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 1.00× to 1.48×.

slowdown
telco, fannkuch, nbody, scimark 1.00–1.04× arithmetic in tight loops
json_dumps, pyflate, chaos, float, go, spectral_norm 1.12–1.25×
raytrace, hexiom, deltablue, richards 1.36–1.48× 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. Reproduce with scripts/bench_alternatives.py:

wall time vs vanilla
vanilla 0.97 s —
trace0 (protobuf) 1.16 s 1.20×
trace0 (json) 1.35 s 1.40×
cProfile 2.36 s 2.45×
profile 19.11 s 19.8×

trace0 adds about a seventh of cProfile's overhead and keeps every event, where cProfile only keeps per-function aggregates. That protobuf trace was 210 MB — a full timeline is not free, it is just cheaper in time than in disk. The same run as JSON is 3 GB.

For the callback in isolation, scripts/bench_producer.py reports the difference against the same workload untraced: ~4 ns per event at 8 threads, ~13 ns on a single thread. An event is one PY_START or PY_RETURN, so a function call is two.

Try it

No install — 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 name what it imports with --with, and the interpreter with --python:

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

Free-threaded builds (3.13t, 3.14t) are a first-class target — every thread is traced, not just the calling one.

Install

Install it for the Tracer API, which also puts the same CLI on your project's path. Requires Python 3.13+.

uv add trace0

from trace0 import Tracer

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

Output is Perfetto protobuf by default. --format json, or Tracer(..., format="json"), writes Chrome Trace Event instead — larger and slower to write, but human-readable and diffable.

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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