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