perf: cache the entry tree directory icon to eliminate severe repaint lag on Windows - #328
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Cache the icon the same way the extension icons are cached, and clear both caches when the style changes so they do not go stale. The caching itself is platform independent, but the problem it solves was only measured on Windows (Qt 6.8.3, Windows 10); other platforms may or may not have a cheap standardIcon(SP_DirIcon). EntryTreeModel::data() returned QApplication::style()->standardIcon(SP_DirIcon) for every directory row without memoizing it, while the file icons directly below it are cached. Qt asks the model for the decoration of each visible row on every repaint, so each repaint re-extracted the platform folder icon once per visible directory. On Windows that lookup goes out to the shell and measures about 4.3 ms per call here, on every style. With a viewport holding a few dozen rows that is well over 100 ms of icon extraction per repaint, so anything that repaints the tree - scrolling, dragging the splitter, resizing the window - stalls. A CPU sampling trace over a large archive showed that roughly a fifth of all samples sit in user32!ConvertPNGToDIBIcon, WindowsCodecs PNG inflate and the kernel alpha blit (win32kfull!ProcessAlphaBitmap, vAlphaPerPixelOnly), while Qt's own painting accounts for about 2%.
Owner
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Thank you for this, didn't think it was platform specific |
Owner
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I'm on vacation but will merge as soon as I can |
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Cache the icon the same way the extension icons are cached, and clear both caches when the style changes so they do not go stale. The caching itself is platform independent, but the problem it solves was only measured on Windows (Qt 6.8.3, Windows 10); other platforms may or may not have a cheap standardIcon(SP_DirIcon).
EntryTreeModel::data() returned QApplication::style()->standardIcon(SP_DirIcon) for every directory row without memoizing it, while the file icons directly below it are cached. Qt asks the model for the decoration of each visible row on every repaint, so each repaint re-extracted the platform folder icon once per visible directory.
On Windows that lookup goes out to the shell and measures about 4.3 ms per call here, on every style. With a viewport holding a few dozen rows that is well over 100 ms of icon extraction per repaint, so anything that repaints the tree - scrolling, dragging the splitter, resizing the window - stalls. A CPU sampling trace over a large archive showed that roughly a fifth of all samples sit in user32!ConvertPNGToDIBIcon, WindowsCodecs PNG inflate and the kernel alpha blit (win32kfull!ProcessAlphaBitmap, vAlphaPerPixelOnly), while Qt's own painting accounts for about 2%.