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A comparison Of Hard Real-Time Linux Alternatives

timothy posted more than 9 years ago | from the close-to-stock dept.

Linux 12

An anonymous reader writes "This study compares the real-time capabilities of various Linux kernels. It was part of a project to upgrade the control software in water-wave generators at research institutions around the world. The results of the study were used by Akamina for the selection of a new RTOS for the control system upgrade of Canada's largest hydraulics and coastal engineering laboratory, the National Research Council Canadian Hydraulics Centre in Ottawa."

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Erster Beitrag (-1, Offtopic)

Anonymous Coward | more than 9 years ago | (#10909307)

oT

LKML discussing RTOS as well. (2, Informative)

Curtman (556920) | more than 9 years ago | (#10909318)

Kernel Traffic has a pretty lengthy summary [kerneltraffic.org] of some discussion on the Linux Kernel Mailing List about the state of Real Time capability in the kernel as well that I found pretty interesting.

Summary: RTAI wins... (3, Interesting)

quamaretto (666270) | more than 9 years ago | (#10909331)

It would have been nice to see how all of these stack up to QNX and other real-time systems.

Meanwhile, I'll keep this article in mind for if I can ever get a better job position than "ASP.net slave".

Re:Summary: RTAI wins... (1)

bhima (46039) | more than 9 years ago | (#10909727)

yeah... I took "Alternatives" to mean alternatives to Linux before I RTFA. Still it's a good read and worthwhile to me, despite the fact that only about 5% of my job is real time dependant.

A full blown RTOS can be overkill (4, Informative)

geirt (55254) | more than 9 years ago | (#10909385)

A full blown RTOS is overkill for many RT applications.

Many RT tasks (like the one used in this article) can be described as:

Wait for IRQ. Do something *NOW*. Wait for IRQ

These tasks can be supported by the rtirq-patch [t-online.de] . rt-irq is a very small patch that allows just that (and nothing more). It would be nice to add rtirq to the comparison.

Not good result is it? (3, Informative)

raffe (28595) | more than 9 years ago | (#10909452)

Conclusions

Based on the latency measurements made:

1. Of the options considered, only Linux 2.4 with RTAI meets the latency requirements for a real-time 100-Hz control system
2. Only Linux 2.4 with RTAI provides what could be considered deterministic interrupt response times and task switch times
3. Linux 2.6 is the next best option for real-time control
4. The results for Linux 2.4 with LXRT indicate that LXRT can not be used for hard real-time systems
5. Linux 2.4 can not be used for hard real-time systems

Re:Not good result is it? (1)

amorsen (7485) | more than 9 years ago | (#10911072)

Linux is a general purpose OS. It would be surprising if it did work for hard real-time out of the box. That said, the Linux kernel really does suck more than necessary when it comes to soft real-time. The low-latency patches for 2.4 help, but it is only recently that Ingo Molnar started doing the hard work for 2.6. He is going way further than the 2.4 low-latency patches ever did. If he manages to get his work into a shape fit for inclusion into the kernel, Linux will be very impressive for soft real-time. On the other hand, his patches also change pretty fundamental things like locking rules and there are many many ways they could end up being detrimental to overall performance or simply increase the complexity of the kernel too much. Ingo obviously believes that the problems can be solved and that his design is fundamentally better in the end. Let us all hope that he proves that he is right.

Re:Not good result is it? (1)

hethopus (806708) | more than 9 years ago | (#10911265)

It would be much nicer if they would compare to RTLinux, which is also linux-based though paid. RTLinux has a patented linux-as-a-process scheme and they claim to be ultra-fast...

Re:Not good result is it? (1)

cherberos (262597) | more than 9 years ago | (#10911672)

which is basicly what RTAI also does. RTAI is even a fork of RTLinux ( if I recall correctly), although nowadays RTAI incorporates some things different. They were also the first with 2.6 patches, if I recall correctly. Btw, FSM labs, who maintain RTLinux, also offer a GPL-edition for download.

Missing RTLinux (2, Interesting)

Anonymous Coward | more than 9 years ago | (#10910857)

How can you do a good comparison of Real Time Linux capabilities, without including RTLinux? RTLinux is used in Bluecat RT, one of the more widely used commercial Real Time linux distros. For that matter, LynxOS would have also been a good one to use. It is difficult to make a comparison of Hard Real Time Linux, when you only have one Real Time Linux version to compare with... odd.

Re:Missing RTLinux (1)

dmh20002 (637819) | more than 9 years ago | (#10925735)

problem is RTLinux isn't free as in beer. or if it is, where do I download the iso's.

Re:Missing RTLinux (0)

Anonymous Coward | more than 9 years ago | (#10951593)

Well, I am not clear to what you are asking. There is a FreeAsInSpeech release, under GPL: ftp://ftp.rtlinux-gpl.org/pub/rtlinux . FSMLabs includes value-added support and tools for a charge. No ISOs are necessary, since RTLinux is mostly just a kernel patch. Use whatever distro you'd like, and patch the kernel... That is how RTAI is as well.

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