magic-trace
A low-overhead, Intel PT-based tracing tool that captures all control flow with ~40ns resolution, enabling post-mortem and performance debugging via interactive timelines.
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v1.2.4 · 6.5 MB
A high-resolution tracing tool using Intel PT to capture all function calls with ~40ns granularity, enabling interactive post-mortem debugging without code changes.
Core Features
- Captures every function call in compiled languages (C, C++, Rust, etc.) with ~40ns resolution
- No application modifications required – attach to a running process
- Two snapshot triggering modes: manual Ctrl+C or automatic by calling a specific function
- Interactive trace viewer with zoom, measurement, and call-stack navigation
- Low overhead of 2-10% thanks to hardware-assisted Intel Processor Trace
What It Can't Do
- •Requires Intel Skylake or newer CPU and Linux (no virtual machines supported)
- •Kernel tracing needs root privileges
- •The trigger function must be a non-inlined empty function
- •Overhead of 2-10% is typical; evaluate impact for latency-sensitive workloads
Use Cases
- Identify root cause of latency spikes in production services by comparing fast vs. slow requests
- Reconstruct the last milliseconds of execution before a crash, providing context beyond a single stack frame
Detailed Introduction
magic-trace is a high-resolution tracing tool from Jane Street that leverages Intel Processor Trace to record every function call executed by a process into a ring buffer. Upon a trigger (Ctrl+C or a designated function), it snapshots the last ~10ms of control flow and renders an interactive call-stack timeline. Unlike traditional sampling profilers like perf, magic-trace captures 100% of control flow, making it possible to see exactly what happened in the microseconds leading up to an event. It requires no application code changes, runs with 2-10% overhead, and supports multiple triggers including fuzzy symbol matching. Ideal for investigating production performance anomalies, understanding complex code execution, and crash recovery scenarios.
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Getting Started
Install the software
Install the appropriate package for your distro (dpkg / rpm / AppImage)
Download the latest binary and make it executable with 'chmod +x magic-trace'
Launch your target program and run 'magic-trace attach -pid $(pidof your_program)'
Wait a few seconds, then press Ctrl+C to stop tracing – a file 'trace.fxt.gz' is created
- Download the latest binary and make it executable with 'chmod +x magic-trace'
- Launch your target program and run 'magic-trace attach -pid $(pidof your_program)'
- Wait a few seconds, then press Ctrl+C to stop tracing – a file 'trace.fxt.gz' is created
Latest Release Notes
## What's Changed
* Add support for compressed output files, whether gzip (supported by Pe… by @bnigito in https://github.com/janestreet/magic-trace/pull/306
* Handle TSX transactions by @bnigito in https://github.com/janestreet/magic-trace/pull/308
* Fix symbol resolution for Rust binaries by @JosiahWhite in https://github.com/janestreet/magic-trace/pull/321
* Reap zombie `perf` processes from capability detection by @Xyene in https://github.com/janestreet/magic-trace/pull/316
* Recognize `int3` instructions in trace parser by @Xyene in https://github.com/janestreet/magic-trace/pull/322
* Add a `MAGIC_TRACE_NO_OCAML_EXCEPTION_DEBUG_INFO` environment variable by @bnigito in https://github.com/janestreet/magic-trace/pull/305
## New Contributors
* @bnigito made their first contribution in https://github.com/janestreet/magic-trace/pull/305
* @JosiahWhite made their first contribution in https://github.com/janestreet/magic-trace/pull/321
**Full Changelog**: https://github.com/janestreet/magic-trace/compare/v1.2.3...v1.2.4
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