[mythtv-users] How to minimize CPU usage

[Jerry Rubinow]

On 2/4/06, Steven Adeff <adeffs.mythtv at gmail.com> wrote:
> On 2/4/06, Kevin Kuphal <kuphal at dls.net> wrote:
> > Steven Adeff wrote:
> > > On 2/4/06, Kevin Kuphal <kuphal at dls.net> wrote:
> > >
> > >> Jerry Rubinow wrote:
> > >>
> > >>> On 1/30/06, Boleslaw Ciesielski <bolek-mythtv at curl.com> wrote:
> > >>>
> > >>>
> > >>>> Jerry Rubinow wrote:
> > >>>>
> > >>>>
> > >>>>> PVRs have existed for a while, but not HD-resolution PVRs - they're
> > >>>>> pretty recent.  If X is in a 24-bit depth mode, then I don't see where
> > >>>>> my numbers are off - 24 (bit depth) * 1280 * 720 (screen res) * 30
> > >>>>> (fps) = 630+ Mbits/sec.
> > >>>>>
> > >>>>>
> > >>>> 720p is (usually) 60fps.
> > >>>>
> > >>>> Bolek
> > >>>>
> > >>>>
> > >>> That's what I thought, but that would mean 24 * 1280 * 720 * 60 = 1260
> > >>> Mbits/second going to the video card.  There'd be no way that would
> > >>> work over PCI - it'd be dropping every other frame (not to mention
> > >>> that X reports 32 bpp, so 24 seems low), but it seemd like there was
> > >>> much less dropage than that.  So there's something I'm not accounting
> > >>> for, but I don't know what (this is without XvMC).
> > >>>
> > >>>
> > >>>
> > >> But here's something.  If PCI HDTV tuners can write the data just fine
> > >> to the disk, how is it that much harder for the video card to push that
> > >> much data along the bus as well?  Assuming XvMC takes care of the CPU
> > >> load, even if the data on the bus is the same, a PCI video card should
> > >> be able to handle as much as a PCI HDTV tuner can...
> > >>
> > >> Kevin
> > >>
> > >
> > > DVB cards send an mpeg stream through the PCI bus, the data being sent
> > > from the cpu to the  video card is "raw uncompressed" video
> > > information.
> > >
> > >
> > So I guess the question still remains, does XvMC make a difference in
> > this case...
> >
> > Kevin
>
> From what I understand of how XvMC works, it should, as much of the
> "decompression" of the video data is done on the card.
>
> --
> Steve

I would think that the GPU on the video card is being used for the
heavy lifting of decompression, as you say.  But is the buffer holding
the output of those calculations on the video card or system RAM?  And
if it is on the video card, does it have to travel back to the system
first before being sent to the video card's output buffer, or does it
go directly there without traveling back and forth over the bus first?

I would guess for the first question that the GPU only operates on
memory on the video card.  For the second question, it depends on how
sophisticated the code is and the algorithm.

-Jerry