What is JetStream?

JetStream 3 is a JavaScript and WebAssembly benchmark suite. For more information see the index and in-depth pages of the deployed benchmark.

A preview of the current main branch is available at https://webkit-jetstream-preview.netlify.app/.

Open Governance

See Governance.md for more information.

Getting Started, Setup Instructions

• Install Node.js and (optionally) jsvu for conveniently getting recent builds of engine shells.
• npm install the necessary dependencies.
• npm run server for starting a local development server, then browse to http://localhost:8010.
• npm run test:shell for running the benchmark in engine shells, or alternatively running directly, e.g., via jsc cli.js.

See package.json and .github/workflows/test.yml for more details and available commands.

Shell Runner

For the shell runner, see the available options by passing --help to cli.js. (Note that this requires -- for JavaScriptCore and V8 to separate VM arguments from script arguments.):

$ v8 cli.js -- --help
JetStream Driver Help

Options:
    --help                 Print this help message.
    --iteration-count      Set the default iteration count.
...

Available tags:
   all
...

Available tests:
   8bitbench-wasm
...

Browser Runner

The browser version also supports passing parameters as URL query parameters, e.g., pass the test parameter (aliases are tests or testList) with a comma-separated list to run only specific workloads: https://webkit-jetstream-preview.netlify.app/?test=8bitbench-wasm,web-ssr. See utils/params.js and JetStreamDriver.js for more details.

Technical Details

The main file of the benchmark harness is JetStreamDriver.js, which lists the individual workloads and their parameters, implements measurement and scoring, etc. The individual workloads are in subdirectories.

Preloading and Compression

The JetStream driver (both in the browser and shell runners) preloads some large assets and source files. That is, it reads those files from disk or fetches them from the network before running the workloads. This is in order to exclude network latency and disk I/O from the benchmark measurements and reduce variance. Otherwise, OS scheduling or CPU frequency scaling may affect the measurement.

Some workloads also utilize large assets (e.g., ML models, heavy JavaScript bundles in the order of 10s of MBs). In order to limit the repository size and network transfers, such large assets are stored as compressed .z files. Preloading handles the decompression of these assets (using DecompressionStream or a Wasm-based zlib polyfill) upfront so that decompression overhead does not affect the benchmark score.

Both preloading and compression can be disabled, e.g., to inspect raw files or because it sometimes helps with debugging (e.g., proper URLs instead of Blobs for resources).

• Compression: Run npm run decompress to decompress all .z files before running the benchmark.
• No prefetching for shells: Pass the --no-prefetch flag, e.g., jsc cli.js -- --no-prefetch.
• No prefetching in browsers: Append the query parameter ?prefetchResources=false to the URL.

See JetStreamDriver.js and utils/compress.mjs for more details.

Score Calculation

Scores in JetStream are dimensionless floats, where a higher score is better. When scores are aggregated (e.g., multiple sub-scores for each workload, or to determine the total score of the full benchmark suite), we use the geometric mean. The geometric mean ensures that a multiplicative improvement of any individual score has the same effect on the aggregated score, regardless of the absolute value of the individual score. For example, an improvement by 5% of the sub score of benchmark A has the same effect on the total score as an improvement by 5% of the sub score of benchmark B.

See the in-depth.html and JetStreamDriver.js for more details.