FYI - made a Rust port of this project

[rrnewton]

Well, Claude did: https://crates.io/crates/rt-app-rs
I run a fuzzer to help ensure that it is a drop-in replacement for rt-app.

The reason was that I wanted to add a feature. I'm using the rt-app workload description language in my simulator for running sched_ext schedulers: sched-ext/scx#3352

I was having problems where the calibration process is not good enough. A task that is supposed to take 10 seconds will be off by a significant margin. I introduced calibration: precise which actually just has the worker spin on reading the clock instead of spinning in a dummy loop. That way it takes almost exactly the amount of time it's supposde to take.

[jlelli]

Hummm, don't we have the runtime event just for that?

[vingu-linaro]

yes, the run event aims to run the right duration on the biggest CPU at the highest frequency in order to emulate the fact that running at a lower freq or a smaller CPU will take more time.
runtime is there for case where we want to run a specific time whatever the CPU or the freq

[rrnewton]

Hah, nice. Sorry I missed that. I will delete that feature.

My own ./target/release/rtapp --print-template prints out the JSON schema (in color!) and shows the "runtime" option right there ;-P.

[rrnewton]

FYI, there is some accuracy tradeoff, where the "runtime" event, both in the C and Rust implementation has a bit of overshoot compared to just spinning on the clock. (This is from a AMD EPYC 9D85 158-Core Processor.)

I guess there's some concern that you still want to do waste_cpu_cycles to activate the FPU and generate heat?

  50ms target (50000 usec):
  ┌─────────────────┬─────────┬─────────────┬───────────────────┐
  │ Implementation  │  Event  │    Range    │       Error       │
  ├─────────────────┼─────────┼─────────────┼───────────────────┤
  │ Rust precise    │ run     │ 50000       │ 0%                │
  ├─────────────────┼─────────┼─────────────┼───────────────────┤
  │ Rust calibrated │ runtime │ 50001-50031 │ +0.002% to +0.06% │
  ├─────────────────┼─────────┼─────────────┼───────────────────┤
  │ C calibrated    │ runtime │ 50008-50033 │ +0.016% to +0.07% │
  ├─────────────────┼─────────┼─────────────┼───────────────────┤
  │ Rust calibrated │ run     │ 45970-46548 │ -8.1% to -6.9%    │
  ├─────────────────┼─────────┼─────────────┼───────────────────┤
  │ C calibrated    │ run     │ 52051-52428 │ +4.1% to +4.9%    │
  └─────────────────┴─────────┴─────────────┴───────────────────┘

  1ms target (1000 usec):

  ┌─────────────────┬─────────────┬───────────┬────────────────┐
  │ Implementation  │    Event    │   Range   │     Error      │
  ├─────────────────┼─────────────┼───────────┼────────────────┤
  │ Rust precise    │ run/runtime │ 1000      │ 0%             │
  ├─────────────────┼─────────────┼───────────┼────────────────┤
  │ Rust calibrated │ runtime     │ 1000-1020 │ 0% to +2%      │
  ├─────────────────┼─────────────┼───────────┼────────────────┤
  │ C calibrated    │ runtime     │ 1000-1033 │ 0% to +3.3%    │
  ├─────────────────┼─────────────┼───────────┼────────────────┤
  │ Rust calibrated │ run         │ 952-1055  │ -4.8% to +5.5% │
  ├─────────────────┼─────────────┼───────────┼────────────────┤
  │ C calibrated    │ run         │ 1033-1097 │ +3.3% to +9.7% │
  └─────────────────┴─────────────┴───────────┴────────────────┘