add derivation parser benchmark

the current identified bottlenecks are parseString in derivations.cc and dirOf (because of std::filessystem creation).

doc/manual/source/SUMMARY.md.in

@@ -128,6 +128,7 @@
 - [Development](development/index.md)
   - [Building](development/building.md)
   - [Testing](development/testing.md)
+  - [Benchmarking](development/benchmarking.md)
   - [Debugging](development/debugging.md)
   - [Documentation](development/documentation.md)
   - [CLI guideline](development/cli-guideline.md)

doc/manual/source/development/benchmarking.md

@@ -0,0 +1,187 @@
+# Running Benchmarks
+
+This guide explains how to build and run performance benchmarks in the Nix codebase.
+
+## Overview
+
+Nix uses the [Google Benchmark](https://github.com/google/benchmark) framework for performance testing. Benchmarks help measure and track the performance of critical operations like derivation parsing.
+
+## Building Benchmarks
+
+Benchmarks are disabled by default and must be explicitly enabled during the build configuration. For accurate results, use a debug-optimized release build.
+
+### Development Environment Setup
+
+First, enter the development shell which includes the necessary dependencies:
+
+```bash
+nix develop .#native-ccacheStdenv
+```
+
+### Configure Build with Benchmarks
+
+From the project root, configure the build with benchmarks enabled and optimization:
+
+```bash
+cd build
+meson configure -Dbenchmarks=true -Dbuildtype=debugoptimized
+```
+
+The `debugoptimized` build type provides:
+- Compiler optimizations for realistic performance measurements
+- Debug symbols for profiling and analysis
+- Balance between performance and debuggability
+
+### Build the Benchmarks
+
+Build the project including benchmarks:
+
+```bash
+ninja
+```
+
+This will create benchmark executables in the build directory. Currently available:
+- `build/src/libstore-tests/nix-store-benchmarks` - Store-related performance benchmarks
+
+Additional benchmark executables will be created as more benchmarks are added to the codebase.
+
+## Running Benchmarks
+
+### Basic Usage
+
+Run benchmark executables directly. For example, to run store benchmarks:
+
+```bash
+./build/src/libstore-tests/nix-store-benchmarks
+```
+
+As more benchmark executables are added, run them similarly from their respective build directories.
+
+### Filtering Benchmarks
+
+Run specific benchmarks using regex patterns:
+
+```bash
+# Run only derivation parser benchmarks
+./build/src/libstore-tests/nix-store-benchmarks --benchmark_filter="derivation.*"
+
+# Run only benchmarks for hello.drv
+./build/src/libstore-tests/nix-store-benchmarks --benchmark_filter=".*hello.*"
+```
+
+### Output Formats
+
+Generate benchmark results in different formats:
+
+```bash
+# JSON output
+./build/src/libstore-tests/nix-store-benchmarks --benchmark_format=json > results.json
+
+# CSV output
+./build/src/libstore-tests/nix-store-benchmarks --benchmark_format=csv > results.csv
+```
+
+### Advanced Options
+
+```bash
+# Run benchmarks multiple times for better statistics
+./build/src/libstore-tests/nix-store-benchmarks --benchmark_repetitions=10
+
+# Set minimum benchmark time (useful for micro-benchmarks)
+./build/src/libstore-tests/nix-store-benchmarks --benchmark_min_time=2
+
+# Compare against baseline
+./build/src/libstore-tests/nix-store-benchmarks --benchmark_baseline=baseline.json
+
+# Display time in custom units
+./build/src/libstore-tests/nix-store-benchmarks --benchmark_time_unit=ms
+```
+
+## Writing New Benchmarks
+
+To add new benchmarks:
+
+1. Create a new `.cc` file in the appropriate `*-tests` directory
+2. Include the benchmark header:
+   ```cpp
+   #include <benchmark/benchmark.h>
+   ```
+
+3. Write benchmark functions:
+   ```cpp
+   static void BM_YourBenchmark(benchmark::State & state)
+   {
+       // Setup code here
+
+       for (auto _ : state) {
+           // Code to benchmark
+       }
+   }
+   BENCHMARK(BM_YourBenchmark);
+   ```
+
+4. Add the file to the corresponding `meson.build`:
+   ```meson
+   benchmarks_sources = files(
+       'your-benchmark.cc',
+       # existing benchmarks...
+   )
+   ```
+
+## Profiling with Benchmarks
+
+For deeper performance analysis, combine benchmarks with profiling tools:
+
+```bash
+# Using Linux perf
+perf record ./build/src/libstore-tests/nix-store-benchmarks
+perf report
+```
+
+### Using Valgrind Callgrind
+
+Valgrind's callgrind tool provides detailed profiling information that can be visualized with kcachegrind:
+
+```bash
+# Profile with callgrind
+valgrind --tool=callgrind ./build/src/libstore-tests/nix-store-benchmarks
+
+# Visualize the results with kcachegrind
+kcachegrind callgrind.out.*
+```
+
+This provides:
+- Function call graphs
+- Instruction-level profiling
+- Source code annotation
+- Interactive visualization of performance bottlenecks
+
+## Continuous Performance Testing
+
+```bash
+# Save baseline results
+./build/src/libstore-tests/nix-store-benchmarks --benchmark_format=json > baseline.json
+
+# Compare against baseline in CI
+./build/src/libstore-tests/nix-store-benchmarks --benchmark_baseline=baseline.json
+```
+
+## Troubleshooting
+
+### Benchmarks not building
+
+Ensure benchmarks are enabled:
+```bash
+meson configure build | grep benchmarks
+# Should show: benchmarks true
+```
+
+### Inconsistent results
+
+- Ensure your system is not under heavy load
+- Disable CPU frequency scaling for consistent results
+- Run benchmarks multiple times with `--benchmark_repetitions`
+
+## See Also
+
+- [Google Benchmark documentation](https://github.com/google/benchmark/blob/main/docs/user_guide.md)