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diff --git a/browser/components/tabunloader/docs/index.rst b/browser/components/tabunloader/docs/index.rst new file mode 100644 index 0000000000..534a10b4fd --- /dev/null +++ b/browser/components/tabunloader/docs/index.rst @@ -0,0 +1,58 @@ +Tab Unloading +============= + +Tab Unloading is a feature that automatically unloads tabs to prevent Firefox +from crashing due to insufficient memory when the system’s available memory is +low. + +The feature consists of two parts: memory pressure detector and tab unloader. +When the memory pressure detector detects a low memory situation, it triggers +the tab unloader that prioritizes tabs and chooses a tab to unload, or if there +are no unloadable tabs, triggers the internal memory-pressure warning allowing +the browser’s subsystems to reduce their memory use. + +There are two modes to prioritize tabs. + +Firefox basically unloads tabs in least-recently-used order, excluding +tabs playing media, using Picture-in-Picture, or using WebRTC. Pinned +tabs are deprioritized and are less likely to be unloaded. + +When there are more tabs opened, in most cases when there are more than +eleven tabs, Firefox does extra calculations identifying processes hosting +tabs and estimates memory usage of each tab, and then unloads tabs with +larger memory and more processes that will be terminated by tab unloading. + +You may disable the feature by setting the preference +``browser.tabs.unloadOnLowMemory`` to ``false``. + +about:unloads +------------- + +The about:unloads page shows how Firefox prioritizes tabs and which tab will +be unloaded when the tab unloader is triggered. You can trigger tab unloading +manually by clicking the **Unload** button in the page. + +The page contains a table where existing tabs are displayed in the same order +used by Firefox to choose the next tab to unload. When you click the button, +a tab shown in the first row, which has the lowest value in the **Priority**, +is unloaded. If the value of **Priority** is a hyphen (-), the corresponding +tab is not unloadable. + +In the first of the two modes mentioned above, Firefox calculates **Last Accessed** +and **Base Weight** for each tab and orders tabs by those values, not calculating +the other attributes such as **Secondary Weight** to save CPU power. Below is +an example of this case. + +.. image:: lightmode.png + +In the second mode, Firefox identifies processes hosting each tab and shows +their process IDs in the **Process IDs** column. Process IDs are displayed in +**bold** when they are hosting the tab’s top frame, and in *italic* when the +process is shared between different tabs. + +After identifying the processes of all tabs, Firefox estimates memory usage of +tabs and calculates the secondary weight for tabs that are not recently accessed. +For recently accessed tabs, Firefox does not calculate the **Secondary Weight** +and **Memory**, leaving those columns empty. + +.. image:: fullmode.png |