[etherlab-users] Jitter when trying to use EtherLab Master at hard real-time 1 kHz under RT-PREEMPT

[Florian Pose]

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Hello,

Am 01.03.2012 20:59, schrieb Soo-Hyun Yoo:
> My tenuous understanding of this is that since the EtherCAT
> character device tries to use the kernel's ioctl calls, the
> kernel's ioctl "interface" (?) needs to be patched. Since
> RT_PREEMPT is the only patch that does so, while LXRT provides its
> own interface, the former is the only one capable of real-time user
> space operation while the latter is limited to kernel space. Based
> on this assumption, I am hoping that the EtherLab Master is capable
> of real-time operation in user space under RT_PREEMPT.

it definitely is: The user-space library is designed for it. We have
serveral applications running with jitter < 10 us.

> What can I do to lessen this jitter? Does anyone have a working
> EtherCAT master running at 1 kHz with jitter under 50 us?

Did you follow the instructions from the RT-PREEMPT-HOWTO?
https://rt.wiki.kernel.org/articles/r/t/_/RT_PREEMPT_HOWTO_6bc9.html

I. e. setting priority, avoiding page-faults, locking memory, etc.

Sure, the user-space library makes use of ioctl()s, but since these
are handled in the context of your (priorized) process, they cause no
measurable latency.

BTW, we have made bad experiences with packaged kernels, please try
the latest stable (3.0-rt) and compile it by yourself.

- -- 
Viele Grüße,
Florian
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