structures. These markers are cheap to create and discard, allowing the system to track register validity without the overhead of full register state checkpointing
2Reliability
If architectural registers are checkpointed during miss-lookahead mode, then correct restoration of processor state is achieved, but power consumption increases
Solution Approach 1:
The patent extracts the power-intensive register checkpointing operation from the miss-lookahead protocol, retaining only the essential program counter checkpointing and using lightweight invalidation markers to track register state, thereby dramatically reducing power consumption while maintaining correct state restoration
Solution Approach 2:
The patent changes the state representation from full register values to simple invalidation bits. This parameter change reduces the amount of data that needs to be saved and restored, directly reducing power consumption associated with memory accesses and register file operations
3Productivity
If miss-lookahead mode is enabled for multiple concurrent strands, then processor utilization is improved, but hardware resources are depleted and other strands are slowed down
Solution Approach 1:
The patent uses lightweight copies of control state (invalidation bits and program counter) instead of full register state copies. This allows multiple strands to maintain their own miss-lookahead state independently with minimal resource consumption, enabling better concurrent utilization without depleting shared hardware resources
Data Source
AI summary
A system that executes program instructions on a processor is described. During a normal-execution mode, the system issues instructions for execution in program order. Upon encountering an unresolved data dependency during execution of an instruction, the system speculatively executes subsequent instructions in a lookahead mode to prefetch future loads. While executing in the lookahead mode, if the processor determines that the lookahead mode is unlikely to uncover any additional outer-level cache misses, the system terminates the lookahead mode. Then, after the unresolved data dependency is resolved, the system recommences execution in the normal-execution mode from the instruction that triggered the lookahead mode.