[mythtv-users] Odd pauses when starting recorded shows...

Michael T. Dean mtdean at thirdcontact.com
Sat Sep 10 17:04:07 UTC 2005


mythtv at varga.co.uk wrote:

>Now that my Myth box seems to be fairly stable, I'm slowly starting to
>migrate recording shows off the Tivo and onto the Myth box. As I'm
>actually using Myth a lot more, I've noticed a few problems...
>
>1) When I select a recorded show to play from the menu, on about 20% of
>them, the screen goes black and there is no sound, it sits there like this
>for anything from 10-45 seconds and then the show starts and plays as
>normal...
>I've run the database check and it's all fine and playing the files over a
>samba share is OK too. Playing videos in MythVideo has no problems...
>  
>
This is most likely swapping all the "unnecessary" stuff out of 
buffers/cache and swapping the playback code back into memory.  
Basically, Myth is a long-running process and the most-used parts of it 
are not the TV-watching parts, but the recording, commflagging, 
transcoding, etc.  So, when you haven't watched TV for 20 hours, the 
system has to make all that "infrequently-used" code available.  You can 
affect the delay by setting kernel options for "swapiness," 
buffering/caching, ...  (How to do so is left as an exercise to the 
reader.  ;)  Do a Google search to see just how much discussion exists 
on the right way to tune your system (and read them all since they all 
tend to disagree).)  For example, I travel a lot for work, and when 
returning from a one-week trip, it can take so long to load the code for 
playing back a recording that Myth gives up and tells me I finished 
watching the recording and asks if I'd like to delete it.  I just say no 
and try playing a second time and it begins playing instantly.  On a 
more typical once-a-day playback (i.e. haven't watch TV for almost 24 
hours), I'll get a more reasonable delay like the ones you mentioned above.

If this is happening multiple times in a single TV-watching session, it 
may be caused by an I/O wait situation.  For example, if you delete a 
recording, and then attempt to play back another recording, you might 
get stuck waiting for the disk to finish the delete before it provides 
any new data (i.e. the next recording) for playback.  This is especially 
noticeable using a filesystem that is not known for fast deletes of 
large files, i.e. ext3.  Even with other filesystems, it may cause some 
delay.

>2) Scrolling up and down the recorded shows menu, occasionally pauses for
>a few seconds, with what sounds like harddrive thrashing about, before
>continuing as normal, this is a minor problem, but it's SOOOOOO
>annoying...
>  
>
The first time you scroll past a show, Myth creates a thumbnail image of 
that show.  (And, I think with Risto Treksler's new "preview image 
honors commflag/cutlist" stuff in recent versions, if you create a 
thumbnail by scrolling over a show before commflagging finishes, it will 
create a new thumbnail the first time you scroll over the show after it 
has been flagged.)  This is causing the delay.

You can make this less noticeable by "frequently" clicking on the 
"Recorded Programs" link in MythWeb (which also causes thumbnails to be 
created).  That way, every program that's been recorded (and 
commflagged) at the point when you click the link will have a preview 
image, so you won't see the delay.  While I wouldn't recommend setting a 
cron job to do this automatically, I typically load the page before I do 
any long-term TV watching--especially if I plan to browse through shows 
to see what sounds interesting or if I've recently recorded a lot of 
shows.  I always load this page once after returning from a trip...

You can also probably eliminate this delay by unchecking the options 
"Generate thumbnail preview images of recordings" (and, possibly, 
"Generate preview image from a bookmark if possible") in your frontend 
settings.  However, I'll admit that I've never tried this since I like 
having the thumbnails.

I've thought about trying to modify the playbackbox to skip displaying 
the preview thumbnail if the user has selected "Display live preview of 
recordings," and instead generate the thumbnail in the background while 
the preview is playing.  Similarly, modifying it to display the program 
information and only displaying the preview thumbnail once it has been 
generated would make sense.  However, in truth--with the MythWeb 
Recorded Programs workaround--this has been extremely low priority for me...

>3) When using the 30 second skip, it skips and then kind of does slow
>motion for about 1 sec or 2 and then plays properly. Is this normal? or is
>there a problem? It's annoying after using the 30sec skip on the Tivo for
>years...
>  
>
Never seen this, so if it is normal, my system isn't.  :)  It sounds 
like a problem with your sound settings.  For example, make sure you 
don't enable, "Aggressive Sound card Buffering."

HTH.

Mike


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