both better and worse interrupt latency with the "realtime" patces

[Bo Brantén]

Hello,

I have measured the interrupt latency on a few different kernels.

The setup is a function generator connected to ACK on the printerport and 
an line added to the interrupt handler that toggles an data pin on it. 
Both signals is viewed on an oscilloscope with storage. (The computer was 
an old Athlon 800MHz but the results should be considered relative to each 
other)

Linux 2.4.27:
    most often 15us.
    often 10us or 15us
    time distribution decreases with time, didn't see any longer
    than 250us

Linux 2.6.9:
    most often 15us.
    else almost always 10us to 30us
    didn't see any longer than 75us
This is an improvement over the 2.4.x kernel

realtime-preempt-2.6.10-rc1-mm2-V0.7.1 by Ingo Molnar:
    often up to 50us
    every few seconds one interrupt takes 400us to 600us

MontaVistas 4 patces from 08 Oct againts Linux-2.6.9-rc3:
    most often 10us, 15us or 25us
    almost always below 30us
    but every few seconds an interrupt takes 600us.
Except for the sporadic long latency this patch would be the best, it 
collects almost all interrupts below 30us.

Bo Branten

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