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How To Profile Benchmark Hot Paths

Use this guide when benchmark ratios point at a real hotspot and you need call-stack evidence before changing the runtime.

What this workflow does

The repo ships a focused benchmark-runner profile mode plus a perf wrapper script:

bash scripts/profile-benchmark-hot-path.sh codecmapper-serialize 120000 person-batch-25

That command:

  • runs one deterministic benchmark operation repeatedly in Release
  • captures perf stat counters into perf.stat.txt
  • captures sampled stacks into perf.data
  • emits both .NET perf maps and JIT dump metadata so managed frames can be symbolized
  • renders a text call-stack report into perf.report.txt

Artifacts land under .artifacts/profiling/<operation>-<scenario-or-records>-<iterations>iters/.

Supported operations

  • codecmapper-serialize
  • codecmapper-deserialize-bytes
  • stj-serialize
  • stj-deserialize
  • newtonsoft-serialize
  • newtonsoft-deserialize
  • our-parser-scan-bytes
  • utf8jsonreader-scan-bytes
  • typed-experiment-deserialize-bytes

Typical workflow

Profile the CodecMapper path first:

bash scripts/profile-benchmark-hot-path.sh codecmapper-serialize 120000 person-batch-25
bash scripts/profile-benchmark-hot-path.sh codecmapper-deserialize-bytes 40000 person-batch-25-unknown-fields

Then capture the System.Text.Json baseline on the same scenario:

bash scripts/profile-benchmark-hot-path.sh stj-serialize 120000 person-batch-25
bash scripts/profile-benchmark-hot-path.sh stj-deserialize 40000 person-batch-25-unknown-fields

Read perf.stat.txt for high-level counters and perf.report.txt for the hottest call paths.

Notes

  • The profile wrapper now defaults to the person-batch-25 scenario from the shared benchmark matrix.
  • Pass a scenario name such as telemetry-500 or escaped-articles-20 to profile one of the standard workloads.
  • The diagnostics matrix now also includes decode-focused scenarios such as decode-strings-1000, decode-flat-objects-400, and decode-flat-objects-400-unknown-fields when you need to isolate string handling, flat record assembly, or unknown-field skipping.
  • Passing a plain integer as the third argument still uses the legacy nested-record batch with --records <n>.
  • The wrapper sets DOTNET_PerfMapEnabled=3 and COMPlus_PerfMapEnabled=3 so perf inject --jit has the metadata it needs for managed symbol names.
  • If perf record is blocked by local kernel permissions, the script will fail before writing perf.report.txt. In that case, fix local perf permissions first and rerun the same command.
  • Keep comparisons on the same power profile and CPU governor, otherwise the counter deltas are noisy enough to mislead.