[mythtv] Maybe it *is* interrupt latency

[John Patrick Poet]

On Mon, 10 Jan 2005, Kyle Rose wrote:

> > I saw this with an early version of the HD3000 driver.  Have not
> > seen it since v1.3, however.
>
> On HD-3000 or 2000?  I haven't tried the 3000 card yet: it may react
> better.  That's something to try.


My backend:

 DFI PRO875B motherboard
 3.0 GHz HT P4 cpu
 1GB RAM
 Three HD-3000 cards
 Fedora Core 2 with stock 2.6.7 kernel (custom configured)
 Myth CVS as of Jan 8th.

I can record three shows simultaneously, while watching a fourth, with
absolutely no data loss or coruption.

>
> > Does it happen with ALL of your stations?
>
> Well, it happens at least with ABC, FOX and PBS-HD.  Haven't tried the
> others, but ABC and PBS-HD have very high signal strength (>90%) and
> FOX has sufficient signal (>75%).


That should be fine.


> > What kernel are you running?  I have custom configured 2.6.7 and
> > 2.6.9 kernels.  The 2.6.7 kernel works better.  That may be because
> > I have "pci=noapic" for the 2.6.7 kernel, and not for the 2.6.9
> > kernel, however.
>
> I am using 2.6.9 with noapic nolapic (seems this behavior isn't quite
> as bad with APIC support off) and with acpi on (some things, notably
> on-board USB, seem not to work with acpi off).

2.6.9 does work for me, so that is probably not the problem.


> > Jack (at pcHDTV.com) suggested turning *off* preemptive scheduling,
> > which I have done in both.
>
> I had CONFIG_PREEMPT off initially; didn't notice much difference when
> I turned it on, so maybe I'll shut it off again.
>
> Given how intermittently this happens (once every few seconds) I think
> I'm right on the cusp of being able to handle it with the current
> system.  Would suck to have to get a faster processor just to mitigate
> high interrupt latencies when an 1800+ is overkill for the work this
> machine needs to do 99% of the time.

If you think it is a PCI latency issue, search the Myth users archive for
subject "PCI/AGP Latency".

http://www-106.ibm.com/developerworks/linux/library/l-hw2.html


John