Added tracing page to the candle book. (#2922)

* tracing page

* warned about asynchronous execution

* cleanup

* added Nsignt Systems recommendation

candle-book/src/SUMMARY.md

@@ -16,6 +16,7 @@
 - [Running a model](inference/inference.md)
     - [Using the hub](inference/hub.md)
 - [Error management](error_manage.md)
+- [Tracing](tracing.md)
 - [Training](training/training.md)
     - [Simplified](training/simplified.md)
     - [MNIST](training/mnist.md)

candle-book/src/tracing.md

@@ -0,0 +1,68 @@
+# Tracing
+
+Tracing is a powerful tool for identifying performance issues and bottlenecks in code.
+
+> Profiling on GPUs is trickier due to asynchronous execution, see the [GPU section](#gpu).
+
+## Overview
+
+Candle uses the [tracing](https://docs.rs/tracing/latest/tracing/) crate for instrumentation.
+
+To try it out, run an example in `candle-examples` with the `--tracing` flag. 
+This generates a trace file, typically named `trace-<timestamp>.json`. 
+You can view the trace in Chrome by navigating to `chrome://tracing/`, clicking **Load**, and selecting the generated trace file.
+
+## Adding Tracing
+
+Candle includes built-in tracing for many internal operations, using [spans](https://docs.rs/tracing/latest/tracing/struct.Span.html) to mark key points of execution.
+
+To add custom tracing in your code, you can define a span like this:
+
+```rust
+let span = tracing::span!(tracing::Level::TRACE, name);
+```
+
+Then, to record the span during execution, create a guard:
+
+```rust
+let _enter = span.enter();
+```
+
+This guard will record the span's duration, from when it is created to when it is dropped, into a global data structure managed by the tracing crate.
+
+## Recording and Saving a Trace
+
+To capture and save trace data, you need to configure the tracing system with an output format. Candle uses the [tracing_subscriber](https://docs.rs/tracing-subscriber/latest/tracing_subscriber/) and [tracing_chrome](https://docs.rs/tracing-chrome/latest/tracing_chrome/) crates.
+
+The snippet below sets up a Chrome compatible recorder that logs all tracing activity between creation and drop of the guard:
+
+```rust
+use tracing_chrome::ChromeLayerBuilder;
+use tracing_subscriber::prelude::*;
+
+let _guard = {
+    let (chrome_layer, guard) = ChromeLayerBuilder::new().build();
+    tracing_subscriber::registry().with(chrome_layer).init();
+    guard
+};
+```
+
+## GPU
+
+When using CUDA, Metal, or other asynchronous GPU backends, tracing may produce misleading timing data because operations are queued rather than executed immediately.
+
+### CUDA
+
+For CUDA-specific profiling, you have two options:
+
+1. Set the environment variable `CUDA_LAUNCH_BLOCKING=1` which forces synchronous execution. This makes trace timings more accurate, at the cost of reduced performance.
+2. Use [NVIDIA's Nsight Systems](https://developer.nvidia.com/nsight-systems) (`nsys profile` and `nsys-ui`) which are designed specifically for profiling asynchronous CUDA executions.
+
+We recommend using NVIDIA's Nsight Systems when possible, as it offers accurate performance data without altering typical execution patterns. In contrast, setting the `CUDA_LAUNCH_BLOCKING` environment variable forces synchronous execution, which can significantly alter execution behavior.
+
+#### Performance Profiling with NVIDIA Nsight Systems
+
+1. Generate an `.nsys-rep` file containing performance data ([docs](https://docs.nvidia.com/nsight-systems/UserGuide/index.html#example-single-command-lines))
+   - Run `nsys profile --trace cuda,nvtx,osrt --gpu-metrics-device=all --output profile_run ./target/debug/... --prompt "whatever "`
+1. Open the generated `.nsys-rep` report file in Nsight Systems GUI
+    - File > Open