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[Merged by Bors] - docs: Add section about using Tracy for profiling #4534

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[Merged by Bors] - docs: Add section about using Tracy for profiling #4534

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c079018
docs: Add section about using Tracy for profiling
superdump Apr 19, 2022
e118139
Clarify the location of the Tracy statistics button in its UI
superdump Jul 4, 2022
18a5c5f
Restructure Tracy capabilities and usage
superdump Jul 4, 2022
aaa2de6
Improve alt-text on Tracy profile timeline image
superdump Jul 4, 2022
1e45271
Merge branch 'main' into docs-profiling-tracy
superdump Jul 4, 2022
f8796e5
Clarify Tracy capture.exe name on Windows
superdump Jul 4, 2022
f7a5d81
Improve Tracy example image alt-text
superdump Jul 4, 2022
9d3c7be
Strip some trailing whitespace
superdump Jul 4, 2022
fb22d0e
Add a section about how to capture wgpu spans when profiling
superdump Jul 4, 2022
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profiling.md: Use markdown inline image syntax instead of HTML
superdump Jul 4, 2022
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28 changes: 28 additions & 0 deletions docs/profiling.md
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Expand Up @@ -12,6 +12,34 @@ After running your app a `json` file in the "chrome tracing format" will be prod

![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 be used for capturing with minimal disruption to the execution of graphical applications, saving the profile data to a file. It also has a GUI that can be used to inspect these profile files, or live capture them, though running the live capture on the same machine will be a competing graphical application, which may impact results. As such, @superdump tends to use the command line to for capturing, and the GUI tool for inspecting.
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Please line break your .md paragraphs; it makes for cleaner git changes.

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I disagree for text because then I end up re-joining and re-splitting all the lines whenever any words change, which creates a lot of noise. If they are on one line, in GitHub's UI only the changed words are highlighted. Plus none of the other existing paragraphs in this document are split across multiple lines. I won't resist on this point if you want us to consistently split paragraphs onto multiple lines in documentation though as I respect your ownership of docs more than my own opinions on it. :)

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What does "capturing" mean?

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I'll clarify this, but it means 'receiving and recording profile data.' The Tracy tool is called 'capture'.

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@superdump superdump Jul 4, 2022 •
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I feel like nicopap's rewording already makes this much clearer so I left it as-is after their change was committed. Let me know if you think it needs further clarification.


In one terminal, run:
`./capture-release -o my_capture.tracy`
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This will sit and wait for a tracy-instrumented application to start, and when it does, it will automatically connect and start capturing.

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:

<img width="1840" alt="Tracy timeline" src="https://user-images.githubusercontent.com/302146/163988636-25c017ab-64bc-4da7-a897-a80098b667ef.png">
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There is a button to display statistics of mean time per call (MTPC) for all systems:

<img width="1086" alt="Tracy span MTPC overview" src="https://user-images.githubusercontent.com/302146/163988302-c21102d8-b7eb-476d-a741-a2c28d9bf8c1.png">

Or you can select an individual system and inspect its statistics 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:
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<img width="514" alt="Tracy span detailed statistics" src="https://user-images.githubusercontent.com/302146/163988464-86e1a3ee-e97b-49ae-9f7e-4ff2b8b761ad.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).
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