[mythtv-users] h264 "Prebuffering" pauses (Non XvMC, Non NFS)

[Steven Ellis]

On 30/08/2008, at 10:08 PM, Robert Longbottom wrote:

> Kingsley Turner wrote:
>> Hi,
>>
>> I'm getting "pre-buffering" pauses playing live-tv.  I know this  
>> because
>> it's in the mythfrontend logfile.
>> What does this actually mean?  What's it waiting on?  Reading from  
>> the
>> tv-card?
>>
>> BBC HD works ok, but Swiss HD has 1/2 second pauses every few seconds
>> when there's high motion displayed (I assume the greatest bitrate),
>>
>> The video signal is a 1280x720 50Hz h264 stream, running at around
>> 10kbit (or is this mbit?) according to VLC.
>>
>> I can playback the recorded HD stream OK though, it's only live TV  
>> that
>> stutters.
>>
>> I disabled all non-used devices in the BIOS.
>> I turned off OpenGL synch
>> I set both RTC and HPET to 1024 in /etc/sysctl.conf
>>
>> This is a dual-core (10 week old) core2-duo (E7200 at 2.53GHz) with 2G  
>> ram,
>> SATA2 disk.  CPU use during playback is around 80-105%  So it's using
>> about 1/2 it's processing power.
>>
>> The video-playback configuration is set to use "linear-blend'
>> deinterlacer, with a fallback of "none".
>>
>> It drawing to an onboard nvidia 7100, but i just tried a separate
>> pci-x-16 8600GT with absolutely no change in symptoms.
>>
>> Does commercial flagging run during live TV?  Or is this only
>> post-processing for recorded shows?
>>
>> Would throwing more hardware at this help?
>> 1066MHz ram instead of 800?
>> Quad core CPU ?
>>
>> Although it doesn't seem to be CPU bound ... but before the tuning  
>> CPU
>> was getting up to 180% (i.e.: almost maxing out both cores)
>>
>>
>> any hints ?
>
> Sounds like you've already checked a lot of things, but you don't  
> say if
> you've set the "number of CPU's" in Playback settings....  I think  
> it's
> set to 1 by default - you should have changed it to 2.
>
> From what you've said the hardware you've got should be more than
> enough to playback HD, so it must be configuration somewhere.
>

The hardware is fast enough for MPEG-2 HD, but H.264 is a different  
game altogether.

If you are hitting 180% during live TV there is a chance that during  
peaks in CPU load the myth backend is struggling to get the inbound  
signal onto the disk, and back to the frontend.

Here in NZ we have similar issues with a local 1080i stream that  
required more CPU than most of us have at present. You might want to  
take a look at some NZ specific patches that tweak some of the H.264  
settings to give slghtly better performance.

Steve



Steven Ellis - Technical Director
OpenMedia Limited
email   - steven at openmedia.co.nz
website - http://www.openmedia.co.nz

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