[etherlab-dev] Possible Realtime Issues with Ethercat Master and RT Preempt Kernel

[Armin Steinhoff]

Hi Dr. Schöpfer,

please have a look to the test rack at the OASDL homepage and you will 
see that the same release of PREEMPT_RT is performing very different on 
different motherboards.

IMHO ... the most important issue is the SMI interrupt and the triggered 
execution of the low level firmware of the motherboard.
Also a transaction on the PCI bus using DMA transfers can increase the 
response time on a single core ...

Best Regards

Armin

http://www.steinhoff-automation.com


Dr.-Ing. Matthias Schöpfer schrieb:
> Hi Thomas Winding,
>
> thanks for your answer!
>
> We are running kernel 3.12.31-rt45, since the newer kernel versions do
> not seem to run as reliable regarding real time at least on our hardware.
>
> ethercat is from the mecurical repository and we use the e1000e driver.
>
> Most of our problems we might have solved, since we had an issue with
> our cycle time / calculations where sometimes, our cycle lasted 600-800
> microseconds, which seems to be way to long. We are now down to < 220
> microseconds.
>
> In rare cases, when we start the software, we still constantly running
> in the mentioned issues. When we stop and restart, everything is fine. I
> wonder, if we sometimes hit a misfortuned timing. As of lately, I was
> not able to reproduce it.
>
> Regarding your suspicion: I wonder, if it is better to sync the
> transmits to the 1 ms cycle instead of the receives?!
>
> Regards,
>
> 	Matthias Schöpfer
>
> On 02/01/2016 09:07 AM, Thomas Winding wrote:
>> Hi Matthias Schöpfer
>>
>> I have seen the same problem. I am also running at 1 ms and using clock_nanosleep.
>>
>>   I suspect that the problem arise when you send a new telegram before you have received the previously send telegram.
>>
>> Which version of the ethercat are you using?
>> Which version of kernel are you using?
>>
>> Best regard,
>>
>> Thomas Winding
>>
>> -----Original Message-----
>> From: etherlab-dev [mailto:etherlab-dev-bounces at etherlab.org] On Behalf Of Dr.-Ing. Matthias Schöpfer
>> Sent: 26. januar 2016 14:22
>> To: etherlab-dev at etherlab.org
>> Subject: [etherlab-dev] Possible Realtime Issues with Ethercat Master and RT Preempt Kernel
>>
>> Hi!
>>
>> We started using etherlab/ethercat and are quite impressed. Nice work!
>>
>> We are running Linux with a RT_PREEMPT Kernel and e1000e ethercat driver. We have to run at a cycle time of 1ms and we have jitter from clock_nanosleep of about 15 microsecs max.
>>
>> Nevertheless, we suffer from time to time from these:
>>
>> EtherCAT WARNING 0: 2 datagrams UNMATCHED!
>> EtherCAT 0: Domain 0: Working counter changed to 9/9.
>> EtherCAT 0: Domain 0: Working counter changed to 0/9.
>>
>> Especially, when we apply load to the system. From previous projects, I experienced these effects when IRQ/Kernel Thread was not set to appropriate RT Level.
>>
>> My Question: has anybody experienced similar problems, and would it be worth to investigate it. And if I decide to patch the kernel module, where is a good starting point.
>>
>> Thanks and regards,
>>
>> 	Matthias Schöpfer
>>
>> --
>> Dr. Matthias Schöpfer
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