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 internals, and user app code. Enable the `trace` cargo feature to enable Bevy's built-in spans.

If you also want to include `wgpu` tracing spans when profiling, they are emitted at the `tracing` `info` level so you will need to make sure they are not filtered out by the `LogSettings` resource's `filter` member which defaults to `wgpu=error`. You can do this by setting the `RUST_LOG=info` environment variable when running your application.

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)

### Backend: trace_tracy

The [Tracy profiling tool](https://github.com/wolfpld/tracy) is:
> A real time, nanosecond resolution, remote telemetry, hybrid frame and sampling profiler for games and other applications.

There are binaries available for Windows, and installation / build instructions for other operating systems can be found in the [Tracy documentation PDF](https://github.com/wolfpld/tracy/releases/latest/download/tracy.pdf).

It has a command line capture tool that can record the execution of graphical applications, saving it as a profile file. Tracy has a GUI to inspect these profile files. The GUI app also supports live capture, showing you in real time the trace of your app. The version of tracy must be matched to the version of tracing-tracy used in bevy. A compatibility table can be found on [crates.io](https://crates.io/crates/tracing-tracy) and the version used can be found [here](https://github.com/bevyengine/bevy/blob/latest/crates/bevy_log/Cargo.toml#L21).

In one terminal, run:
`./capture-release -o my_capture.tracy`
This will sit and wait for a tracy-instrumented application to start, and when it does, it will automatically connect and start capturing. Note that on Windows, the capture tool is called `capture.exe`.

Then run your application, enabling the `trace_tracy` feature:
`cargo run --release --features bevy/trace_tracy`

After running your app, you can open the captured profile file (`my_capture.tracy` in the example above) in the Tracy GUI application to see a timeline of the executed spans.

Alternatively, directly run the tracy GUI and then run your application, for live capture. However, beware that running the live capture on the same machine will be a competing graphical application, which may impact results. Pre-recording the profile data through the CLI tool is recommended for more accurate traces.

In any case, you'll see your trace in the GUI window:

![Tracy timeline demonstrating the performance breakdown of a Bevy app](https://user-images.githubusercontent.com/302146/163988636-25c017ab-64bc-4da7-a897-a80098b667ef.png)

There is a button to display statistics of mean time per call (MTPC) for all systems:

![A table in the Tracy GUI showing the MTPC (mean time per call) for all instrumented spans in the application](https://user-images.githubusercontent.com/302146/163988302-c21102d8-b7eb-476d-a741-a2c28d9bf8c1.png)

Or you can select an individual system and inspect its statistics (available through the "statistics" button in the top menu) to see things like the distribution of execution times in a graph, or statistical aggregates such as mean, median, standard deviation, etc. It will look something like this:

![A graph and statistics in the Tracy GUI showing the distribution of execution times of an instrumented span in the application](https://user-images.githubusercontent.com/302146/163988464-86e1a3ee-e97b-49ae-9f7e-4ff2b8b761ad.png)

If you save more than one trace, you can compare the spans between both of them by clicking the `Compare` button at the top of the UI. This will open a dialog box asking to load a second trace. From there, it's possible to select any family of spans to more closely compare the timing and distribution of a particular span.

![A graph and statistics in the Tracy GUI comparing the distribution of execution times of an instrumented span across two traces](https://user-images.githubusercontent.com/3137680/205834698-84405b2f-97b5-43a3-9dba-385167ac1db5.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/flamegraph-rs/flamegraph), [enable debug symbols in your release build](https://github.com/flamegraph-rs/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)