Hi Mark, Mark Knecht wrote: > Hi Ingo, Steven, Thomas and other RT developers, > As always, thanks for what you do. I've been subscribed here for a > few years. Don't say much. Do read a lot. > > As a user type I've run the RT kernels for years now. 5 years ago > it was really crucial for my music work. I still use it faithfully. > However on many lists now I'm seeing new people say there's no need > for the RT kernel because they run their applications as root and feel > they get the 'same' or 'good enough' results. I wonder how they know > if they don't run both and compare but that's their business. I'm > suspect but not technically deep enough in this subject to have a > conversation so I thought I'd ask for a little simple (if possible) > education. > > I PROCLAIM - Running an application as root on the standard kernel is > not as real time capable as running the same application as a normal > user with proper priorities & permissions on the RT kernel. > > True or false? > True Running as root, even on preempt-rt, doesnt really buy you anything w.r.t. real-time response. What it does do (aside from introduce the security/stability concerns you mention) is give you implicit permission to change your scheduling policy/priority, which is part of what is needed to actually _become_ real-time. Being a root user itself doesn't actually change anything w.r.t. your scheduling response. And as you noted, there are other preferable ways to give this capability to a user without requiring root, and this is generally advised as a better IT policy. That said: Note that all recent mainline kernels generally employ the same real-time scheduler as the one used in PREEMPT_RT and have a fairly high degree of preempt-ability (when suitably configured). Therefore, you *could* conceivably configure mainline with CONFIG_PREEMPT and set your scheduler policy/priorities appropriately to achieve a moderate amount of deterministic behavior. However, note that mainline to date does not have some of the more fancy RT features found in PREEMPT_RT such as threaded-irqs (read: priortizable), "sleeping spinlocks", and PI mutexes that allow you to get the _really_ low latencies. But it may be "good enough", as your pollees put it, for a given application. "Acceptable latency" is in the eye of the beholder. ;) So the short answer is: Running as root doesnt really do anything for you, but you may be able to get "acceptable" (determined by your app) real-time performance out of mainline without PREEMPT_RT if you know what you are doing. YMMV. HTH -Greg > Is there a source of measured data to show this is true or false? > > I know a lot of what you've done over the years is now mainline but > I don't know how much of it is turned on by default. > > Clearly, I think folks don't have their head screwed on tight if > they are running development apps as root, but people are doing it. > They don't seem to care about security or the obvious fact that > running as root might allow an app to erase their hard drive, etc. > Their minds will change when something goes wrong, but in the meantime > what can I tell them about this subject to get them to use the RT > kernel and be as happy as I am with it. > > Thanks, > Mark > -- > To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html >
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