[mythtv] OpenGL vsync issues

Yeasah Pell yeasah at schwide.com
Mon Jun 4 14:57:24 UTC 2007

>> * Borderline CPU power.  If your system works OK w/o bob but hiccups
>> with bob turned on, you might not have enough CPU to get the job done
>> reliably.  Enabling real-time priority (by setting up pam
>> /etc/security/limits.conf rtprio on a modern distro) can help, but might
>> instead exhibit the dreaded "prebuffering pause" on high-motion scenes.
>> Insufficient CPU power is unlikely if your CPU is less than a couple
>> years old, unless there's something else happening on your system.
> I've got plenty.  AMD X2 3600 ... I only have 20-30% cpu usage with bob 
> enabled, and the CPU is in power saving running at 1000Hz instead of the 
> full 2000Hz.
> I've set the limits so that I can run in real-time priority.

This almost surely isn't the answer to your problem in any way, but 

For months I had a frontend-only system with a persistent but relatively 
infrequent slight judder on certain content. Watching CNN, for example, 
would not give a perfectly smooth crawl at the bottom -- if you watched 
it carefully with your eyes you could see the occasional hitch. For 
whatever reason I'm pretty sensitive to temporal stuff like that, and it 
drove me nuts even though other people didn't notice it at all. It was 
worse (more frequent) in some content than others.

One day, I noticed that the Athlon X2 I had in there was switching back 
and forth between 1GHz and 2GHz occasionally (it spent most of its time 
at 1000Hz, short blips up to 2000), at about the same rate as the 
hitches in the video. Disabling speed switching immediately cleared up 
all the hitches; it's pretty much perfectly smooth now (except when the 
bitstream is corrupted of course.) I'd rather have smooth video AND 
power-saving, but if I have to choose, I'm going to have to go with 
smooth video.

Now, this could well be specific to my setup, but I had that problem 
across different motherboards/CPUs (they were all Athlon 64-based, but 
different chipsets and other hardware) until I disabled the CPU 
frequency switching. On the software side, I always used various Fedora 
versions, from FC3 to FC6, and always nvidia drivers and graphics cards 
(but various kinds -- 6600 AGP, 7800 PCIe, maybe some others)

OpenGL vsync and UseEvents were also both very important for smooth 
video on that frontend (until I added the UseEvents option, the X2 was 
pretty much a requirement for smooth video at HD resolutions; but at 
this point the X2 is clearly overkill)

Anyway, I don't think this is the answer to your problem or anything, 
but more generally, for somebody that has a frontend system that runs in 
low power reduced speed mode most of the time, and experiences slight 
but persistent judder, you might want to check to see if the CPU is 
occasionally switching gears, or just try shutting off speed switching 
and see if it goes away. I apologize if this is some well-known FAQ item 
by now; I haven't really been following mythtv happenings in some time.


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