[mythtv] Re: XvMC and Mythtv

David Shirley dave at cs.curtin.edu.au
Sun May 15 02:38:26 UTC 2005


Hi Daniel,

well when the CPU is at 60% the playback is fine.

There is something that triggers the playback to hit 95% and then its
awfull...

I might add up until a week ago 1080i playback was fine and I never had
any stuttering problems (other than seeking of course)

On the Nvidia thing what exactly do we need to say to them... I dont mind
emailing their support and asking them to increase the XvMC buffer size...
i would have thought this would be pretty easy todo???

Cheers
Dave



> On Sat, 2005-05-14 at 22:07 +0800, David Shirley wrote:
>> hey daniel,
>>
>> i did some more diagnosis... this is with today's CVS...
>>
>> I found that when watching 1080i TV the CPU will sit at 30% for credits
>> (ie mostly black screen), and 50-60% for normal TV.
>
> This is pretty high, I'm thinking maybe the Athlon XP @ 2Ghz isn't
> up to decoding 1080i with 8 buffers, even with XvMC. The only thing
> I can suggest is try enabling real-time threads by setting the suid
> bit on the MythFrontend executable.
>
> The problem his that while it is "only" eating up 60% of your CPU
> MythTV needs the CPU in a bursty fashion in order to get frames
> queued up and displayed in time.
>
> Having eight XvMC buffers really means you only have two free buffers
> in the worst case scenario. Two for OSD, three for I/P-frame past/future
> frames, one for display, and _two_ for filling. If there is a lot
> of motion or some other process is sucking up the CPU when one of
> those two frames becomes available, then you get problems... And
> this "worst case scenario" happens all the time with XvMC. You
> have basically 1/30th of a second (@720p), to fill two buffers,
> and you must do it at a rate of 1 buffer every 1/60th of a second.
> Which means you are really using the fancy soft real-time features
> of the 2.6 kernel just to keep up. This means you can't have any
> non-preemptable drivers, your APIC interrupt situation has to
> be happy, your hard-drive DMA access must be working properly,
> and your driver must unmask interrupts (hdparm -u). It's just to
> much to expect to go right.
>
> I'm hoping that if enough people complain to nVidia about their
> XvMC implementation only having eight buffers they will fix this
> in a future driver... The VIA Unichrome drivers support twice as
> many buffers!
>
> -- Daniel
>
>
>
>
> !DSPAM:4286730b208535847718184!
>
>




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