[mythtv-users] Non-continuous picture when watching TV in MythTV

Unit3 unit3 at demoni.ca
Mon Mar 3 20:50:59 UTC 2003

Ray Olszewski wrote:

> But turning dma on for IDE/UDMA hard disks will change this, won't it? 
> It certainly caused major improvements in my capture capabilities.

DMA is better, it reduces the CPU overhead of disk transfers, but it's 
still not as efficient as SCSI.

However, these days it's getting to the point where IDE is so much 
cheaper than SCSI that it's hard for buying SCSI to be worth it... you 
can get two of the incredibly fast Western Digital IDE drives (8mb of 
cache! woohoo!), use RAID 0 on them, and beat the performance of a lot 
of SCSI systems.

Or, you can get a dual processor box, and then only one CPU is tied up 
with disk related nonsense, leaving the other to do all your encoding 
and whatnot. This is what I'm saving up for at home, assuming I can find 
a cheap dual socket-A board.

> The other odd thing I'm seeing here in tests of various Linux 
> video-recording apps is that raw CPU speed matters far more than CPU 
> type. I'd expected, for example, that a 1 GHz P-III and a 1.7 GHz 
> Celeron would perform about the same, but in fact the Celeron captures 
> and encodes (in tests that use vcr, libavifile, and the win32 codecs - 
> 320x240 NTSC, about 500 MB/hour) at about half the CPU load of the 
> P-III. I can even get, with this software, acceptable recordings from 
> a Celeron 533 (though it runs at about 85% CPU load). 

This is probably due to the fact that the Celeron is the exact same chip 
as the P3 (AFAIK), just with no L2 cache. When you're simply pushing 
through megabytes of data, the cache can't help you all that much, so 
the clock speed makes the most difference.

> Perhaps there are some least-common-denominator issues with the 
> pre-compiled apps (in Debian) that cause them not to take proper 
> advantage of the better CPUs? How do the pre-compiled .debs for MythTV 
> fare in this regard?

I'd assume they're just compiled for 386. However, if you're using 
Debian, I'd suggest playing with a tool called apt-build. It lets you 
effortlessly recompile packages for your specific architecture, and then 
it treats your locally compiled package as just another apt source, 
which makes upgrading to them a cinch. I've noticed big performance 
gains after using it to recompile and reinstall apache and postgresql.


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