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RTLinux vs. RTAI - RTLinux vs RTAI...

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RTLinux versus RTAI By Ismael Ripoll (2002). License: OPL . In this document I have tried to put together the most relevant (in my opinion) characteristics of RTLinux and RTAI. I did my best, but I know there may be errors... this document do not intend to be the last word on this issue. Introduction Two approaches to real-time Linux There are two different approaches to provide real-time performance with Linux: Improving the Linux kernel preemption. 1. Adding a new software layer beneath Linux kernel with full control of interrupts and processor key features. 2. There are two working groups on each approach: TimeSys and Linux kernel preemption project (supported by MontaVista ) took the first approach, both groups are working in parallel to produce a similar result. The second approach is supported by RTLinux and RTAI. A detailed analysis of the pros and cons of the two alternatives can be found in the paper Comparing two approaches to real-time Linux by Tim Bird. How Real-Time performance is achieved in both, RTLinux and RTAI The answer to this question can be found in the RTAI programming Guide : "The real-time Linux scheduler treats the Linux operating system kernel as the idle task. Linux only executes when there are no real time tasks to run, and the real time kernel is inactive. The Linux task can never block interrupts or prevent itself from being preempted. The mechanism that makes this possible is the software emulation of interrupt control hardware." There are some intrinsic real-time features that are achieved by just executing real-time tasks in kernel space: Real-Time tasks (threads) are executed inside kernel memory space, which prevents threads to be swapped-out and also also the number of TLB misses is reduced. Threads are executed in processor supervisor mode (i.e. ring level 0 in i386 arch), have full access to the underlying hardware. Since the RTOS and the application are linked together in a "single" execution space, system Index RTLinux vs. RTAI http://www.linuxdevices.com/files/misc/ripoll-rtl-v-rtai.html 1 of 10 1/27/09 9:12 AM
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call mechanism is implemented by mean of a simple function call (not using a software interrupt which produces higher overhead). Licenses and patents RTLinux: RTLinux is released under two different licenses: Open RTLinux Patent License . This license allow you to use RTLinux without fee in two situations: By software licensed under the GPL; or 1. By software that executes within an Open RTLinux Execution Environment - whether that software is licensed under the GPL or not. 2. where " Open RTLinux Execution Environment " means: A computer hardware system where the interrupt control hardware of processors and system boards is under the direct control of unmodified Open RTLinux Software in binary form.
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