L2 Cache Store Buffer Reflections for Memory Ordering Enforcement
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Solution Overview
Problem
The disparity between microprocessor clock speeds and memory access speeds leads to significant performance issues, particularly in enforcing memory-reference ordering requirements like Total Store Ordering (TSO), as existing speculative execution techniques struggle to ensure correct memory models without degrading system performance.
Innovation Solution
A system that enforces memory-reference ordering requirements at the L2 cache level by performing lookups in reflections of store buffers to detect potential conflicts, causing loads and stores to wait or be reordered based on priority, and using a miss buffer to manage pending operations, thereby avoiding violations of memory models.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If speculative execution techniques are used to hide cache-miss latency, then processor productivity is improved, but the ability to enforce memory models like TSO deteriorates
Solution Approach 1:
The patent introduces an intermediary mechanism at the L2 cache level that mediates between speculative execution requests and memory ordering requirements. The L2 cache acts as a mediator that can detect potential TSO violations and intervene by stalling or reordering operations, thus allowing speculative execution to proceed while maintaining memory model correctness.
Solution Approach 2:
The patent segments the memory hierarchy into distinct levels (L1 cache, L2 cache, main memory) with different responsibilities. The L2 cache is specifically equipped with store buffer reflections and conflict detection logic, separating the enforcement function from both the L1 cache and main memory, allowing each component to operate independently with appropriate optimizations.
2Reliability
If stores are held in store buffer until L2 cache miss completes, then memory model correctness is improved, but system performance deteriorates
Solution Approach 1:
The patent performs preliminary actions by pre-loading store buffer reflections into the L2 cache before actual store operations complete. This allows the L2 cache to have advance knowledge of pending stores and make proactive decisions about load ordering, rather than waiting passively for store completion.
Solution Approach 2:
The patent applies partial action by only stalling loads that would actually violate TSO ordering, rather than stalling all loads behind stores. The conflict detection logic identifies specific problematic cases and intervenes only when necessary, allowing other loads to proceed without delay.
3Productivity
If out-of-order execution is used to hide cache-miss latency, then processor productivity is improved, but device complexity increases
Solution Approach 1:
The L2 cache acts as an intermediary that handles the complexity of memory ordering enforcement, allowing the L1 cache and processor core to maintain simpler out-of-order execution logic. The mediation function isolates complexity to a specific component rather than distributing it throughout the entire system.
Data Source
AI summary
One embodiment of the present invention provides a system that enforces memory-reference ordering requirements at an L2 cache. During operation, the system receives a load at the L2 cache, wherein the load previously caused a miss at an L1cache. Upon receiving the load, the system performs a lookup for the load in reflections of store buffers associated with other L1 caches. These reflections are located at the L2 cache, and each reflection contains addresses for stores in a corresponding store buffer associated with an L1 cache, and possibly contains data that was overwritten by the stores. If the lookup generates a hit, which indicates that the load may potentially interfere with a store, the system causes the load to wait to execute until the store commits.