Video stutter / mouse jumps in Kubuntu Intrepid

[Jakob Petsovits]

Binary package hint: kubuntu-desktop

Since I updated to Kubuntu Intrepid (which was about a month before its release or so), Kubuntu shows a two weird bugs, with a pretty clear sign that those are related and probably stem from the same issue.

Symptom 1.
Video playback (with all kinds of different players, including Dragon, Kaffeine, VLC and Flash Player videos) stutters every few (5-8?) seconds or so, for a fraction of a second, plays back nicely otherwise. Sound works flawlessly. The issue also occurs when simply running glxgears, and brief testing on the desktop would suggest that this is not a video playback issue at all but that the whole display is on hold for that short time.

The forum thread at http://ubuntuforums.org/showthread.php?t=939027 found out that this does not happen in fluxbox, or in GNOME using compiz. I can confirm that video playback does not stutter in fluxbox. It still happens with compiz in a KDE session, and also with KWin trunk. KWin is therefore innocent. I also noticed Symptom 2 very early in the session startup, so it's definitely not caused by a normal application but by some permanent ("startup") kded service or other fundamental stuff like Plasma.

Furthermore, the reports in that thread indicate that this might be an issue of the i945 graphics driver, and I've also got that piece of hardware. Given that I haven't yet found a report of this issue happening with other graphics cards, this issue should be assumed to be specific to i945 cards.

Symptom 2.
When I leave the mouse pointer without moving, it jumps about 3-5 pixels to the right. When I move the cursor again, it starts from its original position further left, as if it had never jumped right. Steadily reproducible - I can somehow live with it, but it still gets on my nerves. No mouse events seem to be sent to the system for the mouse cursor jump. (I tried on the Krita canvas which has an associated X/Y pixel position display, and it does not change when the cursor jumps.) The jumps occurs after a similar timespan as the video stutter does (I watched it a few times, it goes around 4-7 seconds).

Now here comes the interesting part.

The thread http://ubuntuforums.org/showthread.php?t=953052 recognizes that the mouse jump coincides with the video stutters. I can confirm that too, easily reproducible with glxgears. Furthermore, I just analyzed the mouse jump times a bit more, and the jump happens all 10 seconds (currently at 8, 18, 28, 38, 48 and 58 seconds in any given minute, but I believe that depends on when the timer starts). So there is good reason to believe that a timer triggers every 10 seconds, and causes the delay by whatever action it executes.

I had been running the same version of KDE 4 on Hardy where both of these issues did not show up, still they don't manifest outside of a KDE session. Therefore, it seems probable to me that that some KDE component interacts with the graphics subsystem (or whatever) in Intrepid, in a different way than it did in Hardy. That makes the X update and KRandR service a prime suspect, going to test that sucker right now. Could be something else, though.

[Jakob Petsovits (jpetso)]

Bingo! Disabling the KRandR service (in the "Service Manager" module in System Settings) really does the trick - the cursor doesn't jump anymore, and the video plays back fluently. I guess there's a downside to disabling this kded though, isn't it?

Well I'm throwing a movie night tomorrow anyways, time to test how X does with enabling my video output (which also worked flawlessly in Hardy, and has given me a rough time in Intrepid so far).

[Jakob Petsovits (jpetso)]

Obviously related to bug #287128, bug #278471 and (in turn) bug #245383.