...Linux does have a RT version, in part supported by Ingo Molnar and Theodore Ts'o, but it does not see heavy use. In part, this has been because for a realtime OS to be successful, all the parts have to play ball, not just some...
Not really, the actual requirement is that nothing can block a real time process, which I believe the linux real time patches do in fact achieve quite reliably. It isn't important whether Firefox hangs up for a while, to anybody except the Firefox user. Perhaps realtime Linux has maximum latency higher than QNX (nobody has posted benmarks so far...) but certainly well within the range required for real time control of noncritical systems.
The real reason that the realtime patch set is not wildly popular is that a lot of devs and users tend to labor under the misaprehension that real time response isn't needed, because after all, their computer works pretty well without it. Actually, it doesn't as anybody involved with professional quality audio can attest, and if said users and devs would quit eding away the the evidence of their own eyes, they would notice frequent instances of nasty, unacceptable interactive latency. Amazing how selective memory works, isn't it?
For critical systems like engine and braking control, it is obvious that a "pure" realtime control system is needed, and be running on its own dedicated processor. For that, even QNX is arguably too complex, and too difficult to analyze to the extent required. But such critical systems have now become just a small part of what is going on in a modern car, with all the nav, infortainment, climate control, etc. For all this big bloaty stuff, realtime Linux is perfectly appropriate and would probably be a better choice overall than QNX due to the far greater range of hardware supported, more networking protocols, more nifty accelerators for applications, etc etc. Regards of whether Linux is the best choice (it is) QNX is still a damn good choice. Light years beyond anything Microsoft has had their fat hands in.
