[mythtv] NVidia XvMC Prebuffering Pauses

Ian Dall ian at beware.dropbear.id.au
Wed May 18 15:22:49 UTC 2005

David Shirley writes:
 > Hey Don,
 > I have been getting this of late (last 2 weeks) as well.
 > One fix which will help but not solve the problem is in nvidia-settings
 > ensure Texture Vsync -> Blank is off (under one of the menu's you will see
 > what I mean).
 > At the moment (as per Daniel's instructions) I am going to be doing
 > profiling on my mythtv install to find out where the CPU usage is going..

I've been doing that. XvMC offloads the motion compensation and IDCT
well enough. You will also notice that XvMC drops the Xserver load
significantly (ie you will underestimate the saving if you only look at
mythfrontend). The dominant remaining time seems to be in doing the
huffman decoding inside of mpeg2_decode_block_intra() and
mpeg2_decode_block_non_intra(), particularly the GET_RL_VLC macro.

I have to say here that I am using a 1.2GHz C3 Nehemiah processor, but
NOT an epia-M, so I don't have XvMC VLC. The C3 has a very small cache,
and the table lookups seem likely to be cache busters. So what I see may
not be typical of other processors.

I have found an alternate algorithm in a paper
<http://www.ece.queensu.ca/symposium/papers/7C_2.pdf>, which looks
more cache friendly. I was planning on coding this up and doing a
comparison, but if anyone else want to beat me to it, feel free!


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