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openPOWERLINK in Linux Userspace: Implementation and Performance Evaluation of the Real-Time Ethernet Protocol Stack in Linux Userspace

Wolfgang Wallner, Wolfgang Seiss, Josef Baumgartner, Bernecker + Rainer, Austria

The RT-Preempt patch for Linux turns the Linux operating system into a real-time operating system and therefore is an ideal platform to implement a real-time Ethernet protocol stack like openPOWERLINK. The initial implementation of the openPOWERLINK stack on X86 Linux was developed as a kernel module. The solution completely bypasses the Linux network stack and achieves maximum performance through the usage of its own network interface drivers. However, this limits the protocol stack to the few openPOWERLINK network interface drivers currently available and also makes the protocol very dependent on the used kernel version. To circumvent these drawbacks, the whole protocol stack was implemented as Linux userspace library. As most of the necessary real-time features are also available in userspace and many applications do not need the performance level of the kernelspace implementation, this solution is adequate for a lot of applications.

This paper describes the porting of the openPOWERLINK stack to userspace and examines the performance of the userspace implementation. Therefore, the influence of the userspace implementation on the network jitter and on the generated system load is analyzed and compared with the kernelspace implementation. Due to the long term goal to integrate the lower level layers of the openPOWERLINK stack into the mainline Linux kernel, in this paper it is furthermore discussed how the protocol stack could be segmented into a kernel part that would be integrated into the Linux kernel and a user part that is provided as a userspace library.