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/* -*- mode: js; indent-tabs-mode: nil; js-indent-level: 2 -*- */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
/*
* TabUnloader is used to discard tabs when memory or resource constraints
* are reached. The discarded tabs are determined using a heuristic that
* accounts for when the tab was last used, how many resources the tab uses,
* and whether the tab is likely to affect the user if it is closed.
*/
const lazy = {};
ChromeUtils.defineESModuleGetters(lazy, {
PrivateBrowsingUtils: "resource://gre/modules/PrivateBrowsingUtils.sys.mjs",
webrtcUI: "resource:///modules/webrtcUI.sys.mjs",
});
// If there are only this many or fewer tabs open, just sort by weight, and close
// the lowest tab. Otherwise, do a more intensive compuation that determines the
// tabs to close based on memory and process use.
const MIN_TABS_COUNT = 10;
// Weight for non-discardable tabs.
const NEVER_DISCARD = 100000;
// Default minimum inactive duration. Tabs that were accessed in the last
// period of this duration are not unloaded.
const kMinInactiveDurationInMs = Services.prefs.getIntPref(
"browser.tabs.min_inactive_duration_before_unload"
);
let criteriaTypes = [
["isNonDiscardable", NEVER_DISCARD],
["isLoading", 8],
["usingPictureInPicture", NEVER_DISCARD],
["playingMedia", NEVER_DISCARD],
["usingWebRTC", NEVER_DISCARD],
["isPinned", 2],
["isPrivate", NEVER_DISCARD],
];
// Indicies into the criteriaTypes lists.
let CRITERIA_METHOD = 0;
let CRITERIA_WEIGHT = 1;
/**
* This is an object that supplies methods that determine details about
* each tab. This default object is used if another one is not passed
* to the tab unloader functions. This allows tests to override the methods
* with tab specific data rather than creating test tabs.
*/
let DefaultTabUnloaderMethods = {
isNonDiscardable(tab, weight) {
if (tab.undiscardable || tab.selected) {
return weight;
}
return !tab.linkedBrowser.isConnected ? -1 : 0;
},
isPinned(tab, weight) {
return tab.pinned ? weight : 0;
},
isLoading() {
return 0;
},
usingPictureInPicture(tab, weight) {
// This has higher weight even when paused.
return tab.pictureinpicture ? weight : 0;
},
playingMedia(tab, weight) {
return tab.soundPlaying ? weight : 0;
},
usingWebRTC(tab, weight) {
const browser = tab.linkedBrowser;
if (!browser) {
return 0;
}
// No need to iterate browser contexts for hasActivePeerConnection
// because hasActivePeerConnection is set only in the top window.
return lazy.webrtcUI.browserHasStreams(browser) ||
browser.browsingContext?.currentWindowGlobal?.hasActivePeerConnections()
? weight
: 0;
},
isPrivate(tab, weight) {
return lazy.PrivateBrowsingUtils.isBrowserPrivate(tab.linkedBrowser)
? weight
: 0;
},
getMinTabCount() {
return MIN_TABS_COUNT;
},
getNow() {
return Date.now();
},
*iterateTabs() {
for (let win of Services.wm.getEnumerator("navigator:browser")) {
for (let tab of win.gBrowser.tabs) {
yield { tab, gBrowser: win.gBrowser };
}
}
},
*iterateBrowsingContexts(bc) {
yield bc;
for (let childBC of bc.children) {
yield* this.iterateBrowsingContexts(childBC);
}
},
*iterateProcesses(tab) {
let bc = tab?.linkedBrowser?.browsingContext;
if (!bc) {
return;
}
const iter = this.iterateBrowsingContexts(bc);
for (let childBC of iter) {
if (childBC?.currentWindowGlobal) {
yield childBC.currentWindowGlobal.osPid;
}
}
},
/**
* Add the amount of memory used by each process to the process map.
*
* @param tabs array of tabs, used only by unit tests
* @param map of processes returned by getAllProcesses.
*/
async calculateMemoryUsage(processMap) {
let parentProcessInfo = await ChromeUtils.requestProcInfo();
let childProcessInfoList = parentProcessInfo.children;
for (let childProcInfo of childProcessInfoList) {
let processInfo = processMap.get(childProcInfo.pid);
if (!processInfo) {
processInfo = { count: 0, topCount: 0, tabSet: new Set() };
processMap.set(childProcInfo.pid, processInfo);
}
processInfo.memory = childProcInfo.memory;
}
},
};
/**
* This module is responsible for detecting low-memory scenarios and unloading
* tabs in response to them.
*/
export var TabUnloader = {
/**
* Initialize low-memory detection and tab auto-unloading.
*/
init() {
const watcher = Cc["@mozilla.org/xpcom/memory-watcher;1"].getService(
Ci.nsIAvailableMemoryWatcherBase
);
watcher.registerTabUnloader(this);
},
isDiscardable(tab) {
if (!("weight" in tab)) {
return false;
}
return tab.weight < NEVER_DISCARD;
},
// This method is exposed on nsITabUnloader
async unloadTabAsync(minInactiveDuration = kMinInactiveDurationInMs) {
const watcher = Cc["@mozilla.org/xpcom/memory-watcher;1"].getService(
Ci.nsIAvailableMemoryWatcherBase
);
if (!Services.prefs.getBoolPref("browser.tabs.unloadOnLowMemory", true)) {
watcher.onUnloadAttemptCompleted(Cr.NS_ERROR_NOT_AVAILABLE);
return;
}
if (this._isUnloading) {
// Don't post multiple unloading requests. The situation may be solved
// when the active unloading task is completed.
Services.console.logStringMessage("Unloading a tab is in progress.");
watcher.onUnloadAttemptCompleted(Cr.NS_ERROR_ABORT);
return;
}
this._isUnloading = true;
const isTabUnloaded =
await this.unloadLeastRecentlyUsedTab(minInactiveDuration);
this._isUnloading = false;
watcher.onUnloadAttemptCompleted(
isTabUnloaded ? Cr.NS_OK : Cr.NS_ERROR_NOT_AVAILABLE
);
},
/**
* Get a list of tabs that can be discarded. This list includes all tabs in
* all windows and is sorted based on a weighting described below.
*
* @param minInactiveDuration If this value is a number, tabs that were accessed
* in the last |minInactiveDuration| msec are not unloaded even if they
* are least-recently-used.
*
* @param tabMethods an helper object with methods called by this algorithm.
*
* The algorithm used is:
* 1. Sort all of the tabs by a base weight. Tabs with a higher weight, such as
* those that are pinned or playing audio, will appear at the end. When two
* tabs have the same weight, sort by the order in which they were last.
* recently accessed Tabs that have a weight of NEVER_DISCARD are included in
* the list, but will not be discarded.
* 2. Exclude the last X tabs, where X is the value returned by getMinTabCount().
* These tabs are considered to have been recently accessed and are not further
* reweighted. This also saves time when there are less than X tabs open.
* 3. Calculate the amount of processes that are used only by each tab, as the
* resources used by these proceses can be freed up if the tab is closed. Sort
* the tabs by the number of unique processes used and add a reweighting factor
* based on this.
* 4. Futher reweight based on an approximation of the amount of memory that each
* tab uses.
* 5. Combine these weights to produce a final tab discard order, and discard the
* first tab. If this fails, then discard the next tab in the list until no more
* non-discardable tabs are found.
*
* The tabMethods are used so that unit tests can use false tab objects and
* override their behaviour.
*/
async getSortedTabs(
minInactiveDuration = kMinInactiveDurationInMs,
tabMethods = DefaultTabUnloaderMethods
) {
let tabs = [];
const now = tabMethods.getNow();
let lowestWeight = 1000;
for (let tab of tabMethods.iterateTabs()) {
if (
typeof minInactiveDuration == "number" &&
now - tab.tab.lastAccessed < minInactiveDuration
) {
// Skip "fresh" tabs, which were accessed within the specified duration.
continue;
}
let weight = determineTabBaseWeight(tab, tabMethods);
// Don't add tabs that have a weight of -1.
if (weight != -1) {
tab.weight = weight;
tabs.push(tab);
if (weight < lowestWeight) {
lowestWeight = weight;
}
}
}
tabs = tabs.sort((a, b) => {
if (a.weight != b.weight) {
return a.weight - b.weight;
}
return a.tab.lastAccessed - b.tab.lastAccessed;
});
// If the lowest priority tab is not discardable, no need to continue.
if (!tabs.length || !this.isDiscardable(tabs[0])) {
return tabs;
}
// Determine the lowest weight that the tabs have. The tabs with the
// lowest weight (should be most non-selected tabs) will be additionally
// weighted by the number of processes and memory that they use.
let higherWeightedCount = 0;
for (let idx = 0; idx < tabs.length; idx++) {
if (tabs[idx].weight != lowestWeight) {
higherWeightedCount = tabs.length - idx;
break;
}
}
// Don't continue to reweight the last few tabs, the number of which is
// determined by getMinTabCount. This prevents extra work when there are
// only a few tabs, or for the last few tabs that have likely been used
// recently.
let minCount = tabMethods.getMinTabCount();
if (higherWeightedCount < minCount) {
higherWeightedCount = minCount;
}
// If |lowestWeightedCount| is 1, no benefit from calculating
// the tab's memory and additional weight.
const lowestWeightedCount = tabs.length - higherWeightedCount;
if (lowestWeightedCount > 1) {
let processMap = getAllProcesses(tabs, tabMethods);
let higherWeightedTabs = tabs.splice(-higherWeightedCount);
await adjustForResourceUse(tabs, processMap, tabMethods);
tabs = tabs.concat(higherWeightedTabs);
}
return tabs;
},
/**
* Select and discard one tab.
* @returns true if a tab was unloaded, otherwise false.
*/
async unloadLeastRecentlyUsedTab(
minInactiveDuration = kMinInactiveDurationInMs
) {
const sortedTabs = await this.getSortedTabs(minInactiveDuration);
for (let tabInfo of sortedTabs) {
if (!this.isDiscardable(tabInfo)) {
// Since |sortedTabs| is sorted, once we see an undiscardable tab
// no need to continue the loop.
return false;
}
const remoteType = tabInfo.tab?.linkedBrowser?.remoteType;
if (tabInfo.gBrowser.discardBrowser(tabInfo.tab)) {
Services.console.logStringMessage(
`TabUnloader discarded <${remoteType}>`
);
tabInfo.tab.updateLastUnloadedByTabUnloader();
return true;
}
}
return false;
},
QueryInterface: ChromeUtils.generateQI([
"nsIObserver",
"nsISupportsWeakReference",
]),
};
/** Determine the base weight of the tab without accounting for
* resource use
* @param tab tab to use
* @returns the tab's base weight
*/
function determineTabBaseWeight(tab, tabMethods) {
let totalWeight = 0;
for (let criteriaType of criteriaTypes) {
let weight = tabMethods[criteriaType[CRITERIA_METHOD]](
tab.tab,
criteriaType[CRITERIA_WEIGHT]
);
// If a criteria returns -1, then never discard this tab.
if (weight == -1) {
return -1;
}
totalWeight += weight;
}
return totalWeight;
}
/**
* Constuct a map of the processes that are used by the supplied tabs.
* The map will map process ids to an object with two properties:
* count - the number of tabs or subframes that use this process
* topCount - the number of top-level tabs that use this process
* tabSet - the indices of the tabs hosted by this process
*
* @param tabs array of tabs
* @param tabMethods an helper object with methods called by this algorithm.
* @returns process map
*/
function getAllProcesses(tabs, tabMethods) {
// Determine the number of tabs that reference each process. This
// is stored in the map 'processMap' where the key is the process
// and the value is that number of browsing contexts that use that
// process.
// XXXndeakin this should be unique processes per tab, in the case multiple
// subframes use the same process?
let processMap = new Map();
for (let tabIndex = 0; tabIndex < tabs.length; ++tabIndex) {
const tab = tabs[tabIndex];
// The per-tab map will map process ids to an object with three properties:
// isTopLevel - whether the process hosts the tab's top-level frame or not
// frameCount - the number of frames hosted by the process
// (a top frame contributes 2 and a sub frame contributes 1)
// entryToProcessMap - the reference to the object in |processMap|
tab.processes = new Map();
let topLevel = true;
for (let pid of tabMethods.iterateProcesses(tab.tab)) {
let processInfo = processMap.get(pid);
if (processInfo) {
processInfo.count++;
processInfo.tabSet.add(tabIndex);
} else {
processInfo = { count: 1, topCount: 0, tabSet: new Set([tabIndex]) };
processMap.set(pid, processInfo);
}
let tabProcessEntry = tab.processes.get(pid);
if (tabProcessEntry) {
++tabProcessEntry.frameCount;
} else {
tabProcessEntry = {
isTopLevel: topLevel,
frameCount: 1,
entryToProcessMap: processInfo,
};
tab.processes.set(pid, tabProcessEntry);
}
if (topLevel) {
topLevel = false;
processInfo.topCount = processInfo.topCount
? processInfo.topCount + 1
: 1;
// top-level frame contributes two frame counts
++tabProcessEntry.frameCount;
}
}
}
return processMap;
}
/**
* Adjust the tab info and reweight the tabs based on the process and memory
* use that is used, as described by getSortedTabs
* @param tabs array of tabs
* @param processMap map of processes returned by getAllProcesses
* @param tabMethods an helper object with methods called by this algorithm.
*/
async function adjustForResourceUse(tabs, processMap, tabMethods) {
// The second argument is needed for testing.
await tabMethods.calculateMemoryUsage(processMap, tabs);
let sortWeight = 0;
for (let tab of tabs) {
tab.sortWeight = ++sortWeight;
let uniqueCount = 0;
let totalMemory = 0;
for (const procEntry of tab.processes.values()) {
const processInfo = procEntry.entryToProcessMap;
if (processInfo.tabSet.size == 1) {
uniqueCount++;
}
// Guess how much memory the frame might be using using by dividing
// the total memory used by a process by the number of tabs and
// frames that are using that process. Assume that any subframes take up
// only half as much memory as a process loaded in a top level tab.
// So for example, if a process is used in four top level tabs and two
// subframes, the top level tabs share 80% of the memory and the subframes
// use 20% of the memory.
const perFrameMemory =
processInfo.memory /
(processInfo.topCount * 2 + (processInfo.count - processInfo.topCount));
totalMemory += perFrameMemory * procEntry.frameCount;
}
tab.uniqueCount = uniqueCount;
tab.memory = totalMemory;
}
tabs.sort((a, b) => {
return b.uniqueCount - a.uniqueCount;
});
sortWeight = 0;
for (let tab of tabs) {
tab.sortWeight += ++sortWeight;
if (tab.uniqueCount > 1) {
// If the tab has a number of processes that are only used by this tab,
// subtract off an additional amount to the sorting weight value. That
// way, tabs that use lots of processes are more likely to be discarded.
tab.sortWeight -= tab.uniqueCount - 1;
}
}
tabs.sort((a, b) => {
return b.memory - a.memory;
});
sortWeight = 0;
for (let tab of tabs) {
tab.sortWeight += ++sortWeight;
}
tabs.sort((a, b) => {
if (a.sortWeight != b.sortWeight) {
return a.sortWeight - b.sortWeight;
}
return a.tab.lastAccessed - b.tab.lastAccessed;
});
}