[mythtv] HDTV CPU utilization

Brad Templeton brad+mydev at templetons.com
Mon Jan 17 17:03:02 EST 2005


On Mon, Jan 17, 2005 at 01:59:26PM -0700, John Patrick Poet wrote:
> That all being said, if you have a 3.0GHz HT P4, why would you want to use
> XvMC?  That much horsepower is plenty to decode HD material without
> resorting to XvMC, and the quality will be much better.  *Not* using XvMC
> will result in most of your CPU power being used -- it should not be
> "pegged", but it will use most of it..

I have to admit that while I have only done cursory examinations, I haven't
seen a big quality difference (in mplayer) between xvmc mpeg decoding and
the software mpeg decoding.   Is this just a question of not looking hard
enough?

Because otherwise, I would say there are many motivations to use xvmc
even when you have the CPU available.  It means more CPU available for
other things -- commercial scan, transcode and general system operations,
and I have indeed seen the video skip on a 3ghz HT P4 when there are other
demands on the CPU, even niced demands on it.

Plus on some processors (mostly mobile) the CPU had different heat output
when idle than when heavily active, which means all sorts of things.  Many
of these things might even merit a small drop in quality of mpeg decode,
though I haven't done a full study of that.

Now on the other hand, not being able to seek in the video (a reported
problem with the mythplayer, mplayer is fine on that) is of course
a deal-killer.

And finally, many would answer that if they were to use xvmc, they might
simply use a lower-powered processor in their HD system, to be cheaper,
and lower heat, and often effectively free because you have it sitting
around.  (Or close to free... Fry's sells a 1.6hgz duron plus motherboard
for $40, and that might be enough to do hdtv with xvmc, though I have not
tried it.)  Of course, in this case the original author has a 3ghz chip
so this doesn't apply, though he might repurpose that for another use
and slow down his myth frontend box with xvmc.


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