[mythtv-users] ffmpeg and multiple CPUs. Was: Should I be switching to 0.21?

[Tom Dexter]

On Sun, Apr 13, 2008 at 1:33 PM, Tom Dexter <digitalaudiorock at gmail.com> wrote:
>
>  I just wanted to report back on my experience regarding CPU usage.  I
>  upgraded today from 0.20.2 to 0.21 SVN 16944 under Gentoo.  First of
>  all, the upgrade from everything I've tested appears to have gone
>  flawlessly.
>
>  My frontend (dedicated frontend only) is a Dell Dimension 4700C 3.0
>  Ghz with hyper threading enabled.  Under 0.20.2 I was using libmpeg2
>  and bob de-interlacing.  It should be noted that I'm using Viktor's
>  patch to allow bob de-interlacing on my 1080i interlaced display:
>
>  http://svn.mythtv.org/trac/ticket/2903
>
>  ...though other than allowing me to use bob, that shouldn't be a factor.
>
>  First thing after upgrading I setup my own custom playback profile
>  group to mimic my old settings with only one entry as follows:
>
>  Resolution: >= 0 0
>  Decoder: libmpeg2
>  Video Renderer: xv-blit
>  OSD Renderer: softblend
>  OSD fade: ON
>  Deinterlacer: bobdeint
>  Secondary Deinterlacer: none
>  Filters: none
>
>  For max cpus under the Decoder I assumed 2 made sense for my hyper-threaded CPU.
>
>  Because of all the talk about increased CPU usage in 0.21, I did
>  several checks before upgrading, noting the CPU usage on the frontend
>  for 720p (that doesn't require de-interlacing) as well as 1080i (which
>  of course does) for several different OTA channels, both watching
>  recordings and livetv (though those really didn't differ).
>
>  I can say beyond question that the CPU usage under 0.21 with the above
>  profile is identical to that under 0.20.2.  As a matter of fact, on
>  occasion when watching 720p it's slightly less.
>

I was experimenting with some changes to my above profile and I've
made a rather interesting discovery.

On my frontend I found that using the Standard (ffmpeg) decoder in
place of libmpeg2 in the above profile used a fairly significant
amount of extra CPU.  In 0.20.2 I also found it used more, but this
difference was quite a bit more significant.

Playing 720p recordings with my above profile (with libmpeg2) used
18-20% of both hyper-threaded CPUs, while the exact same profile with
ffmpeg used around 38%.  Playing 1080i recordings with my above
profile (with libmpeg2) used slightly less than 30% of both CPUs,
while that profile with ffmpeg used 50% or more, with mythfrontend
often using 100% of one CPU.  I believe that the difference in 0.20.2
was more in the 10% range as apposed to this near 20% change.

However...and this is the interesting part...if I use ffmpeg but
switch the number of CPUs to 1 instead of 2 the performance of the two
is virtually identical.  It seems that, at least with my
hyper-threaded 3 Ghz P4, ffmpeg works much better with a single core.

Very cool...now I can used captions if I want.  Speaking of ATSC
captions though...is there any way to control the font color of ATSC
captions?  I see all sorts of choices for fonts but nothing for color.
 The grayish color I get right now over most video content can be
unreadable.  I also wish I could control the position...is that
controlled by the broadcaster?  I often have the captions chopped off
due to overscan.

Tom