[LinuxPPS] So far, so good

[Miroslav Lichvar]

On Mon, Aug 09, 2010 at 03:11:59PM -0700, Hal V. Engel wrote:
> > Isn't that a change in frequency caused by increased temperature
> > rather than increased latency?

> Both probably come into play.  If it were mostly temperature it would take a 
> little while for the oscillator to be affected but I find that my system will 
> start showing increased offsets with in a few seconds of the change in system 
> load and then stabilize in a few minutes.  If the primary issue was 
> temperature related then I would expect it to take several minutes for the 
> temperature of the oscillator to change enough to cause this but since it is 
> almost instantaneous I think it is mostly caused by the increased latency of 
> the interrupt handler.   Also if I do a single threaded build on my dual core 
> system I don't see the quick change in the offset that I do with a multi-
> threaded build.  Of course the single threaded build generates less heat but 
> it also will have much less impact on the latency of the interrupt handler 
> since one core is sitting idle and can service the interrupt very quickly.

Interesting. What sign has the offset you see? If it was an increase
in the latency I'd expect it to be in the opposite direction than the
change caused by increased temperature.

> Of course we can do little more than speculate on this since we don't have any 
> way to measure the interrupt handler latency.  About the only way to test this 
> would be to use a machine with a temperature controlled oscillator as a test 
> bed.  If loading such a machine causes significant changes to the offset then 
> that affect can probably be attributed to latency rather than temperatures.  
> But I don't have a machine like this to test with.

It should be also possible to get an upper bound by flipping a bit on
parallel port in the interrupt handler and measuring the delay on a
scope.

-- 
Miroslav Lichvar