PreloadServingMetrics: Logs/metrics related to preload serving

Design doc: https://docs.google.com/document/d/1bBhfhO7BotUB7Myy_8mtFF_4lI5N8hUyNayV_gI019Y

This document will become truth once the CL https://crrev.com/c/6884362 has landed. TODO(crrev.com/360094997): Remove the above.

Goal

Enable collecting, recording, and analyzing serving metrics of preloads. For example:

• Continuously collect page load metrics for navigations under different conditions: without preloads, with prefetch used, and with prerender used. (E.g. to evaluate improvements of preload execution stacks.)
• Investigate the cause of regressions from new features by understanding what is happening during the serving phase.

Overview

Logs/metrics are processed in three steps:

1. Log objects are recorded in each component of preloads.
2. During a navigation, log objects are collected and associated with that navigation.
3. At the end of the navigation, the collected logs are recorded as metrics (UMAs/UKMs) if certain conditions are met. (E.g. a prerender initial navigation is cancelled.)

Logs

PreloadServingMetrics collects per-navigation, serving-related data about preloads, and contains logs objects collected from several preloading components.

While PreloadServingMetrics must be serving-side metrics, it also collects and uses non-serving-side data (e.g., data about prefetching or the initial prerender navigation) insofar as it relates to the serving navigation.

(Some of the non-serving-side data contained in log objects are also used for non-serving-side metrics outside PreloadServingMetrics.)

Examples:

• PreloadServingMetrics: Log of preloads associated with a navigation. May hold:
  • PrefetchMatchMetrics and
  • PreloadServingMetrics for prerender initial navigation (if the navigation is prerender activation navigation).
• PrefetchMatchMetrics: Log of prefetch matching. May hold PrefetchContainerMetrics.
• PrefetchContainerMetrics: Log of PrefetchContainer.
  • This contains data about the prefetching phase.
  • This is used for PreloadServingMetrics, only in the context of serving metrics (e.g., when the corresponding prefetch received the non-redirect header relative to the serving/navigation timing).
  • This is also used for metrics outside PreloadServingMetrics (including prefetching-side metrics).

The log objects are always:

• struct
• Passed by std::unique_ptr<>.
• Not movable.

Each log object is built by a related component, and becomes read-only outside that component. For example, PrefetchContainerMetrics is built by PrefetchContainer. It will be copied to a field of PrefetchMatchMetrics, and read-only after the copy. See each log object's comment to identify its builder component.

Life of PreloadServingMetrics {#life-of-PreloadServingMetrics}

A PreloadServingMetrics object is created per-navigation and is owned by PreloadServingMetricsHolder until navigation commit or failure.

After that, the ownership is moved to:

• PrerenderHost for committed prerender initial navigation.
  • The PreloadServingMetrics from the initial prerender navigation is then moved to the activation navigation's PreloadServingMetrics and processed along with it.
• PreloadServingMetricsPageLoadMetricsObserver (via PreloadServingMetricsCapsule that exposes PreloadServingMetrics to //content public) for committed non-prerender-initial navigation.
  • PreloadServingMetricsPageLoadMetricsObserver records UMAs/UKMs based on PreloadServingMetrics, when a navigation is committed and FCP is received.
• Discarded with recording UMAs/UKMs if it is a failed prerender initial navigation.
• Discarded without recording UMAs/UKMs otherwise.

Non-prerender navigation:

flowchart LR
  subgraph PreloadServingMetricsPageLoadObserver
    PreloadServingMetricsCapsule
  end
  subgraph non-prerender-navigation
    PreloadServingMetricsHolder
  end
  PreloadServingMetricsHolder --> PreloadServingMetricsCapsule

• If navigation is not committed, PreloadServingMetrics is not taken.

Prerender navigations:

flowchart LR
  subgraph PreloadServingMetricsPageLoadObserver
    PreloadServingMetricsCapsule
  end
  subgraph prerender initial navigation
    PreloadServingMetricsHolder-1
  end
  subgraph prerender activation navigation
    PreloadServingMetricsHolder-2
  end
  PreloadServingMetricsHolder-1 --> PrerenderHost
  PrerenderHost --> PreloadServingMetricsHolder-2
  PreloadServingMetricsHolder-2 --> PreloadServingMetricsCapsule

• If the prerender initial navigation is not committed and fails, PreloadServingMetrics is taken by PrerenderHost and used to report metrics for failure.
• If the prerender initial navigation is committed but PrerenderHost is cancelled, the same applies.

Test

Tests are managed in

• //content/browser/preloading/preload_serving_metrics_unittest.cc
  • Detailed checks of the correspondence between the PreloadServingMetrics structure and the recorded metrics.
• //content/browser/preloading/prerenderer_impl_browsertest.cc
  • End-to-end checks for basic scenarios.
• //chrome/browser/preloading/search_preload/search_preload_browsertest.cc
  • End-to-end checks that include PreloadServingMetricsPageLoadMetricsObserver.