0.12: Tracing Span Caching (#774)

Co-authored-by: Carter Anderson <[email protected]>
Co-authored-by: Pascal Hertleif <[email protected]>

content/news/2023-10-21-bevy-0.12/index.md

@@ -356,12 +356,22 @@ Try the new `time` example to get a better feel for these resources.
 
 The fix for the windup problem was limiting how much `Time<Virtual>` can advance from a single frame. This then limits how many times `FixedUpdate` can be queued for the next frame, and so things like frame lag or your computer waking up from a long sleep can no longer cause a death spiral. So now, the app won't freeze, but things happening in `FixedUpdate` will appear to slow down since it'll be running at a temporarily reduced rate.
 
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+## Reduced Tracing Overhead
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+<div class="release-feature-authors">authors: @hymm, @james7132</div>
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+Bevy uses the [tracing](https://crates.io/crates/tracing) library to measure system running time (among other things). This is useful for determining where bottlenecks in frame time are and measuring performance improvements. These traces can be visualized using the [tracy](https://github.com/wolfpld/tracy) tool. However, using tracing's spans has a significant overhead to it. A large part of the per-span overhead is due to allocating the string description of the span. By caching the spans for systems, commands, and parallel iteration, we have significantly reduced the CPU time overhead when using tracing. In the PR that introduced system span caching, our "many foxes" stress test went from 5.35 ms to 4.54 ms. In the PR that added caching for the parallel iteration spans, our "many cubes" stress test went from 8.89 ms to 6.8 ms.
+
+![tracing overhead](tracing-overhead-reduction.png)
+
 ## <a name="what-s-next"></a>What's Next?
 
 We have plenty of work that is pretty much finished and is therefore very likely to land in **Bevy 0.13**:
 
 Check out the [**Bevy 0.13 Milestone**](https://github.com/bevyengine/bevy/milestone/17) for an up-to-date list of current work being considered for **Bevy 0.13**.
 
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