[mythtv] OpenGL vsync issues
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
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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