[mythtv-users] mythbackend in a KVM VM (thread started out as "in docker container")

[Paul Gardiner]

Thanks for the continued help.

On 23/05/2024 02:29, Stephen Worthington wrote:
> On Wed, 22 May 2024 17:05:16 +0100, you wrote:
> 
>> Yes, it wasn't minisatip I was suspecting as the core of the problem. I
>> was thinking that the tuner drivers may be failing to react to
>> interrupts fast enough because of the virtualisation overheads. When you
>> were running minisatip on that old system, was it running under a VM or
>> was that old system acting as a host for other software running in VMs
>> symultaneously with running minisatip?
> 
> My main MythTV box does not normally also run VMs.  I do occasionally
> run a VirtualBox VM on it, with a basic Windows 10 inside, to allow me
> to run the software to configure my Corsair RM650i power supply via a
> USB connection.  I have run that VM at times when the box is busy in
> the middle of the evening (running multiple recordings), and I believe
> I would have noticed if there was a problem with recordings done then.

But did you do that with that back when you were running a much older 
system?

> The idea that the data loss is coming from slow response to interrupts
> in the VM would only apply when running the tuners in the VM.  It
> would not affect using SAT>IP tuners where the drivers are running in
> the normal system environment.

I wondered if there might be overheads just because the system was 
hosting VMs that might sometimes add a little extra latency to the 
handling of interrupts. There would be in a single CPU system because on 
occasion the CPU would have to change context to service the interrupt, 
but you likely know more how it all works than I do.

>> Strangely the drop out seems largely independent of number of
>> simultaneous recordings. Most of my initial tests were with live TV,
>> with nothing else recording. More recently, I've tried three concurrent
>> recordings, and the drop out seemed about the same. I'm using just one
>> hard drive for recordings. It's the one I've been using for many years -
>> moving it between servers each time I perform an upgrade.
> 
> Do you normally use LiveTV at all?  I only use it for testing, myself.
> It has a long history of problems, mainly because it is rarely used in
> real life.

I record almost everything I watch, but there is one thing I watch part 
of live about 5 days a work. LiveTV seems to work well for me.

> It is better to use normal recordings to test with.  A while ago I
> wrote a little Python program that allows me to create a manual
> recording from the command line.  It is very useful for setting up
> test recordings in a situation like this.
> 
> http://www.jsw.gen.nz/mythtv/start-recording.py
> 
> I normally use it in a script to set up multiple 10 minute recordings,
> some starting at the same time, and some a minute or two later.
> 
> I have the "-v record" option set on my mythbackend command line:
> 
> root at mypvr:/etc/mythtv# cat additional.args
> ADDITIONAL_ARGS=-v record,dvbcam
> 
> which logs a bit more data about each recording.  Then I also have a
> script that checks the mythbackend.log file for the quality report
> that get generated at the end of each recording (which may need -v
> record).  It does that with this command:
> 
> grep -a "overall_score=\"0" /var/log/mythtv/mythbackend.log
> 
> That tells me if there are any recordings where mythbackend knows
> there were problems, even if the level of problem was below the limit
> that would cause it to automatically re-record.  However, all LiveTV
> recordings will show up in that test, as they normally do not start
> and stop at the correct times for shows in the EPG data, so they are
> deemed to be missing heaps of data when it comes time to calculate
> their quality score.  There can still be small data losses or
> corruption that mythbackend does not know about and which do not show
> up in the log, but they are normally real tuner problems (eg rain fade
> or a cable going bad), rather than losses in the recording process.
> 
> One thing that just occurred to me - how are you moving the aerial
> connections to the tuners when they are in the problem system?  Are
> the two PCs near each other so you can just use the same cables?  Or
> do you have to re-route the signals over different cables?  Could
> there be a problem with the cables?

No, not that sadly. I've been swapping the locations of the machines for 
testing.