Hi all,
I have and stange behavior that I fail to understand:
For performance measurement purpose, I'm using the 'old' parrallel port interface to generate IRQs on a debian kernel 3.2.0-4-amd64 (I am using an externel signal generator connected to tha ACK pin).

I wrote my own kernel module (top half only) to handle the interrupt and send an external signal back to the parrallel port and display both signals on a oscilloscope so I can measure the kernel response time.
Everything works as expected and I can see an average 70 µs of time response with some 'burst' of 20µs . I'm running on a "Intel(R) Core(TM) i3-3240 CPU @ 3.40GHz".

Now, the "unexplained" part.
If I load the CPU, memory and I/O using the "stress" program, I expected the average time to be worst , but the opposit happens: my average response time drops to 20µs.
I tried on 3 differents kernel:
vanilla, PREEMT-RT anf vanilla with NO_HZ option set to false.
Can someone explain the magic of this ?

Are you by any chance using the ondemand governor for powersaving? If so, try to set that to performance (CPU always runs with maximum clockspeed) and see if that fixes that behavior.

1 members found this post helpful.

It's a Bingo!
I wasn't aware of this "governor" thing.
I can see the CPU frequency changing from 1.6Ghz of 3.4 Ghz with "ondemand" governor.
Thanks for your fast answer.

well I concluded a little bit to fast.
The governor tweak is not the answer. My system has still better performance when under stress.
Any other explaination ?

(is it possible to unmark the 'solved' attribute of my question ?)