bevy/docs/profiling.md

# Profiling

## Runtime Flame Graph: `tracing` spans

Bevy has built-in [tracing](https://github.com/tokio-rs/tracing) spans to make it cheap and easy to profile Bevy ECS systems, render logic, engine iternals, and user app code. Enable the `trace` cargo feature to enable Bevy's built-in spans. You also need to select a `tracing` backend using the following cargo features:

### Backend: trace_chrome

`cargo run --release --features bevy/trace_chrome`

After running your app a `json` file in the "chrome tracing format" will be produced. You can open this file in your browser using <https://ui.perfetto.dev>. It will look something like this (make sure you expand `Process 1`):

![image](https://user-images.githubusercontent.com/2694663/141657409-6f4a3ad3-59b6-4378-95ba-66c0dafecd8e.png)

### Adding your own spans

Add spans to your app like this (these are in `bevy::prelude::*` and `bevy::log::*`, just like the normal logging macros).

```rust
{
  // creates a span and starts the timer 
  let my_span = info_span!("span_name", name = "span_name").entered();
  do_something_here();
} // my_span is dropped here ... this stops the timer


// You can also "manually" enter the span if you need more control over when the timer starts
// Prefer the previous, simpler syntax unless you need the extra control.
let my_span = info_span!("span_name", name = "span_name");
{
  // starts the span's timer 
  let guard = my_span.enter();
  do_something_here();
} // guard is dropped here ... this stops the timer
```

Search for `info_span!` in this repo for some real-world examples.

For more details, check out the [tracing span docs](https://docs.rs/tracing/*/tracing/span/index.html).

## `perf` Runtime Flame Graph

This approach requires no extra instrumentation and shows finer-grained flame graphs of actual code call trees. This is useful when you want to identify the specific function of a "hot spot". The downside is that it has higher overhead, so your app will run slower than it normally does.

Install [cargo-flamegraph](https://github.com/killercup/cargo-flamegraph), [enable debug symbols in your release build](https://github.com/killercup/cargo-flamegraph#improving-output-when-running-with---release), then run your app using one of the following commands. Note that `cargo-flamegraph` forwards arguments to cargo. You should treat the `cargo-flamegraph` command as a replacement for `cargo run --release`. The commands below include `--example EXAMPLE_NAME` to illustrate, but you can remove those arguments in favor of whatever you use to run your app:

* Graph-Like Flame Graph: `RUSTFLAGS='-C force-frame-pointers=y' cargo flamegraph -c "record -g" --example EXAMPLE_NAME`
* Flat-ish Flame Graph: `RUSTFLAGS='-C force-frame-pointers=y' cargo flamegraph --example EXAMPLE_NAME`

After closing your app, an interactive `svg` file will be produced:
![image](https://user-images.githubusercontent.com/2694663/141657609-0089675d-fb6a-4dc4-9a59-871e95e31c8a.png)

## Project Compile Times

Append `--timings` to your app's cargo command (ex: `cargo build --timings`).
If you want a "full" profile, make sure you run `cargo clean` first (note: this will clear previously generated reports).
The command will tell you where it saved the report, which will be in your target directory under `cargo-timings/`.
The report is a `.html` file and can be opened and viewed in your browser.
This will show how much time each crate in your app's dependency tree took to build.

![image](https://user-images.githubusercontent.com/2694663/141657811-f4e15e3b-c9fc-491b-9313-236fd8c01288.png)