Speculative Execution Resource Allocation via Temporal Ordering Policy
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current speculative execution techniques, such as scout mode, face issues like live-lock and early-prefetch eviction due to cache line evictions and sub-optimal performance, especially when processors generate checkpoints at launch instructions or run for extended times in scout mode, leading to repeated cache requests and inefficiencies.
Innovation Solution
Implementing a temporal ordering policy for allocating limited processor resources by using speculative-use indicators to manage cache lines and other resources, ensuring that resources allocated during speculative-execution mode are protected from eviction, thereby preventing live-lock and optimizing prefetch operations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the processor generates a checkpoint at the launch instruction before commencing scout mode execution, then the processor can resume normal-execution mode from the launch instruction, but the cache line required by the launch instruction is re-read and may be evicted during scout mode, causing live-lock
Solution Approach 1:
The processor asserts a speculative-use indicator on the cache line before entering scout mode execution. This preliminary action protects the cache line from eviction during scout mode, ensuring it remains available when the processor returns to normal-execution mode and needs to re-execute the launch instruction.
Solution Approach 2:
The speculative-use indicator acts as an intermediary mechanism between the cache line and the eviction policy. This indicator mediates the conflict by preventing the cache line from being evicted during scout mode execution, thus resolving the live-lock problem without requiring changes to the fundamental scout mode operation.
2Productivity
If the processor runs for an extended time in scout mode to prefetch future loads, then more work is completed during stall conditions, but early prefetches are evicted from the cache by later prefetches, causing sub-optimal performance
Solution Approach 1:
The processor asserts speculative-use indicators on cache lines before entering scout mode and maintains them throughout scout mode execution. This preliminary protection ensures that early prefetches remain in the cache and are not evicted by later prefetches, improving cache utilization efficiency while maintaining high productivity.
Solution Approach 2:
The speculative-use indicator provides differentiated treatment to different cache lines based on their temporal relationship to the launch instruction. Cache lines that are needed for the launch instruction or early prefetches receive protection, while other cache lines follow normal eviction policies. This local quality approach optimizes cache behavior for critical operations without unnecessarily restricting overall cache flexibility.
3Loss of time
If the processor uses out-of-order execution to hide cache-miss latency, then instructions can be issued when operands become available, but hardware complexity grows quadratically with the size of the issue queue
Solution Approach 1:
The processor separates execution into two distinct modes: normal-execution mode for standard instruction processing and scout mode for speculative prefetching. This segmentation allows the processor to handle cache-miss latency differently depending on the execution context, avoiding the need for a large out-of-order issue queue while still achieving latency hiding through speculative execution.
Solution Approach 2:
Instead of using out-of-order execution to hide cache-miss latency, the processor inverts the approach by using speculative execution in scout mode to perform prefetches during stall conditions. This inversion achieves the same latency hiding goal with reduced hardware complexity by avoiding quadratic growth associated with large out-of-order issue queues.
Data Source
AI summary
One embodiment provides a system which uses a temporal ordering policy for allocation of limited processor resources. The system starts by executing instructions for a program during a normal-execution mode. Upon encountering a condition which causes the processor to enter a speculative-execution mode, the processor performs a checkpoint and commences execution of instructions in the speculative-execution mode. Upon encountering an instruction which requires the allocation of an instance of a limited processor resource during the execution of instructions in the speculative-execution mode, the processor checks a speculative-use indicator associated with each instance of the limited processor resource. Upon finding the speculative-use indicators asserted for all instances of the limited processor resource which are available to be allocated for the instruction, the processor aborts the instruction. On the other hand, upon finding the speculative-use indicator is deasserted for an instance of the limited processor resource which is available to be allocated for the instruction, the processor asserts the speculative-use indicator associated with the instance and executes the instruction.