Hi Marcel,
In my opinion a 70 us jitter is a very good number.
I have run some tests in November 2011. It was different from you, because I
measured the whole chain latency and not only the PC performance.
Indeed, I wondered how many time is required to trigger an action after reading
an event.
So my test was:
- Send a square signal on an ADC and when reproduce the signal with a DAC.
Hardware:
- PC: DELL Precision 390 (Intel(R) Core(TM)2 CPU 6700 @ 2.66GHz)
- Network card: Realtek RT8139D with native ethercat master 1.5 driver (8139too)
- Ethercat terminal from Beckhoff: EK1100 + EL3102 + EL4132
- Frequency generator + oscilloscope
- OS: Debian 6.0 i386
Results:
* with kernel 2.6.32.11 + rtai-3.8.1
- maximum latency 200us, maximum jitter 200us
* with kernel 2.6.31.6 + rt-preempt
- maximum latency 600us, maximum jitter 200us
My conclusion was:
- it does not worth using RTAI and ethercat native network driver below 1KHz.
because RT-PREEMPT+generic_driver is much easier to use and offer quite similar
performances for 1ms cycle time.
regards,
--
sebastien
On 05/30/2013 08:56 PM, Marcel Rothgänger wrote:
Hello everybody,
I have some problems with jitter on the following system:
ASUS P7P55D-E with Intel(R) Core(TM) i7 CPU 860 @ 2.80GHz
82574L Gigabit Network Connection with e1000e
RTL8111/8168B PCI Express Gigabit Ethernet controller with r8169
8 GB RAM
Which possible jitter minimum at 250µs cycle time do you expect? What should
be possible on this system?
As OS I use Ubuntu 10.04.4 and patched the Kernel with RT PREEMPT
2.6.31.6-rt19. Both NIC were tried with both native and generic driver (for
e1000e on another RT Kernel).
The application is the adapted user sample with RT PREEMPT timing. Cycle
time 250 µs (4 kHz). Abstract below. On the oscilloscope I can see very
different signal Jitter of 20µs – 70µs. The Jitter depends on system load
e.g. mouse moving, window moving etc. The ambition is all OS services like
screensaver, window moving and so on should surge but not my cyclic task.
I am interested in your system hardware configuration, the OS, cycle time
and of course your jitter. I think it can help to improve our system.
Regards,
Marcel
Maybe the idea of timing and cyclic task below is not the best:
printf("Started.\n");
clock_gettime(CLOCK_MONOTONIC ,&t);
/* start after one second */
t.tv_sec++;
while(1) {
/* wait until next shot */
clock_nanosleep(CLOCK_MONOTONIC, TIMER_ABSTIME, &t, NULL);
/* do the stuff */
cyclic_task();
/* calculate next shot */
t.tv_nsec += interval;
while (t.tv_nsec >= NSEC_PER_SEC) {
t.tv_nsec -= NSEC_PER_SEC;
t.tv_sec++;
}
}
void cyclic_task()
{
int i;
// receive process data
ecrt_master_receive(master);
ecrt_domain_process(domain1);
blink = !blink;
if (counter) {
counter--;
} else { // do this at 1 Hz
counter = FREQUENCY;
check_master_state();
}
// write process data
EC_WRITE_U8(domain1_pd + off_dig_out, blink ? 0x06 : 0x09);
// send process data
ecrt_domain_queue(domain1);
ecrt_master_send(master);
}
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