Detector performance monitoring

The detectorPerf feature measures synchronous detector execution time and reports threshold crossings through webEvents. Native EventHub consumes those events and aggregates them into periodic telemetry.

The feature is remote-config gated. When it is disabled or unavailable, detector behavior is unchanged and no performance data is attached to breakage reports.

Instrumented detectors

Periodic telemetry uses Web Detection group labels such as adwalls, captcha, commentsDisabled, and other.

Detectors within a group are pooled for periodic event names, reducing telemetry size while distinguishing broad detector families. The exact ID, such as adwalls.generic_en, is retained for single-run severe-event attribution and breakage-report data.

The on-demand bot and fraud detectors are not instrumented because their request-driven sampling is not comparable with automatically running Web Detection. Their detection results remain in breakage reports independently of detector performance monitoring. The YouTube interference detector is also excluded because its recurring sweep does not have the same run and accumulated-total semantics.

Settings

• defaults.singleRunThresholdsMs controls worst-single-run event edges.
• defaults.totalPerPageThresholdsMs controls accumulated edges for each frame. The setting retains its original PerPage name for config compatibility.
• combinedThresholdsMs controls accumulated edges across all instrumented detectors in a frame.
• singleRunSevereThresholdMs makes every crossed single-run edge at or above the cutoff emit immediate severe telemetry.
• totalPerPageSevereThresholdMs does the same for crossed per-group accumulated edges.
• combinedSevereThresholdMs does the same for crossed combined accumulated edges.
• detectorOverrides can replace the single-run or accumulated edges for one detector group.
• maxSeverePerPage caps immediate severe events in each frame. It likewise retains its original name for config compatibility.

Invalid threshold lists fall back to C-S-S defaults. Threshold lists are normalized to ascending, unique, positive finite values. An absent or invalid severe cutoff retains the previous behavior of emitting only at the highest edge.

Event contract

All detector performance event types use the reserved detectorPerf_ prefix:

• detectorPerf_measured — the top frame initialized the feature
• detectorPerf_<group>_ran — a detector in the named group ran
• detectorPerf_<group>_failed — a detector in the named group threw during evaluation
• detectorPerf_<group>_over<N>ms — one run in the group exceeded threshold N
• detectorPerf_<group>_total_over<N>ms — the group's accumulated time in the frame exceeded N
• detectorPerf_combined_over<N>ms — accumulated time for all instrumented detectors in the frame exceeded N
• detectorPerf_severe — an immediate event for each crossed single-run, per-group total, or combined edge at or above its severe cutoff

Thresholds are configuration values embedded in event names. Every threshold change therefore requires corresponding EventHub sources in privacy configuration. C-S-S unit tests enumerate the possible output types, while privacy-configuration tests verify both directions of the contract: every possible event has a consumer and no stale detectorPerf_ source remains.

Failed runs still contribute to run and duration events because failed detector work consumes CPU. Failure events fire at most once per frame, so their periodic counters measure pages containing one or more failures rather than the exact number of exceptions.

detectorPerf_severe includes:

{
    "kind": "single",
    "detector": "adwalls.generic_en",
    "thresholdMs": 150
}
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kind is single, total, or combined. Single-run events identify the exact config-driven detector. Totals use the detector group because the accumulator is shared within that group; combined crossings use the literal combined. Severe events are deduplicated per detector, kind, and threshold. When several edges cross together, the highest edges emit first so the frame cap preserves the strongest signals.

Frames and EventHub deduplication

Each injected frame owns its own accumulators and at-most-once guards. Only the top frame emits detectorPerf_measured, preventing subframes from inflating the page denominator.

Native EventHub owns page-level deduplication. Consequently, periodic counters represent pages where at least one frame crossed a threshold, not the number of frame emissions.

Breakage reports

User-initiated breakage reports include the reporting frame's exact accumulated Web Detection values:

• run count
• total duration
• worst duration
• combined duration across instrumented detectors

These values are rounded to 0.1 ms. Unlike periodic telemetry, config-driven detectors retain their exact IDs in breakage reports.

Debug test-page event

When the platform debug flag is enabled, the feature dispatches a page-observable detectorPerfDebugStats CustomEvent after each recorded run. Production builds must never enable this flag on user pages.

The listener and visual overlay live in the privacy-test-pages repository at features/detector-perf/overlay.js. C-S-S integration tests also listen for the event to verify that it is emitted only in debug mode.