Runners

Three engines, one interface: every runner produces an mpsc::Receiver<LogLine> that streams output live and closes when the run ends. Pick one via ExecutionPlan.

Native step engine (step::runner)

Pure Rust, no external binary. run_steps(steps, config, tx) spawns config.vus tokio tasks; each loops over the step list until duration expires, sharing a metrics collector.

• Per-VU Context — step outputs and ${{ }} interpolation are isolated between VUs, persistent across iterations of the same VU
• HTTP timings from std/http@v1 feed the shared metrics; other actions (WebSocket, gRPC, TCP/UDP) contribute counters and latency histograms (e.g. ws_msg_rtt, grpc_req_duration) through the same collector
• Ends with the k6-compatible summary block + Done — Xs wall clock
• vus: 0 is clamped to 1; duration strings parse via parse_duration_secs ("90", "1m30s", "1h" — minimum 1s)

k6 (runner::k6)

Wraps an existing k6 install:

1. the script is written to $TMPDIR/perfscale-<uuid>.js
2. k6 run --no-color <script> is spawned with piped stdio
3. stdout/stderr stream as LogLines; the temp file is removed on exit

Two modes:

Function	Returns	Use
run_streaming(script)	Receiver<LogLine>	live output
run_oneshot(script)	RunResult { exit_code, success, stdout, stderr, script }	collect-then-inspect

Load configuration (VUs, stages, thresholds) belongs in the script's own options block — perfscale does not inject k6 flags.

Missing binary → k6 not found in PATH — install from https://k6.io/....

locust (runner::locust)

Wraps an existing locust install in headless mode:

locust -f <script> --headless -u <users> -r <spawn_rate> -t <duration> --csv <tmp-prefix> [--host <host>]

LocustOpts { users, spawn_rate, duration, host } maps from a generic RunConfig via LocustOpts::from_run_config (vus → users and spawn_rate).

While running, locust's own stdout/stderr stream through. After exit, the runner parses the Aggregated row of <prefix>_stats.csv and emits the same summary block the other engines produce:

http_req_duration......: avg=42.50ms p(50)=40ms p(90)=60ms p(95)=68ms p(99)=85ms min=10ms max=120ms
http_req_failed........: 2.00%
http_reqs..............: 100 10.50/s

Temp CSV files (_stats, _stats_history, _failures, _exceptions) are cleaned up afterwards. A missing/short CSV yields a system line (failed to read locust stats: ...) rather than an error — the process output has already been streamed.

Missing binary → locust not found in PATH — install with pip install locust.

Choosing an engine

	native	k6	locust
Install needed	none	k6 binary	python + locust
Scenario language	YAML steps	JavaScript	Python
Scripting power	low (4 actions)	high	high
Load model	fixed VUs × duration	stages/thresholds/scenarios	users/spawn-rate
Best for	smoke tests, CI gates, simple API flows	complex k6 suites you already have	python-centric teams