[mythtv-users] Commercial Detection during recording

[Michael T. Dean]

On 03/11/2013 01:18 PM, Simon Hobson wrote:
> Robert Dege wrote:
>> Does enabling "Auto-commercial-detection jobs when recording starts" add a significant amount of overhead to the system?
>> Right now, I can simultaneously record three shows (via HD Prime) onto two physical hard drives, while I'm watching a previously recorded show without issue. I'm just wondering if the additional commercial detection (3 max) would be too much for the system.
> You'd have to try it with your hardware, your signal sources, and your recording profile - no-one else will have exactly the same setup as you.
>
> Commflagging during record will actually result in less I/O than doing it afterwards. As long as the process can keep up (more or less) then the data it's working with will already be in memory as it will have been cached when the record process wrote it to disk.
> So, for example, if you currently commflag after the recording has finished, you'll be doing a lot of I/O (reads mostly) while you are recording the next program. Switch to commflagging while recording and you may find I/O goes down and you can handle more concurrent tasks.

Far more importantly, on any properly-chosen system (i.e. not severely 
underpowered) it results in significantly less I/O because you're 
constrained by wall clock time, not CPU time.  I.e. you'll be spreading 
the detection of commercials for a 1hr show over 60 minutes of work.

That said, if you have more concurrent recordings than running 
commercial detection jobs, you'll lose all benefits of enabling 
"Auto-commercial-detection jobs when recording starts" after the first 
recording ends (when MythTV will begin detecting commercials in 
already-finished recordings and no longer be constrained by the wall 
clock).  Regardless, make sure the number of jobs and the CPU usage 
settings are correct.  See 
http://www.gossamer-threads.com/lists/mythtv/users/499517#499517 .

So, it all comes down to "test it out and if it hurts, don't do it."

Mike