document how to do good performance work

Author: sylvestre

docs/src/performance.md

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+# Performance Profiling Tutorial
+
+## Effective Benchmarking with Hyperfine
+
+[Hyperfine](https://github.com/sharkdp/hyperfine) is a powerful command-line benchmarking tool that allows you to measure and compare execution times of commands with statistical rigor.
+
+### Benchmarking Best Practices
+
+When evaluating performance improvements, always set up your benchmarks to compare:
+
+1. The GNU implementation as reference
+2. The implementation without the change
+3. The implementation with your change
+
+This three-way comparison provides clear insights into:
+- How your implementation compares to the standard (GNU)
+- The actual performance impact of your specific change
+
+### Example Benchmark
+
+First, you will need to build the binary in release mode. Debug builds are significantly slower:
+
+```bash
+cargo build --features unix --release
+```
+
+```bash
+# Three-way comparison benchmark
+hyperfine \
+  --warmup 3 \
+  "/usr/bin/ls -R ." \
+  "./target/release/coreutils.prev ls -R ." \
+  "./target/release/coreutils ls -R ."
+
+# can be simplified with:
+hyperfine \
+  --warmup 3 \
+  -L ls /usr/bin/ls,"./target/release/coreutils.prev ls","./target/release/coreutils ls" \
+  "{ls} -R ."
+```
+
+```
+# to improve the reproducibility of the results:
+taskset -c 0
+```
+
+### Interpreting Results
+
+Hyperfine provides summary statistics including:
+- Mean execution time
+- Standard deviation
+- Min/max times
+- Relative performance comparison
+
+Look for consistent patterns rather than focusing on individual runs, and be aware of system noise that might affect results.
+
+## Using Samply for Profiling
+
+[Samply](https://github.com/mstange/samply) is a sampling profiler that helps you identify performance bottlenecks in your code.
+
+### Basic Profiling
+
+```bash
+# Generate a flame graph for your application
+samply record ./target/debug/coreutils ls -R
+
+# Profile with higher sampling frequency
+samply record --rate 1000 ./target/debug/coreutils seq 1 1000
+```
+
+## Workflow: Measuring Performance Improvements
+
+1. **Establish baselines**:
+   ```bash
+   hyperfine --warmup 3 \
+     "/usr/bin/sort large_file.txt" \
+     "our-sort-v1 large_file.txt"
+   ```
+
+2. **Identify bottlenecks**:
+   ```bash
+   samply record ./our-sort-v1 large_file.txt
+   ```
+
+3. **Make targeted improvements** based on profiling data
+
+4. **Verify improvements**:
+   ```bash
+   hyperfine --warmup 3 \
+     "/usr/bin/sort large_file.txt" \
+     "our-sort-v1 large_file.txt" \
+     "our-sort-v2 large_file.txt"
+   ```
+
+5. **Document performance changes** with concrete numbers
+   ```bash
+   hyperfine --export-markdown file.md [...]
+   ```