Cache Coherence Protocol Trace Logging for Multi-Core Debugging
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current debugging tools face significant challenges in efficiently recording and replaying program execution due to the tradeoff between robust trace data and performance overhead, particularly in tracing across multiple processor caches, which results in large trace files and performance impacts.
Innovation Solution
The proposed solution involves modifying processor hardware and microcode to record bit-accurate 'time travel' trace recordings by tracing cache misses and cache coherence protocol transitions across multiple layers of processor caches, reducing the performance impact and trace file size by several orders of magnitude.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional software-based tracing is used to record program execution, then robust trace data can be obtained, but performance overhead increases significantly and trace file size becomes large
Solution Approach 1:
The patent introduces hardware-based cache coherence protocol (CCP) tracking as an intermediary mechanism that captures execution traces at the cache level rather than through software instrumentation. The CCP logic monitors cache state transitions (Modified, Exclusive, Shared, Invalid) and generates trace events when coherence state changes occur, providing robust trace data with minimal impact on program execution since the tracking is performed by dedicated hardware logic rather than software overhead
Solution Approach 2:
The patent replaces software-based tracing mechanisms with hardware-based CCP tracking. Instead of using software debuggers or instrumentation to monitor program execution, the system utilizes the existing hardware cache coherence protocol infrastructure to automatically generate traces. This substitution eliminates software overhead and performance penalties while maintaining trace robustness, as the hardware CCP logic operates transparently alongside normal execution
2Loss of information
If comprehensive tracing of all cache operations is performed, then detailed execution data is captured, but trace file size increases by several orders of magnitude
Solution Approach 1:
The patent extracts and records only the essential coherence state transitions from the full cache operation log. Instead of tracing every cache access, the system selectively captures events where the CCP state changes (e.g., from Invalid to Shared, or from Exclusive to Modified). This extraction of critical transition points maintains execution data completeness for debugging purposes while dramatically reducing trace file size by eliminating redundant information about unchanged cache states
Solution Approach 2:
The patent applies partial action by recording only a subset of cache operations—specifically, those that result in coherence protocol state changes. Rather than comprehensively logging all cache reads and writes, the system captures only the meaningful transitions that affect program execution semantics. This partial recording approach suffices for debugging needs while avoiding the excessive data generation that would occur with complete operation logging
3Productivity
If hardware assistance is added to processor for trace recording, then performance impact is reduced, but device complexity and hardware cost increase
Solution Approach 1:
The patent leverages the existing cache coherence protocol hardware infrastructure to serve dual purposes: maintaining cache consistency across multiple processors and generating execution traces simultaneously. The CCP logic, which is already present in modern multi-processor systems for ensuring data consistency, is enhanced to also monitor and record state transitions. This multi-functionality approach reduces hardware complexity by reusing existing components rather than adding dedicated trace recording hardware
Solution Approach 2:
The system enables the cache coherence protocol hardware to self-generate trace data without requiring external trace recording circuitry. The CCP logic automatically monitors its own state transitions and generates trace events when coherence states change. This self-service capability eliminates the need for separate hardware trace generators, reducing overall device complexity while maintaining improved trace recording performance
Data Source
AI summary
Trace recording based on data influxes to an outer-level cache and cache coherence protocol (CCP) transitions between inner caches. Example computing device(s) include a plurality of processing units, a plurality of (N-1)-level caches, and an N-level cache that is associated with two or more of the (N-1)-level caches and that is a backing store for the two or more (N-1)-level caches. Based at least on detecting influx(es) of data to a location in the N-level cache during execution across the processing units, the computing device(s) causes the influx(es) of data to be logged. The computing device(s) also causes one or more (N-1)-level CCP transitions between the two or more (N-1)-level caches to be logged. The (N-1)-level CCP transitions result from the location being accessed by two or more of the processing units.