Re: PREEMPT_RT vs I-PIPE: the numbers, part 2

[Ingo Molnar <mingo@elte.hu>]

* Paul E. McKenney <paulmck@us.ibm.com> wrote:

> Any way of getting the logger's latency separately?  Or the target's?

with lpptest (PREEMPT_RT's built-in parallel-port latency driver) that's 
possible, as it polls the target with interrupts disabled, eliminating 
much of the logger-side latencies. The main effect is that it's now only 
a single worst-case latency that is measured, instead of having to have 
two worst-cases meet.

Here's a rough calculation to show what the stakes are: if there's a 
1:100000 chance to trigger a worst-case irq handling latency, and you 
have 600000 samples, then with lpptest you'll see an average of 6 events 
during the measurement. With lrtfb (the one Karim used) the chance to 
see both of these worst-case latencies on both sides of the measurement 
is 1:10000000000, and you'd see 0.00006 of them during the measurement.  
I.e. the chances of seeing the true max latency are pretty slim.

So if you want to reliably measure worst-case latencies in your expected 
lifetime, you truly never want to serially couple the probabilities of 
worst-case latencies on the target and the logger side.

	Ingo
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