[time-nuts] DPC Latency

[David J Taylor]

> Hi
>
> DPC Latency = Deferred Procedure Call Latency = how long it takes 
> Windows to get around to doing what it should do real time. Put another 
> way - how lazy is your computer?
> It may not be a big issue, but it's probably worth checking for free:
> http://www.thesycon.de/deu/latency_check.shtml
> We don't do audio or video since we're serious Time Nuts:)... The same 
> issue would mess up our stuff though. It certainly would be useful to 
> know when playing with NTP.
> Usual disclaimer - if your PC explodes don't blame me. There are lots of 
> other programs out there that do the same thing. I have no connection 
> with these guys. It's just the first one I found.
>
> Bob

Bob,

Yes, that's a program which produces some interesting results, and has 
been useful for detecting bad behaviour by some poorly written drivers 
(e.g. when receiving masses of data over a satellite DVB link).

For NTP, I think you may find that all the time-critical stuff is done at 
high-priority, and it's only the non-time critical stuff which is done at 
DPC time.  This is how the DPC is intended to be used, of course.  VMS has 
a similar mechanism, ASTs, if I recall correctly, but that was a long time 
ago.

On Windows with NTP, you can get within a couple of hundred microseconds 
(XP) or perhaps 50 microseconds (Windows-7) if my graphs here are any 
guide:

  http://www.satsignal.eu/mrtg/performance_ntp.php

The best I can offer is within 10 microseconds or so on an Intel Atom 
system running FreeBSD.  I'm sure that others can do better!

Cheers,
David
-- 
SatSignal software - quality software written to your requirements
Web:  http://www.satsignal.eu
Email:  david-taylor at blueyonder.co.uk