[mythtv-users] [ATrpms-users] Confused: why did video performance DECREASE (up to 70%) after upgrading video? PARTIALLY SOLVED
george_mythusers at mari1938.org
Tue Dec 9 15:01:08 UTC 2008
Jeffrey J. Kosowsky wrote:
> > Hmmm...I agree it could be either audio or video, or the combination of
> > trying to decode and playback both at the same time, while keeping the
> > two in sync. My next suggestion would be to try changing your audio
> > output device to NULL, from the current ALSA:default. I'm not 100%
> > sure, but I believe this will disable all audio decoding by myth. By
> > taking that out of the picture (so to speak) maybe we can see what
> > happens to the video quality. If it still stutters and generates
> > pre-buffering pauses, then the audio is not the issue. If the video
> > quality improves, and there are no more pre-buffering pauses, I'd say
> > it's time to try installing a PCI sound card instead of using your
> > built-in motherboard audio.
> Interestingly, I didn't see a big difference in setting it to
> NULL. Also, I'm not sure I understand exactly how the "extra audio
> buffering" helped the problem.
OK - main thing is, it's working reasonably well now. I think the myth
player tries to keep audio and video in sync more than other players.
If the audio is "slow", or cannot be decoded and played back in time
because of driver or hardware issues, it will "pause" the video, which I
think results in stuttering. This is just my understanding / theory.
> This is my typical CPU usage (from top):
> Cpu(s): 61.6%us, 9.7%sy, 0.0%ni, 27.3%id, 0.5%wa, 0.5%hi, 0.5%si,
> 20 0 326m 103m 34m S 62.0 5.1 14:52.29 mythfrontend
> 20 0 364m 82m 11m S 9.0 4.1 1:58.87 mythbackend
> 20 0 56312 5432 3688 S 6.0 0.3 1:57.03 pulseaudio
> 20 0 59240 23m 9088 S 4.6 1.1 1:42.21 X
> I'm curious now how you seem to be getting better performance. Could
> it be that your 2.2Ghz Athlon 64 is just that much faster than my
> 2.8GHz P4? (and I am using XvMC).
Probably not - CPUs are about the same. My understanding is P4s are
generally slower per-clock than Athlon 64 chips, so your 2.8ghz P4 is in
the same ballpark as my 2.2ghz Athlon 64. Motherboard / chipset might
be a different story. They're probably in the same ballpark, I've just
been tweaking mine for mythtv longer than you have.
I have pulseaudio installed, but it's not running. Also, my system is a
dedicated frontend only - no backend running.
> Two questions that remain:
> - Any other suggestions about things I can do to tweak my mythtv/video
> setup to grind out a few more marginal cpu or video points?
1. Are you running hyper-threading on the P4? There have been a few
reports on the list of people getting better performance - better
throughput - with hyperthreading turned off. Or, if you already have it
off, try turning it on.
2. Have you tried adjusting the PCI latency on any of your PCI devices?
This made a big difference for me on my dedicated backend - an old Dual
600mhz-P3. I have a PVR-350 in it for recordings, and it was very
glitchy when I first set it up - there were frequently messages in the
system log from the ivtv driver about buffer underruns or overruns (I
forget which), until I maxed out the PCI latency on the device - and it
has worked perfectly, glitch free ever since.
You might want to experiment with increasing the latency on your audio
device and/or video card.
There is a link to good article on PCI latency and how it works at IBM's
website at the bottom of the wiki article.
> - Would upgrading further to a 7600gs card be likely to help or is
> this likely to be more of a CPU issue? (note: I clearly got some
> benefit in moving from the 4600 to the 6200 card and turning on XvMC)
Not sure if the video overlay on the 7 series offers any more offload
than the 6 series. However, if you try to use any of the OpenGL
rendering methods for Mythtv, the 7600 would likely give you more oomph
for this than the 6200. OpenGl performance is more closely related to
GPU clock speed, memmory bandwidth, etc. I believe a 7 series card is
reported to be good enough for OpenGl rendering of up to 720P video.
Again, this is just my understanding - no direct experience with this yet.
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