[Bug 89233] New: Very slow Xorg performace when drawing Qt5's QPainter

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=89233

            Bug ID: 89233
           Summary: Very slow Xorg performace when drawing Qt5's QPainter
           Product: xorg
           Version: unspecified
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Driver/Radeon
          Assignee: xorg-driver-ati at lists.x.org
          Reporter: madcatx at atlas.cz
        QA Contact: xorg-team at lists.x.org

Created attachment 113671
  --> https://bugs.freedesktop.org/attachment.cgi?id=113671&action=edit
MWE

My Qt5 based application updates contents of a widget as the mouse moves over
it. Unfortunately the drawing operation causes inexplicably high load and
everything slows down to a crawl. During that time I observe Xorg fully loading
one CPU core, the application itself does not seem to consume much CPU. My
entire desktop is almost unusable until Xorg sorts itself out after a few
seconds.

As of now I found two possible workarounds:
1) Use Qt4 toolkit instead of Qt5
2) Set "Option" "NoAccel" to "true" in Xorg.conf

I believe that this issue has to do with either GLAMOR or xf86-ati drivers
because the application behaves normally on a machine with nVidia card or in
Windows.

I attached a small MWE which demonstrates the problem. Build the MWE with Qt5
toolkit, maximize the window and move mouse over the green area.

Up-to-date Fedora 21
xorg-x11-server 1.16.3
mesa 10.4.3
xorg-x11-drv-ati 7.5.0

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