Are there any open source real time operating systems out there? I've heard of real-time Linux, but most implementations seem to really be a proprietary RTOS (that you have to pay for) that run Linux as a process -- much the same way Ardence's RTX real-time system works for Windows.
EDIT: I should clarify that I'm looking for RTOS to work with multi-core x86-family CPUs.
Check out eCos free, open source and real-time operating system. (Supports x86, not sure about multi-core)
RTLinux is also available
There is the S.Ha.R.K. Project. It works with x86 CPUs but I don't know if it handles all cores of a CPU.
FreeRTOS, it provides the underlying kernel. I've used it in some embedded apps and it seems robust. But, it really depends on your application.
Well this is not Open Source, but did you know that Windows CE is a hard real time operating system and that it does have a x86 port? I don't know however if it can support multi core CPUs. If it is a commercial project, you definitely should consider it.
There is also MicroC/OS-II, which has a x86 port, but as above, I don't know if it supports multi cores. It is free for non-commercial applications.
What are your exact requirements? Perhaps you can use vanilla Linux - it doesn't provide real-time guarantees but might be good enough. Some people find that it's not as bad as the real-time vendors try to make out.
Vanilla Linux DOES have different scheduling policies as well, but not a lot of people know that.
eCos is free (but you can get paid support). It supports Intel x86 architecture. It supports multi-processor systems. Depending on your timing requirements, I've had not too good experience with real-time Linux systems. Although response time may be good in average, I've seen cases where the worst case over a few days may be 10 or even 100 times as much. I guess this partly depends on the quality of the drivers, partly on the scheduler itself.
But I guess it boils down to whether your system demands hard or soft real-time, what the timing constraints are, what kind of application you need to run. And how streamlined development system you require.
There are hard real-time extensions to the Linux kernel. You might want to check some of those out.
Good examples are RTAI and LXRT
There are real-time extensions to Linux, as already mentioned by someone else. Have a look at xenomai.org.
I'm not so sure about the multiprocessor issue. What exactly do you want to do on your multiple processors?
OpenSolaris has real-time capabilities, however you should watch out if you decide to use it for real-time development: pretty much all I/O can cause priority inversions in the kernel (low-priority system worker threads can starve and cause high priority threads to be blocked, e.g. in STREAMS code).
I have also been using the FreeRTOS operating system that is available either for free under a modified GNU licence, a paid commercial licence version or an expensive safety certified version (SafeRTOS)
From the web-site there is an x86 port as follows
* Supported processor families: Any x86 compatible running in Real mode only, plus a Win32 simulator
* Supported tools: Open Watcom, Borland, Paradigm, plus Visual Studio for the WIN32 simulator
This OS provides the pre-emptive or co-operative task scheduling with queues, semaphores and priority setting for the tasks. It does not provide the sort of I/O or file library functions that come with other larger OS implementations like Linux.