To me the difference is that a (or all) scheduler will not be able to
'continue' (and thus migrate) the job it preempted on any of the cpu's
available to the scheduler.
You might find more info on this in this thread:
http://www.beowulf.org/archive/2007-April/018245.html
toon
Mark Hahn wrote:
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.
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