trace0

A low-overhead tracing profiler for Python, written in Rust.

trace0 hooks PEP 669 sys.monitoring, packs each event into 8 bytes in a thread-local buffer, and hands whole batches to a background thread over lock-free SPSC rings. It emits Perfetto-compatible traces — open them at https://ui.perfetto.dev.

How much does it cost?

Across 14 benchmarks from pyperformance, the suite CPython itself is judged on, run unmodified on an M4 Mac:

Median slowdown 1.19×, ranging 0.99× to 1.49×.

slowdown
telco, scimark, fannkuch, nbody 0.99–1.03× arithmetic in tight loops
pyflate, chaos, float, go 1.17–1.22×
raytrace, hexiom, deltablue, richards 1.40–1.49× call-heavy

The spread is the story: a tracer charges per call, so what a program pays depends entirely on how many calls it makes per unit of work, not on how long it runs. Reproduce with scripts/bench_pyperformance.py.

Against the alternatives, on a call-dense workload — transpiling SQL with sqlglot, 10.8M events per second of work, measured with hyperfine:

wall time vs vanilla
vanilla 0.98 s —
trace0 (protobuf) 1.38 s 1.41×
cProfile 2.42 s 2.47×
profile 19.27 s 19.5×

trace0 costs about a third of what cProfile does and keeps every event, where cProfile only keeps per-function aggregates. That trace was 210MB — a full timeline is not free, it is just cheaper in time than in disk.

For the callback in isolation, scripts/bench_producer.py reports the difference against the same workload untraced: ~5 ns per event at 8 threads, ~14 ns on a single thread. An event is one PY_START or PY_RETURN, so a function call is two.

Try it, without installing anything

uvx fetches the wheel, traces your script, and leaves nothing behind:

uvx trace0 run --output trace.pb your_script.py

Drop trace.pb onto https://ui.perfetto.dev and you have a flame chart.

The traced script runs inside the tracer's environment, so give it whatever it imports with --with:

uvx --with httpx --with pandas trace0 run --output trace.pb your_script.py

To trace under a free-threaded interpreter, name it — every thread is traced, not just the calling one:

uvx --python 3.13t trace0 run --output trace.pb your_script.py

Install

Add it to a project when you want the Tracer API:

uv add trace0

Requires Python 3.13+. Free-threaded builds (3.13t, 3.14t) are a first-class target.

from trace0 import Tracer

with Tracer("trace.pb"):
    your_workload()

The same CLI is then on your project's path:

uv run trace0 run --output trace.pb your_script.py

Formats are protobuf (Perfetto, smaller and faster to write, the default) and json (Chrome Trace Event, human-readable and diffable) — pass --format json for the latter.

Status

Early. The tracer works end to end and is covered by tests, but the API is not yet stable and wheels are not yet built for every platform.

Layout

Rust lives under crates/, Python under python/.

crate role
trace0-core clock, event model, event queue, exporter contract
trace0-json Chrome Trace Event output
trace0-proto Perfetto protobuf output
trace0-py the sys.monitoring callbacks and the _core extension

Only trace0-py links against CPython, and it is the one crate with no tests: a cdylib built with extension-module leaves the interpreter's symbols undefined, so it cannot link outside maturin. Everything worth testing lives in the other three, which cargo test runs directly.

License

MIT

Project links

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  • Released:
Newer release available (0.1.11)

1 maintainer

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Avatar for soofgolan from gravatar.com soofgolan

Credits

Author: Soof Golan

GitHub Statistics

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License expression

MIT
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Requires

Python >=3.13

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