[mythtv-users] xscreensaver-command causes Prebuffering Pause

[Robert Humphrey]

I've recently installed Mythbuntu 10.10 on an Epia M10000 to make a combined
front and backend system.  I have managed to get the S-Video output working
without tearing, using quite low CPU usage and realtime priority during
playback.  However, sometimes there are glitches in the output, and
associated prebuffering pause messages in mythfrontend.log.

I've looked through the relevant page of possible causes on the wiki, but am
still not quite sure how to pinpoint the cause in my case.

Firstly, I disabled CPU frequency scaling by setting scaling_min_freq to the
maximum for my processor (997000).  This change didn't seem to notably
reduce the problem,

In a log including playback messages I've noticed that approximately half a
second after each xscreensaver-command entry, at least one prebuffering
pause would occur. eg:

2010-10-25 21:56:44.202 ScreenSaverX11Private: Calling xscreensaver-command
-deactivate
2010-10-25 21:56:44.611 NVP(0): prebuffering pause
2010-10-25 21:56:44.613 NVP(0): Waiting for prebuffer..  0 AAUAAAAAALAdLAA
2010-10-25 21:56:44.708 NVP(0): prebuffering pause
2010-10-25 21:56:44.709 NVP(0): Waiting for prebuffer..  0 AADAALAAALADUAA

This isn't the only time that prebuffering pauses occur, but it accounts for
the majority and so I would like to establish why these ones are occurring
first.

With frequency scaling disabled (as described above) and watching live TV,
top tends to show between 55% and 70% idle.  When the xscreensaver-command
occurs I don't always see idle drop to 0%, but wonder if this is due to the
refresh rate of top.  Therefore, I'm not sure if CPU bottleneck might be
causing the problem.

I don't think swap is an issue because I've 1GB RAM, and top indicates only
about 20MB of swap is used.

I do wonder if the hard drive might be a potential culprit - I'm using a 1TB
WD10EARS partitioned with a system partition (ext3), swap and data (xfs).
This is a sata drive, so I've got a sata<->IDE converter.  Is there a way
for me to check the drive performance?  Alternatively is it possible to get
MythFrontend to buffer more data from disk, so it can cope with longer
interruptions in the data?

If I've missed any important information then just let me know.

Any help would be greatly appreciated.

Thanks,
Rob
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