Getting Started with dotnet-wtrace: A Beginner’s Guide

dotnet-wtrace: Key Features and Best Practicesdotnet-wtrace is an observability tool designed for .NET applications that captures detailed runtime events, thread activity, and call stacks to help developers diagnose performance issues, deadlocks, and unexpected behavior. This article covers the main features of dotnet-wtrace, how it works, best practices for using it effectively, and practical examples for common troubleshooting scenarios.

What dotnet-wtrace captures

dotnet-wtrace gathers a variety of runtime signals useful for deep debugging and performance analysis:

• Event traces: method entry/exit, exceptions, garbage collection notifications, thread pool activity.
• Call stacks: sampled or instrumented stack traces to show hot paths.
• Thread state transitions: blocking, waiting, running, and thread pool scheduling details.
• I/O and synchronization: locks, waits on synchronization primitives, and I/O wait times.
• Performance counters: CPU usage, memory allocation rates, GC pauses, and other metrics.

How it works (high-level)

dotnet-wtrace leverages the .NET runtime diagnostics APIs and event tracing mechanisms (ETW on Windows, EventPipe on cross-platform runtimes) to collect events with minimal overhead. It can operate in two modes:

• Light sampling: periodically captures stack samples to indicate hotspots with low overhead.
• Instrumented tracing: records detailed entry/exit and event data for precise sequencing, which has higher overhead but gives exact timelines.

Captured data is typically written to a trace file (.nettrace or similar) that can be analyzed offline with tools like PerfView, dotnet-trace, or custom parsers.

Installation and setup

1. Install via the recommended distribution (NuGet/global tool or package maintained by your organization).
2. Ensure the target .NET runtime supports EventPipe/ETW for the tracing you need.
3. Run with appropriate permissions — elevated privileges may be required to capture system-wide events.
4. Configure output path, capture duration, sampling frequency, and event filters to balance detail vs overhead.

Example command (conceptual):

dotnet-wtrace collect --process-id 1234 --duration 60s --sample-rate 10ms --output app.nettrace 

Best practices

• Start with sampling: Use sampling mode to find hotspots with minimal impact, then switch to instrumented tracing for focused areas.
• Limit capture duration: Long traces are heavy; capture the smallest window that reproduces the issue.
• Filter events: Collect only the providers and event levels you need to reduce noise and file size.
• Use symbol servers and source indexing: Ensure full stack traces by configuring access to PDB files or symbol servers.
• Reproduce in staging if possible: Avoid tracing in production unless necessary; if you must, prefer sampling and short captures.
• Correlate traces with metrics/logs: Combine dotnet-wtrace data with Prometheus/Grafana metrics or application logs for context.
• Automate trace capture for CI: Capture traces in integration tests for regressions that affect performance.
• Secure trace files: They may contain sensitive information—store and transmit them securely.

Interpreting common results

• High CPU with deep managed stacks: look for tight loops or synchronous I/O on the main thread. Sampled stacks will point to hot methods.
• Long GC pauses: correlate allocation rates with GC events; reduce allocations in hot paths or tune GC settings.
• Thread pool starvation: examine thread pool growth, queue lengths, and tasks that block threads—use async where appropriate.
• Deadlocks or long waits on locks: instrument synchronization points and inspect waiting threads and owners.
• Excessive context switches: may indicate tearing between threads doing fine-grained work—consider batching or coarser-grained scheduling.

Example workflows

1. Performance hotspot hunting

   • Run sampling trace during a high-load window.
   • Identify top stack samples by CPU time.
   • Add targeted instrumented tracing around identified methods and re-run short traces.

2. Diagnosing thread pool exhaustion

   • Capture thread state transitions and thread pool events.
   • Check for long-running blocking calls on thread-pool threads.
   • Convert blocking code to asynchronous patterns or increase thread pool limits if justified.

3. Investigating high allocation rates

   • Capture GC and allocation events.
   • Identify types with the highest allocation frequency.
   • Optimize allocations (reuse objects, use Span/ArrayPool, value types where appropriate).

Tooling and ecosystem

dotnet-wtrace output is compatible with several analysis tools:

• PerfView — deep analysis of .NET traces and GC.
• dotnet-trace/dotnet-counters — complementary .NET diagnostics tools.
• Visual Studio Diagnostic Tools — for interactive investigation.
• Custom parsers — for automated analysis in CI pipelines.

Limitations and considerations

• Overhead: Instrumented tracing can significantly affect performance; always measure the tracing overhead.
• Platform differences: ETW is Windows-specific; EventPipe covers cross-platform scenarios but feature parity may vary.
• Symbol availability: Without PDBs, stacks may show method IDs instead of names—configure symbol servers.
• Privacy/security: Trace files can expose application internals and data—handle them as sensitive artifacts.

Quick checklist before tracing

• Confirm runtime and OS support for EventPipe/ETW.
• Choose sampling vs instrumented mode.
• Set duration and filters to minimize overhead.
• Ensure symbols are available.
• Securely store and share trace files.

dotnet-wtrace is a powerful ally for .NET developers when used carefully: begin with low-overhead sampling, focus traces narrowly, correlate with other telemetry, and ensure symbol availability to get the most actionable insights.