Low-Overhead Memory Atomicity via Store Buffer Delayed Reclamation
Solution Overview
Problem
Conventional processors with robust memory atomicity (RMA) facilities face inefficiencies when executing small atomic regions due to significant processing overhead, making them less performant for frequent, small atomic regions and optimized code with back-to-back atomic operations.
Innovation Solution
The implementation of low-overhead memory atomicity (LOMA) facilities, which use delayed reclamation to shift memory atomicity management from the data cache into load and store buffers, allowing atomic regions to commit before senior pipeline walks occur and avoiding the need for cache updates, thereby reducing processing overhead.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If robust memory atomicity (RMA) facilities are used to execute atomic regions, then atomicity protection is ensured, but processing overhead increases significantly for small atomic regions
Solution Approach 1:
The patent segments memory atomicity management into two distinct paths: RMA facilities for large atomic regions requiring full cache coherence, and LOMA facilities for small atomic regions using delayed reclamation. This segmentation allows each path to be optimized for its specific use case, reducing overhead for small operations while maintaining protection for larger ones.
Solution Approach 2:
The patent changes the parameter of memory management timing by introducing delayed reclamation, where cache line state updates are postponed until the atomic region commits. This parameter change eliminates unnecessary cache update overhead for small atomic regions that complete quickly, while still ensuring atomicity through commit-time validation.
2Stability of the object's composition
If cache updates are performed for each store instruction in an atomic region, then memory consistency is maintained, but processing overhead increases
Solution Approach 1:
The patent performs preliminary actions by recording store instructions and their associated cache line states in buffers during the atomic region execution, but delays the actual cache update action until commit time. This allows the system to prepare for memory consistency without immediately incurring the overhead of cache updates for every store instruction.
Solution Approach 2:
The patent introduces buffers as intermediary structures between the store instructions and the cache system. These buffers temporarily hold store data and cache line states, acting as a mediator that decouples the frequency of store instructions from the frequency of cache updates, thereby reducing overhead while maintaining consistency.
3Stability of the object's composition
If senior pipeline walks are performed after atomic region execution, then cache state is updated, but atomic regions cannot commit before pipeline walks occur
Solution Approach 1:
The patent inverts the traditional sequence by performing commit validation before senior pipeline walks. Instead of waiting for pipeline walks to complete cache state updates before allowing commits, the system validates atomic region correctness at commit time using buffered information, then performs pipeline walks afterward to update cache states. This inversion enables earlier commit timing while maintaining cache accuracy.
Data Source
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AI summary
An integrated circuit with support for memory atomicity comprises a processor core. The processor core comprises a data cache unit (DCU), a store buffer (SB), a retirement unit, and memory atomicity facilities. The memory atomicity facilities are configured, when engaged, to (a) add an SB entry to the SB, in response to the processor core executing a store instruction that is part of an atomic region of code; (b) cause the SB entry to become senior, in response to the retirement unit retiring the store instruction; and (c) cause the SB entry to become walk enabled, in response to the retirement unit committing a transaction associated with the atomic region. Other embodiments are described and claimed.