crossbench

name	crossbench
description	Running benchmarks with Crossbench.

Crossbench

Crossbench is the central chrome runner harness for press benchmarks like JetStream, Speedometer or Motionmark, as well as for loading live pages.

Location and Setup

• Location: It is often installed in ~/crossbench or in CHROMIUM_CHECKOUT/src/tools/perf/cb, but could be anywhere. If not found in standard locations, ask the user for the path.
• Update: Always ensure you are running the latest version.

Basic Usage

Navigate to the crossbench directory and use ./cb.py directly (or via poetry if vpython3 is not available).

# Run the latest stable JetStream with a specific d8 binary
./cb.py jetstream --browser=/path/to/d8 --env-validation=warn

And for chrome:

./cb.py jetstream --browser=out/x64.release/chrome --env-validation=warn

Filtering

• You can run benchmark workload separate with --separate for instance to generate individual profiles for each sub test
• Use --story=XXX to only run a specific test

Crossbench MCP

Use ./cb.py mcp to launch a local MCP to interact with benchmarks.

Profiling with Probes

• Use ./cb.py describe probes to list all probes.
• Use ./cb.py describe probe v8.log to show detailed help for a specific probe.

Important probes for v8 investigation:

• --probe='v8.log' to generate a v8.log file (see v8-log skill for more information on how to analyse the log file)
• --probe='profiling' to generate a Linux perf trace.
• --probe=perfetto to collected perfetto traces

Results:

• Crossbench prints the results directory and provides a summary cb.results.json file listing all probe results

Best Practices

• Use --env-validation=warn to bypass environment input prompts.
• Use ./cb.py describe to understand subcommands and probes.
• Prefer creating JSON files instead of HJSON for configurations to minimize quoting errors.
• Validate generated configs with poetry run cb_validate_hjson -- file.hjson.

Known Stories

• Be aware that 'WSL' in JetStream3 refers to the WebGPU Shading Language workload, not the Windows Subsystem for Linux environment.

Environment Fallbacks

• Suggest install depot_toools if vpython3 is not available
• Use poetry as backup if vpython3 is not available.
• The JetStream benchmark can be run directly with d8