Profiling Your Build

When a build feels slow, nros-build-profile tells you where the time went — codegen vs compile vs link vs flash, which unit dominated, and whether a shared crate was rebuilt redundantly.

It is a passive, read-only tool. Your build runs exactly as today on its native toolchain (west, cmake, idf.py, cargo); the profiler only reads the timing artifacts that build already produced. It never compiles or flashes anything, and it adds no nros build/test command — nros stays setup + codegen.

How it works

Every backend already emits timing data:

The profiler discovers and parses these, then prints a stage table (always) plus an optional per-unit drill-down and diagnostic hints.

Flow 1 — Zephyr / cmake / esp32-idf (no opt-in)

$ west build -b qemu_cortex_a53 examples/zephyr/rust/talker   # build/.ninja_log written
$ just profile examples/zephyr/rust/talker

Backend: ninja (west)        Total: 41.2s

Stage      Duration    %
codegen        1.1s   3%
compile       33.8s  82%
link           6.0s  15%

hints:
  - 1 unit = 61% of compile (libzenoh_pico.a, 20.6s)

Flow 2 — cargo (one flag for per-crate detail)

The coarse table works with any build; the per-crate drill-down needs --timings:

$ cd examples/native/rust/talker && cargo build --timings   # target/cargo-timings/ written
$ just profile examples/native/rust/talker --deep

Backend: cargo               Total: 30.3s

Stage      Duration    %
codegen        1.2s   4%
compile       24.8s  82%
link           3.9s  13%

slowest units:
  zenoh-pico-sys     18.1s ########
  nros-node           2.4s #
  std_msgs            1.1s #
  <80 more>           3.2s

hints:
  - nros-c compiled 3× — examples use isolated target/; pool target_dir to reuse the build

If you build cargo without --timings, you still get the coarse table plus a note telling you how to enable the drill-down — never an error.

Flow 3 — machine-readable (CI regression diffing)

$ just profile examples/zephyr/rust/talker --deep    # then add --json on the bin, or:
$ ./target/debug/nros-build-profile examples/zephyr/rust/talker --json
wrote examples/zephyr/rust/talker/nros-build-profile.json

A CI step can diff nros-build-profile.json across commits to catch build-time regressions (a stage’s seconds or a unit’s share jumping).

External (copy-out) projects

Copy-out example projects have no justfile. Run the bin directly — same output:

$ nros-build-profile <project-dir> --deep

Diagnostics

On top of the numbers, the profiler flags known slow patterns:

• a large native (C/C++/-sys) unit with no incremental → suggest a compiler cache;
• the same crate compiled more than once → suggest pooling target_dir;
• the configured job count vs available RAM (the fixture-build OOM budget).

Silence them with --no-hints.