Cache Agent Trace Engine for Deterministic Post-Silicon Debugging
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Post-silicon debugging of processors has become increasingly complex due to increased microarchitectural complexity, leading to lengthy durations in identifying system failures and their root causes, especially in cache agents, where tracing all instances is not possible due to limited debug bandwidth and lack of determinism, and performance debugging is resource-intensive due to disconnects between manufacturer performance benchmarks and real-world user workloads.
Innovation Solution
A Tracing and Capture (TC) engine for cache agents that takes periodic snapshots of state and losslessly records relevant events, allowing for deterministic replay and capture of internal processor state to assist with functional or performance debugging, enabling monitoring of cache agents without requiring global determinism and supporting a larger number of System-On-Chip (SoC) derivatives without extending post-silicon sighting root-cause duration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional debugging methods are used to trace all cache agent instances, then complete debug information can be obtained, but debug bandwidth is exceeded and debugging duration becomes unacceptably long
Solution Approach 1:
The system segments the debugging process into two distinct phases: a trace phase where events are captured in the cache agent, and a replay phase where the traced events are replayed in a controlled environment. This segmentation allows complete debug information to be collected during the trace phase without time constraints, while the replay phase enables detailed analysis without exceeding debug bandwidth limitations.
Solution Approach 2:
The trace phase performs preliminary action by capturing and storing all relevant events and state information from cache agent instances before the actual debugging analysis begins. This preliminary capture of debug information includes tracing memory access requests, cache hits/misses, and other cache operations, allowing the subsequent replay phase to proceed without time pressure and with complete information available.
2Reliability
If deterministic replay is implemented for cache agents, then root cause analysis is significantly improved, but system complexity increases due to additional tracing infrastructure
Solution Approach 1:
The patent introduces a trace buffer as an intermediary component between the cache agent and the replay logic. This trace buffer stores traced events and state information in a structured format that can be replayed deterministically. The intermediary trace buffer simplifies the overall system architecture by providing a clear separation between the tracing function and the replay function, making the deterministic replay mechanism more manageable despite the increased infrastructure requirements.
Solution Approach 2:
The system creates a copy of the cache agent's operational state and events during the trace phase, storing this copy in the trace buffer. This copied information includes cache line states, memory access patterns, and control signals. During replay, this copied state is restored and replayed in a controlled manner, enabling deterministic analysis without requiring the original live system to be modified or reconfigured.
3Productivity
If performance benchmarks are conducted using manufacturer test workloads, then standardized performance metrics can be obtained, but real-world user workload performance issues remain undetected
Solution Approach 1:
The system dynamically adapts the replay configuration based on the specific performance issue being investigated. Rather than using a fixed benchmarking approach, the replay logic can be configured to replay specific trace data from real-world workloads when performance anomalies are detected. This dynamic flexibility allows the system to maintain efficient standardized benchmarking for routine testing while enabling detailed analysis of real-world performance issues when they occur.
Solution Approach 2:
The deterministic replay infrastructure serves multiple functions: it can replay traces from manufacturer performance benchmarks for standardized testing, replay traces from real-world user workloads for field issue analysis, and replay traces from controlled stress tests. This multi-functional capability allows a single system to address both the efficiency needs of standardized benchmarking and the reliability needs of real-world performance issue detection.
Data Source
AI summary
In one embodiment, a processor comprises a fabric interconnect to couple a first cache agent to at least one of a memory controller or an input/output (I/O) controller; and a first cache agent comprising a cache controller coupled to a cache; and a trace and capture engine to periodically capture a snapshot of state information associated with the first cache agent; trace events to occur at the first cache agent in between captured snapshots; and send the captured snapshots and traced events via the fabric interconnect to the memory controller or I/O controller for storage at a system memory or storage device.