[mythtv-users] tuner use slowing down entire CPU?

Omer Shenker mail at omershenker.net
Mon Aug 4 04:26:19 EDT 2003


Hello,

This may be something that belongs on linux-kernel, but I'm getting some
really strange behavior. I've recently installed a second tuner (thanks to
all those who replied by my question about wiring) and I've noticed that
each tuner I use seems to slow down the CPU. With no tuners in use on my
Athlon XP 2400+, top claims to take 0.7-1.3% CPU (according to itself). With
1 tuner active, top takes 2.2-2.9% and mythbackend takes about 34-40%. With
2 tuners active, top takes 4.2-5.5% CPU and each of the two major
mythbackends takes about 42-48% CPU. (Now, I can understand that top may
take polynomial time in the number of processes, but we're talking an extra
two or three processes out of 100-something, and running extra copies of top
doesn't have the same effect.)

Both tuners are Hauppauge 401s, using btaudio, and there are no unusual
messages in the logs so far as I can tell. This is a real problem because I
can pick a recording setting that looks great when only 1 tuner is active
and takes up well under 50% CPU, yet when I add a second tuner recording
with the same settings, they both start skipping.

This also happens with one recording and one being watched live, though of
course that's really just two recording and one playing. With one recording
and an old recording being watched, top takes 3.1-4.0% CPU and the one major
mythbackend takes 42-60%.

Something isn't scaling here. I don't know if it's mythbackend or the
kernel, but it's not something I see for other parts of the system (e.g.
making lots of requests at Apache might cause it to work more children and
take up more overall CPU, but top (and each individual httpd) doesn't take
up any more CPU). Rebooting, removing/reinsterting various modules,
restarting various processes: none of these make any difference.

Also, my ntp drift decreases from ~+92 down to ~+87 or so after extended
periods of tuner use, though I'm not sure if it doesn't always do that in
response to some other sort of activity (e.g. disk IO).

Any clues?

-- 
Omer Shenker                          http://omershenker.net/





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