[mythtv-users] Playback problem -- random short pauses

[Steven Adeff]

On Mon, May 23, 2011 at 3:06 PM, Daniel Kristjansson
<danielk at cuymedia.net> wrote:
> On Mon, 2011-05-23 at 10:51 -0400, Steven Adeff wrote:
>> On Mon, May 23, 2011 at 9:41 AM, Mark Lord <mythtv at rtr.ca> wrote:
>> > What I would look for in the code, are sync(), fsync(),
>> > and fdatasync() calls.  Those generally force writes to happen
>> > and pause a lot of stuff to wait for them to complete in real-time.
>> >
>> > Which can lead to tiny pauses and glitches elsewhere.
>> >
>> > So just as a trial, perhaps grep through the code looking for those,
>> > and temporarily remove them, to see if the problem then goes away.
>> > That should prove/disprove this theory.
>>
>> my concern with this is other detrimental effects that just removing
>> those calls would cause? I just don't know enough about programming to
>> make this change in a way that would be useful for troubleshooting.
>>
>
> Removing the fsync() call can lead to pauses, in the unlikely event that
> it is being called on a Linux system. The fdatasync() and range syncs
> are mostly no-ops since we're appending to a file, so removing those
> will have no effect. The main reason the fsync() was added was to
> prevent pauses on when the disk I/O scheduler decided to finally write
> hundreds of megabytes of data to disk in one long write and consequently
> starve all readers and so stop video playback. This was accidentally
> subverted when the fdatasync() code was added. But that change happened
> over five years ago so if you are seeing a regression with 0.24, fsync()
> changes are not to blame.

That's what I figured.

It sounds like, from what Mike says, the issue has to do with VDPAU
decoding and the interaction with ffmpeg (Mike, am I understanding
that correctly?). So I don't think chasing i/o is the way to attack
this. It sounds like Mike and the other dev's looking at the issue
have an understanding of what's causing VDPAU to do this. Hopefully a
solution can be found that still lets users take advantage of using
VDPAU with ATOM-like systems.


-- 
Steve
http://www.mythtv.org/wiki/User:Steveadeff
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