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Web Workers · 25 Measuring End-to-End Latency
Series contents · Technology map
Stage: Validating Throughput and Responsiveness · Editable example · Storage ID: web-workers-step-25
Goals and learning outcomes
- Learning outcome: explain the role of Round-trip cost. Input cases: three array sizes.
- Deliverable: a table of round-trip, computation, and remaining intervals. Explain the rationale for handling Clock references, linking it to the code.
Prerequisites
Prepare the experiment results for Races and Atomics from Unit 24, “Atomics and Synchronization”.
Key concepts and explanations
Round-trip cost
Measuring only computation inside the Worker misses serialization and waiting costs. Measure from sending the request through updating the UI.
Clock references
Distinguish the time origins of different contexts. Record main-thread round-trip time and Worker-internal duration separately.
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 existing web-workers-01·02·03 are starting examples. Implement the new assignments in main.js and worker.js in a separate local HTTP project. Preserve CSP and iframe isolation. For SharedArrayBuffer, check COOP and COEP requirements on an external server you control; use the transfer path if they are not met. Measure performance 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 src/day-25.js in a separate practice project, include main-side performance.now measurements and Worker computation duration in the response. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: three array sizes. Baseline: Small array. Run under the baseline conditions, then record observations and errors in notes/day-25.md.
- Variable: Data size; changed value: Large array. Run again through the same input path. Deliverable to save: a table of round-trip, computation, and remaining intervals. Hypothesis to check: The proportions spent on communication and computation 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 |
|---|---|---|---|
| Data size | Small array | Large array | The proportions spent on communication and computation 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 round-trip, computation, and remaining intervals. 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: Round-trip cost and Clock references. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Round-trip cost affect in this exercise? Input cases: three array sizes.
- Variable: Data size. Hypothesis to check: The proportions spent on communication and computation may change. Did the actual results agree with the hypothesis? Explain the evidence in terms of Clock references.
Official references
Linked example and execution scope
Example source · Runtime environments and limits
An interactive Measuring End-to-End Latency experiment with cleanup of the resources it uses.
Source code and records are not overwritten automatically. Resetting an existing example also preserves your notes.
Next learning
Previous: Atomics and Synchronization · Next: Memory and Pool Limits