[mythtv-users] Debugging mythfrontend video freeze
Scott D. Davilla
davilla at 4pi.com
Mon Dec 10 16:18:24 UTC 2007
>I've made some significant progress on this problem that I'd like to
>share with others that might be having similar problems. First the
>symptoms from the logs;
>
>NVP: Waiting for prebuffer.. 0 ALAADAdL
>NVP: prebuffering pause
>
>If you see these, that means the thread filling the video buffer on
>the frontend has tripped some fill threshold and is has sped up
>getting video data from the backend. Normally you will only see
>sporatic bursts this in the log when the video content changes like a
>change to a dark background. The reason is that the mpeg2
>decompression rate has changed and the video buffer is emptying
>faster than before. The thread filling the video buffer re-fills
>according to the decompressed video rate rather than the actual
>buffer in use. This keeps network traffic throttled to the video
>consumption rate.
>
Correction on my understanding, "Waiting for prebuffer" is myth
waiting for a free video frame buffer (using XvMC in my case). This
occurs when there are not enough free video frame buffers. The
interesting part of the log above is "ALAADAdL". "A" indicates an
available video frame buffer. So if in the above prebuffering case
there are four frames available, why is myth waiting for more free
video frames in the first place?
This seems to occur when the video display gets behind and mfe is
trying to catch up. This condition can be triggered by a) new content
being displayed, b) position seeks (ff, rw, etc) and c) glitches in
the mpeg2 decoding due to transmission errors (OTA ATSC capture)
which cause mpeg2 decoder errors.
c) could be helped by having the backend qualify the mpeg2 stream
before saving. That way only known good content is saved. Backends
tend to have more ponies than frontends.
And some more performance notes, while the AppleTV under linux has
enough ponies to handle 720p HD content with software decoding, 1080i
must use XvMC as it's not quite fast enough to handle 1080i with
software decoding (a 1.7GHz pentium-M can). This also suggests that a
Myth frontend on the AppleTV under OSX should be able to handle 720p
HD content decode. 1080i content is not possible until they figure
out how to inject the mpeg2 into the apple dvd decoder framework.
Scott
More information about the mythtv-users
mailing list