[mythtv-users] Latency timings...

Brian Wood beww at beww.org
Fri Apr 14 23:41:45 UTC 2006

On Apr 14, 2006, at 5:22 PM, Steven Adeff wrote:

> On 4/14/06, Brian Wood <beww at beww.org> wrote:
>> On Apr 14, 2006, at 2:47 PM, Steven Adeff wrote:
>>> Couple questions....
>>> 1)
>>> Is there a reason setpci wouldn't work? I run:
>>> setpci -v -s 00:0a.0 latency_timer=80
>>> or
>>> setpci -v -s 00:09.0 latency_timer=80
>>> for my two SATA devices  and it shows:
>>> 00:09.0:0d 80
>>> but when I check with lspci -v they're both still set to zero?
>> Some chipsets seem to report a latency of zero no matter what you do.
>> In those cases you can adjust the values for things capture cards and
>> video cards but can't adjust the disk systems. Playing with latency
>> values is something that sometimes helps some people but is not a
>> panacea.
>>> 2)
>>> The wiki page doesn't give much help in the advice area as to  
>>> latency
>>> values or how to "calculate" them for Myth use (ie DVB cards, etc),
>>> except to say that your harddrives should have a higher value than
>>> your PVR card. Anyone have any advice in this area?
>> It's pretty much a trial and error thing at this point. The mobo
>> manufacturers don't seem to give out much info, they just assume that
>> you will be running Windows and be happy.
>> The "optimum" values are so subject to so many variables that any
>> attempt to give advice is doomed failure, you just want to understand
>> the general principles well enough to make somewhat intelligent
>> adjustments, but the exact correct value will depend on what type and
>> how many video streams are being delivered, what sort of storage
>> system they are being delivered to, the buffer capacity of everything
>> involved in the system, what other devices are competing for time on
>> the PCI bus, what interrupts might be shared and many more that I
>> have not even thought of I'm sure.
>> The purpose of the WiKi entry was basically to let people know what
>> the parameter is, the fact that it can affect performance of a Myth
>> system and what that error you sometimes get from the IVTV drivers in
>> your boot log might mean.
> gotcha. thanks for clearing this up. I'll let it be and be happy
> everything works fine =)
> (damnit steve, stop trying to optimize!)

Yeah, if it works leave it alone, it is very wrong you will know it.

I started looking into this matter when my AMD64 3700+ system with a  
500GB SATA RAID-0 array could not record one SD program while  
watching another. Adjusting the latency values allowed me to record 3  
programs and watch a fourth, with a couple of commflag jobs running  
as well, a significant difference for tweaking two numbers.

 From what I've been able to learn, it seems the VIA chipsets are  
moreprone to these problems than others, but that is from a limited  
data set.

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