On Sat, 30 Oct 2004 15:15:07 +0200
Ingo Molnar <mingo@elte.hu> wrote:

> should have mentioned that in the user-triggered modus you have to do
> the latency measurement in userspace. It is only the trace that is
> correct, for the time being. This trace shows what i'd expect a jackd
> wakeup to look like normally: 13 usecs.
> 
> 	Ingo
> 

Hi,

i thought, why not try to get jack out of the equation first and use a
different irq source. So i wrote a small test app for /dev/rtc to see if i
see the same erratic behaviour [.cc source file atached. find a small
tarball here: http://affenbande.org/~tapas/wakeup.tgz (it has a simple
makefile, plus a script which does the chrt thing to make it SCHED_FIFO)].

This program takes two parameters: the desired freq of the rtc and the
number of irq's to be measured.

Then it polls on the fd of /dev/rtc and takes a cpu cycle count timestamp as
soon as poll returns. At the end of the program i tried to gather some
[useful ???] statistics of the data: 

- the mean difference in cycles between two wakeups

- the max difference in cycles between two wakeups (and how much this
differs from the  mean difference)

- the min difference in cycles between two wakeups (and how much this
differs from the  mean difference)

- the mean difference from the mean difference :)

And alas, wiggling windows screws up the timing on V0.5.6 for this, too.

dunno, if this is any useful to you, but i felt the urge to try it :)

	flo


P.S.: don't forget to make your rtc irq SCHED_FIFO with a high priority, too

here's a sample output with window wiggling in X (rt_wakeup runs chrt -f 90
wakeup, so make rtc at least prio 91):

~/source/my_projects/wakeup$ ./rt_wakeup 1024 6000
freq: 1024 #: 6000
setting up /dev/rtc...
locking memory...
turning irq on, beginning measurement (might take a while)...
...measurement done

mean cycle difference betweem two wakeups: 1.17845e+06 cycles

min. cycle difference betweem two wakeups: 185992 cycles (#: 1563) 
 diff from mean diff: 992461

max. cycle difference betweem two wakeups: 9.73166e+06 cycles (#: 1546) 
 diff from mean diff: 8.5532e+06
                      ^this is bad i suppose :)

mean difference from mean difference: 25279.2 cycles


here's one on a rather idle system:

~/source/my_projects/wakeup$ ./rt_wakeup 1024 6000
freq: 1024 #: 6000
setting up /dev/rtc...
locking memory...
turning irq on, beginning measurement (might take a while)...
...measurement done

mean cycle difference betweem two wakeups: 1.16697e+06 cycles

min. cycle difference betweem two wakeups: 1.1486e+06 cycles (#: 5492) 
 diff from mean diff: 18379.2

max. cycle difference betweem two wakeups: 1.18439e+06 cycles (#: 5491) 
 diff from mean diff: 17417.8

mean difference from mean difference: 1144.82 cycles
~/source/my_projects/wakeup$