Hi Kent, and the list in general,
On 11/02/2012 04:53 PM, Kent A. Reed wrote:
> RTAI - 1ms servo thread/25us base thread - max jitter 4401ns/2378ns
> after 3 hours
>
> Xenomai/kernel - 1ms servo thread/25us base thread - max jitter appeared
> to be running roughly 10000ns/10000ns after 45 minutes *but* then while
> I was doing something random the max jitter jumped to 89590ns/367129ns
> (at which point /proc/xenomai/stat said wait errors=8. last overrun=14,
> total overruns=76). Later, I reran the test unattended for an hour and
> again got roughly 10000ns/10000ns max jitter. Which is good but the
> burst is worrisome.
>
> The strange behavior is that as I increase the base period in 25us steps
> I see both base and servo jitters first increase for several steps then
> fall back somewhat to what seems to be a steady result at longish base
> periods.
>
> Here's what I get for a series of two-minute tests, always with a 1ms
> servo period.
>
> base period---max jitter (servo/base)
> 25us---5672ns/9734ns
> 50us---22037ns/32170ns
> 75us---24328ns/34338ns
> 100us---18712ns/30256ns
> 125us---17051ns/19064ns
> 150us---16775ns/19168ns
> 175us---14389ns/19667ms
> 200us---18403ns/21412ns
>
> I believe the last three or four sets are essentially equivalent.
>
> Does anyone understand why I'm getting this kind of behavior? It seems
> to be repeatable. Is it an artifact of latency-test or is it real? Were
> this a physical system, I'd wave my hands about being the system being
> in saturation at the shortest periods, but here...?
>
> I conjectured some time ago that the ASUS bios is perhaps causing a
> problem with both this board and the Atom board I'll be testing in a
> bit, but I need more data:-)
I know about the 'latency-test' app, which pops up a gui window. Is
there a command-line/textual interface for the app which would be
suitable for input into a log file with time stamps? (Do I need to hack
one up myself? ;)
It could be informative to run a latency test overnight that logged max
jitter into a log file, say, every 10 seconds (resetting the counter
every period, of course). Then spikes like Kent is seeing could be
correlated to system logs, performance monitoring systems, and other
available history of system activity.
John
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