Speculative Load Marking in Multi-Processor Cache Systems
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing techniques for speculative execution in computer systems face performance issues due to the need for dedicated hardware resources and frequent cache line invalidations, leading to a significant number of failed speculative loads and reduced memory bandwidth.
Innovation Solution
A system that efficiently marks cache lines by incrementing a reader count value in metadata, allowing speculative loads without exclusive access, and propagates load-mark metadata to maintain coherence across the memory hierarchy, preventing store operations by other threads.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If dedicated hardware structures are used to maintain speculative load addresses and detect invalidations, then speculative execution capability is improved, but hardware resource consumption increases and scalability deteriorates
Solution Approach 1:
The patent uses a copy of the cache line in the local cache to store speculative load information, rather than maintaining dedicated hardware structures. The local cache copy serves as a lightweight representation that can be updated without complex hardware mechanisms, resolving the contradiction between speculative execution capability and hardware resource consumption
Solution Approach 2:
The local cache acts as an intermediary between the speculative execution mechanism and the cache coherence protocol. By using the existing cache infrastructure as a mediator, the patent avoids the need for dedicated hardware structures while still enabling speculative load tracking and invalidation detection
2Device complexity
If metadata in L1 data cache is used to indicate speculative loads, then hardware resource consumption is reduced, but cache line invalidation frequency increases causing failed speculative loads
Solution Approach 1:
The patent performs preliminary load-marking of cache lines before speculative execution occurs. By pre-marking cache lines that are likely to be accessed speculatively, the system prepares the metadata in advance, reducing the impact of subsequent invalidations and improving speculative load success rates without requiring additional hardware resources
Solution Approach 2:
The patent implements dynamic load-marking where the metadata state changes based on execution context. Cache lines are marked as load-marked when speculative loads are detected and unmarked when execution commits or fails, allowing the system to adapt to varying speculative execution patterns and reduce failed loads while maintaining low hardware overhead
3Measurement precision
If exclusive access to cache line is obtained for metadata update, then metadata update accuracy is improved, but memory bandwidth utilization deteriorates due to access conflicts
Solution Approach 1:
The patent applies partial action by updating only the necessary metadata fields (load-mark status and reader count) in the cache line copy, rather than obtaining full exclusive access to the entire cache line. This selective metadata update maintains accuracy for the specific fields needed while reducing access conflicts and improving memory bandwidth utilization
Data Source
AI summary
One embodiment of the present invention provides a system that efficiently marks cache lines in a multi-processor computer system. The system starts by receiving a load request for a cache line from a requesting thread. Upon receiving the load request, the system loads a copy of the cache line into a local cache for the requesting thread. The system then load-marks the copy of the cache line in the local cache by incrementing a reader count value contained in metadata for the copy of the cache line, regardless of the cache coherency protocol status of the copy of the cache line, whereby the system updates the metadata in the local copy of the cache line without obtaining exclusive access to the cache line.