[mythtv-users] Realtime commflagging....was: How many simultaneous recordings on one drive?

Simon Hobson linux at thehobsons.co.uk
Tue Oct 11 07:08:30 UTC 2011


David Whyte wrote:

>I record a show (formula 1) in HD, and it is set to real-time
>commflag.  When I watch the show at the same as recording it (and
>hence comm-flagging it) I get a bit of stutter now and then.  If I
>look at the load on the server at this time, it has spiked quite a bit
>up to about 5-6 as opposed to 1-2.
>
>Is this extra load being caused by slow access to the disk or the CPU
>not being able to keep up?  I note that the commflag is running at a
>nice value of 17.  Can you think of anything I can do to enable me to
>record, reali-time commflag and watch an HD show and limit the
>stuttering?

How much RAM do you have ? How much space does the recording take (GB/Hr) ?

 From observation, commflag seems to spend a lot of it's time 
searching through the video for certain types of frame.event. I 
suspect it then goes through again doing something else.
With HD and insufficient RAM, this means it will need to re-read 
what's been flushed from RAM to make room for other stuff.

Your stuttering is almost certainly due to I/O limitations. With 
commflag niced, it's going to get paused whenever anything else is 
working, but that doesn't actually do a lot for I/O conflicts. One 
key parameter to watch is how much your system is doing on WIO 
(waiting for I/O) - I suspect that you'll find this goes 
significantly non-zero when your pauses are happening.

-- 
Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
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