L1 Cache Memory Ordering Enforcement via Speculative Execution Failure
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Solution Overview
Problem
Existing methods for enforcing memory reference ordering requirements, such as Total Store Ordering (TSO), in multiprocessor systems face challenges due to the scalability issues of memory-disambiguation buffers and the need to recompute results during speculative execution, which can lead to performance bottlenecks and memory model violations.
Innovation Solution
A system that enforces memory reference ordering requirements at the L1 cache level by using invalidation signals from the cache-coherence system to fail speculative-execution mode if a cache line with a load-mark is updated, thereby preventing memory model violations without relying on memory-disambiguation buffers, and employs checkpointing to resume normal-execution mode.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If memory-disambiguation buffers are used to enforce memory reference ordering requirements, then memory model compliance is improved, but device complexity and scalability deteriorate due to large CAM structures
Solution Approach 1:
The patent extracts the memory ordering enforcement mechanism from the traditional memory-disambiguation buffer (CAM structure) and relocates it to the L1 cache level. By examining invalidation signals at the L1 cache and checking load-marks in cache lines, the system enforces TSO requirements without requiring large, complex CAM structures. This extraction principle resolves the contradiction by maintaining reliability while reducing device complexity.
Solution Approach 2:
The patent changes the dimension of enforcement from the memory buffer level (traditional approach) to the cache line level (L1 cache). By integrating ordering checks into the cache invalidation signal processing path and using load-mark bits in cache lines, the system achieves memory model compliance through a different operational dimension, avoiding the scalability issues of large CAM structures.
2Productivity
If speculative execution is used to hide cache-miss latency, then execution speed is improved, but memory reference ordering requirements may be violated
Solution Approach 1:
The patent implements a feedback mechanism where invalidation signals from the cache-coherence system trigger examination of load-marks in L1 cache lines. When a cache line with a load-mark is invalidated during speculative execution, the system detects this condition and fails the speculative-execution mode, thereby maintaining memory model compliance while allowing speculative execution to proceed when safe.
Solution Approach 2:
The patent sets load-marks in cache lines before speculative execution occurs. These pre-set markers enable the system to quickly detect potential memory model violations during speculative execution by simply checking whether invalidated cache lines have load-marks, allowing fast detection and resolution without complex runtime analysis.
3Productivity
If execute-ahead mode is used to avoid re-computation, then productivity is improved, but it is difficult to ensure memory model adherence
Solution Approach 1:
The patent introduces load-marks as intermediary indicators in cache lines that mediate between speculative execution and memory model enforcement. These load-marks serve as a simple, efficient mechanism to track which cache lines were loaded during speculative execution, enabling the system to maintain execute-ahead mode productivity while ensuring memory ordering compliance through the intermediary marking system.
Data Source
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AI summary
One embodiment of the present invention provides a system that enforces memory reference ordering requirements, such as Total Store Ordering (TSO), at a Level 1 (L1) cache in a multiprocessor. During operation, while executing instructions in a speculative-execution mode, the system receives an invalidation signal for a cache line at the L1 cache wherein the invalidation signal is received from a cache-coherence system within the multiprocessor. In response to the invalidation signal, if the cache line exists in the L1 cache, the system examines a load-mark in the cache line, wherein the load-mark being set indicates that the cache line has been loaded from during speculative execution. If the load-mark is set, the system fails the speculative-execution mode and resumes a normal-execution mode from a checkpoint. By failing the speculative-execution mode, the system ensures that a potential update to the cache line indicated by the invalidation signal will not cause the memory reference ordering requirements to be violated during the speculative-execution mode.