[Beowulf] preemptive kernel and preemptive schedulers

[Mark Hahn]

> RT Linux (I am a newbie to RT=Linux) is based on the concept of Preemptive kernels where the jobs are
> temporally preempted say a FIFO scheduler implemented at the kernel level.

when people say "RT Linux", they normally mean various flavors of hard-RT 
hacks (not meant in an entirely disparaging way;)

on such systems, it much more than simply suspending user-space - back in the
day, the idea of preempting the whole kernel was extremely controversial.
my impression is that since then, the (real, kernel.org kernel) has become 
drastically more predictable in latency, and in many ways more preemptable.

even ~6 years ago, it was quite feasible to run realtime applications on the 
kernel.org kernel - I did realtime video generation and response timing for 
psychophysics labs.  it wasn't hard-RT in the sense of "provably always meets 
all deadlines", but for a lab, it just worked...

> I know that the same is achieved by submitting the job through a job scheduler(ca. PBS Pro) installed
> on top of the OS.

a job scheduler is entirely different from hard-RT stuff.  a job scheduler is
primarily concerned with managing cluster resources, which may indeed include 
suspending jobs.  at least the conventional ones like PBS/LSF/etc are most
definitely _not_ hard-RT - if nothing else, they tend to respond on the order
of several seconds, not microseconds...

> I wanted to know whether schedulers can be alternatives for preemptive kernels.

sure.  in the most abstract sense, they are the same.  the original RTLinux
stuff was based on an executive/hypervisor which preempted the kernel much 
as a normal job scheduler might preempt a job.  these days, the boundaries 
are further blurred by virtualization...

regards, mark hahn.