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

From: Ingo Molnar
Date: Wed Jun 22 2005 - 14:07:16 EST

* Paul E. McKenney <paulmck@xxxxxxxxxx> 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.

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