[Wayland-bugs] [Bug 772075] New: GTK+ uses a lot more CPU under Wayland than under X11

[gtk+ (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=772075

            Bug ID: 772075
           Summary: GTK+ uses a lot more CPU under Wayland than under X11
    Classification: Platform
           Product: gtk+
           Version: 3.21.x
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: Normal
         Component: Backend: Wayland
          Assignee: gtk-bugs at gtk.org
          Reporter: gns at gnome.org
        QA Contact: gtk-bugs at gtk.org
                CC: rob at robster.org.uk, wayland-bugs at lists.freedesktop.org
     GNOME version: ---

When I run gtk3-demo --run=pixbufs on my Gnome on Wayland session I get around
86% CPU usage. If I run it with GDK_BACKEND=x11 it uses only around 25%.

Perf tells me the biggest offender is
bits_image_fetch_bilinear_affine_normal_a8. Setting a breakpoint in gdb I
noticed it is almost all due to gtk_window_draw drawing client-side
decorations, particularly shadows.

Returning at the top of that function after just chaining up to the parent
brings CPU usage down to ~42%. Still higher than under Xwayland, but much lower
than the original 86%.

This is the biggest drag on the WebKitGTK+ performance at the moment, I
believe. While playing a 4k video the browser spends most of its time painting
the window decoration.

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