[mythtv-users] High load, low CPU usage

Ray Olszewski ray at comarre.com
Fri Jul 11 13:42:16 EDT 2003


At 07:45 PM 7/11/2003 +0100, Steve Hill wrote:

>I am recording MPEG4 at 416x576, 2200Kbps and audio quality 7, 32000KHz.
>The hardware is an Athlon XP 2100+ with a WinTV/PCI bt848 tv card.
>The frontend and backend run on the same machine.
>
>While recording, the backend's CPU usage varies between about 15% and
>40% with the load average usually below 0.50.
>
>When playing the frontend uses under 5% CPU and the load average is under
>0.02.
>
>The problems start when I am playing at the same time (e.g. LiveTV) - The
>load average varies anywhere from 0.50 to over 3.00.  This results in
>choppy video, and loss of AV sync.  The front end also sometimes reports
>audio buffer overflows.
>
>DMA mode is turned on on the hard drive, although it isn't hitting the
>disk much anyway.
>
>Does anyone have any ideas what might be causing the problem?  I can't
>understand why it works fine recording and playing separately but
>recording and playing symultaneously causes such a lot of load, especially
>since there is plenty of free CPU time.

1. What is CPU utilization when "playing at the same time (e.g. LiveTV)"? 
You left that detail out, and I'd like to know whether or not there is 
really "plenty of free CPU time" in that setting.

2. Are you certain that you are using the recording profile you describe 
above for *both* the default and the live-TV profiles?

3. I'm puzzled by your saying that Myth "isn't hitting the disk much 
anyway". It has to be writing to the buffer, and I think Isaac said Myth 
was set to force a sync every second (but check the list archive to be sure 
of this). But a DMA problem would show up in recording mode, not just in 
"live" TV mode, so that probably isn't it.

4. XV problems are unlikely in this instance too, since playback would 
never use "under 5% CPU" if it were not running.

5. The extremely high load average suggests that the live-TV choice leads 
to some process that blocks. Offhand I cannot see what it might be; listing 
(say) the top dozen of so processes in a "top" listing might suggest 
something, though.

6. A real longshot here -- is it possible that the bttv and the sound card 
are using the same IRQ? Or that you have a sound card that has trouble with 
full-duplex, something that would show up only when simultaneously 
recording and playing (e.g., "live'" TV)? This is pretty wild guessing, though.

7. Some miscellaneous items: How much RAM is in the system? What gbuffers 
setting do you use with the bttv driver? What sound modules (OSS? ALSA?) ar 
you using?



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