[mythtv-users] Commflagging X 4 = CPU ?

Simon Hobson linux at thehobsons.co.uk
Sat Oct 29 19:19:29 UTC 2011

Craig Treleaven wrote:

>Is the simple rule of thumb that each commflag job should have its
>own core?  I want to try to find a middle point between cost and
>processing power.
>What about disk i/o?  I certainly intend to put the os and MySQL
>database on one disk and the recordings on another.  If there are
>four simultaneous recordings and four simultaneous commflag jobs, is
>that too much for one 3 Gbps SATA disk?

I think CPU load is largely irrelevant - if you run more jobs than 
you have CPU for then they'll just run slower. Disk I/O is probably 
the biggest issue - it is in my experience and discussions here don't 
contradict that. It's rarely raw I/O rates, but the sustainable I/O 
rate while reading/writing multiple streams and thus doing a lot of 
performance sapping seeks.

In that respect, having sufficient memory to run commflagging during 
recording will help enormously. If you have enough memory and CPU, 
then the commflag processes can use the written data while it is 
still in the OS cache - and that avoids the need to read it from disk 
with the additional seeking that would be entailed.

For some time I ran my backend virtualised as a Xen guest on my home 
server which was a single core AMD64 3200. I now have an HP 
Microserver running a dual core AMD64 running at only 1.3G dedicated 
to being the backend. I haven't done any tests yet (I have two single 
tuner DVB-T cards for UK Freeview), but I do know it can handle far 
more than my old system.
My old system (also on a single disk) could not record two programs 
while I watched another recording. I have so far had my current 
system up to four recordings while watching another and commflagging 
one of the recordings in real-time. The OS & DB are on a mirrored 
setup across two disks. Recordings are stored on the same two disks 
(in different partitions), but not raided.
Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
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