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WebAssembly · 25 Total Cost Including Copies
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Stage: Correctness and Bottleneck Evaluation · External example · preview instructions · Storage ID: webassembly-step-25
Goals and learning outcomes
- Learning outcome: explain the role of Kernel time. Input cases: three data sizes.
- Deliverable: a table of JS·kernel·copy-inclusive times. Explain the rationale for handling Total cost, linking it to the code.
Prerequisites
Prepare the experiment results for Warm-up and Equivalent conditions from Unit 24, “Warm Repeats and Warm-up”.
Key concepts and explanations
Kernel time
Input/output copies can dominate the whole path even with a fast function. Comparing kernels alone misses user-facing costs.
Total cost
Include input set and output slice in measurements. Include result-array creation costs in the JS path as well.
Practice setup
First check the execution scope guidance on the example screen. The following describes the environment needed to extend the assignment in a separate project. The iframe preview is an experiment with WebAssembly.Memory and JS reference computations; do not present it as execution of an actual Wasm module. Practice with actual modules in a separate Node 22+ environment or a controlled local web project. Use kernel.wat·kernel.wasm·benchmark.mjs in the existing src/labs/webassembly-02·03 folders, and rebuild with the official wat2wasm tool when modifying WAT. Preserve CSP·isolation, and record measurements yourself.
The linked example is a starting point. It does not implement every task below. Complete work beyond its supported scope in a separate project.
Practice tasks
- In day-25.mjs in a separate practice project, read the existing wasmKernelMeanMs and wasmWarmWithCopiesMeanMs separately. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: three data sizes. Baseline: kernel only. Run under the baseline conditions, then record observations and errors in notes/day-25.md.
- Variable: scope; changed value: including copies. Run again through the same input path. Deliverable to save: a table of JS·kernel·copy-inclusive times. Hypothesis to check: Wasm's comparative advantage may change. Compare it with the actual results, and record whether they agree and why they differ.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| scope | kernel only | including copies | Wasm's comparative advantage may change. |
Predictions in this table are not measured results. Change one condition at a time and record why the actual result differs.
Completion criteria
- Required deliverable: a table of JS·kernel·copy-inclusive times. Results are available for both the baseline and changed conditions, allowing comparison of 2 conditions; input values and the execution environment are recorded together.
- The explanation of differences between the two conditions connects both concepts: Kernel time and Total cost. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Kernel time affect in this exercise? Input cases: three data sizes.
- Variable: scope. Hypothesis to check: Wasm's comparative advantage may change. Did the actual results agree with the hypothesis? Explain the evidence in terms of Total cost.
Official references
Linked example and execution scope
Example source · Runtime environments and limits
Standalone WAT + Node assertions; iframe CSP blocks Wasm compilation.
Source code and records are not overwritten automatically. Resetting an existing example also preserves your notes.
Next learning
Previous: Warm Repeats and Warm-up · Next: Boundary-value Correctness