GeckoView Priority Hint API

Cathy Lu <calu@mozilla.com>, Bug 1764998

May 2nd, 2022

Summary

This document describes the API for setting a process to high priority by applying a high priority hint. Instead of deducing the priority based on the extension’s active priority, this will add an API to set it explicitly.

Motivation

This API will allow Glean metrics to be measured in order to compare performance and stability metrics for process prioritization on vs off. Previously, prioritization depended on whether or not a GeckoSession had a surface associated with it, which lowered the priority of background tabs and needed to be reloaded more often.

Goals

Apps can set priorityHint on a GeckoSession.

Existing Work

In bug 1753700, we added an API in dom/ipc to allow GeckoViewWebExtension to set a specific remoteTab’s boolean priorityHint. This allows tabs that do not have a surface but are active according to web extension to have high priority.

Implementation

In GeckoSession, add an API setPriorityHint that takes an integer as a parameter. The priority int can be PRIORITY_DEFAULT or PRIORITY_HIGH. Specified and active tabs would be PRIORITY_HIGH. The default would be PRIORITY_DEFAULT. The API will dispatch an event GeckoView:SetPriorityHint.

public void setPriorityHint(final @Priority int priorityHint)

Listeners in GeckoViewContent.sys.mjs will set this.browser.frameLoader.remoteTab.priorityHint to the boolean passed in.

case "GeckoView:setPriorityHint":
  if (this.browser.isRemoteBrowser) {
    let remoteTab = this.browser.frameLoader?.remoteTab;
    if (remoteTab) {
      remoteTab.renderLayers.priorityHint = val;
    }
  }
break;

Additional Complexities

Apps that use this API will need to manually use the API to set the priorityHint when the tab goes to foreground or background.