Improve profiling.md

docs/profiling.md

@@ -1,9 +1,51 @@
 # Profiling
 
-* Compile Times: append ```-Ztimings``` to cargo builds
-* Runtime Flame Graph:
-  * Flat-ish: ```RUSTFLAGS='-C force-frame-pointers=y' cargo flamegraph --example EXAMPLE_NAME```
-  * Graph: ```RUSTFLAGS='-C force-frame-pointers=y' cargo flamegraph -c "record -g" --example EXAMPLE_NAME```
-  * built on top of perf, no instrumentation required
-* Runtime Instrumentation:
-  * [thread_profiler](https://github.com/glennw/thread_profiler)
+## 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 trace,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
+
+let my_span = info_span!("span_name", name = "span_name");
+{
+  // starts the span's timer 
+  let guard = my_span.enter();
+} // 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
+
+This requires nightly rust (`rustup default nightly`). Append ```-Ztimings``` to your app's cargo command (ex: `cargo build -Ztimings`). If you want a "full" profile, make sure you run `cargo clean` first. Open the produced `cargo-timing.html` file in your browser of choice. 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)
+
+