Nivedita Singhvi wrote:
On 09/14/2010 07:49 AM, Jeff Angielski wrote:
Isn't any case where preempt-rt does not behave as hard reatlime a
bug in
preempt-rt, that should be reported to this list and eventually fixed?
That is a philosophical question for the preempt-rt maintainers.
I *believe* that the design goal for the preempt rt code is to minimize
kernel latency. It's not to make the kernel deterministic to support
hard realtime.
Er, sort of, not quite.
The design goal of the real-time kernel is most certainly to make the
kernel more deterministic, to the extent we can in a general-purpose way.
Determinism = capping max latencies.
Capping max latencies doesn't help without a good real-time, event
driven scheduler.
But it helps to classify real-time operatings systems as so called
hard or soft real-time operating systems.
IMHO ... there is a common understanding that a RTOS can be
considered as a hard-reatime OS if the
max latency is < 15us because it is able to server 80% (?) of all
hard real-time applications in the field.
All others are considered as soft real-time operating systems. From
this point of view is PREEMPT_RT Linux
a hard real-time OS ... if the hardware base is appropriate.
BTW ... we use PREEMPT_RT Linux as a base for our commercial
solft-PLC called DACHSview: http://steinhoff-automation.com/Programming.htm
It's better to have all hundred iterations of an operation take
45us each than to have 95 iterations take 30us and 5 iterations
take 75us. You want to be able to say, "this operation will take
at _most_ 50us" and have that be as true as possible.
All these numbers have no meaning if you don't specify the hardware
environment.
--Armin
http://www.steinhoff-automation.com
We sacrifice overall throughput (ave latency) for determinism
(low max latency).
Not sure if that's what you intended to say, but hope that helps.
thanks,
Nivedita
--
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
--
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
