Mark Hounschell wrote:
On 04/24/2012 05:08 AM, Lars Segerlund wrote:
This is not based on facts, rt-preempt does provide hard realtime and
strive to provide hard realtime, where have you come up with the
notion that it does not ?
Please, don't spread misinformation, this is pure FUD .......
Check osadl.org and their test rack, it will perhaps shed some light
on the quality assurance they try to do.
It is hard to argue with numbers, also check a recent kernel and a
'good' system.
Some system have latency problems, but most are fine, atleast a
worstcase of 50 usor so under hard load and normal times in the low 10
to 20 us range.
/ regards, Lars Segerlund.
2012/4/24 Mark Hounschell<dmarkh@xxxxxxxxxx>:
On 04/24/2012 01:46 AM, Anisha Kaul wrote:
From:
https://rt.wiki.kernel.org/articles/f/r/e/Frequently_Asked_Questions_7407.html
Real-time only has impact on the kernel; Userspace does not notice
the
difference except for better real time behavior.
Does it mean that if we write the applications in user space, they
won't get the hard real time effect?
The threads running in the userspace won't get the hard real time
effect?
You use the term "hard real time". The RT patch set does not even
come close
to providing a "hard real time" environment, and isn't even
attempting to.
It does however provide user land applications a much better chance
for a
"soft real time" environment. The phrase you quot above just means the
patches are applied to the kernel and there are no patches required
for user
land glibc or your application.
Your in lala land. The Linux kernel, even with the RT patch has so
much "per CPU" crap in it, there is no way to prevent it from steeling
usecs from your application.
Not only the kernel steals CPU time :) Also the SMI interrupt can steal
a lot of time from any CPU core! And these effects are much higher then
the actions at the kernel level.
The OS AND the hardware must provide the environment for hard real-time
processing. The definition of the a deadline includes also a time
resolution (or variation) for meeting the deadlines ... and this is
defined by the real-time application and not by theOS. The OS has just
to profide the technological base to support this requirement. In some
cases is a time resolution of 1ms OK for meeting a deadline ... and
this still a hard-realtime application.
The per CPU timer interrupt alone takes a few usecs away from your
application every HZ. A hard RT env is one in which you can always,
every time, do a predefined work in the same amount of time. Fast or
slow isn't the key. It's determinism. The timer interrupt alone
prevents that. And it's not the only thing. I've got 8 cpus on my
machine but the kernel has to have a piece of every one of them. Until
there is isolation from the kernel, there cannot be "Hard RT". This is
fact.
Your understanding of hard real-time is far too narrow ...
--armin
Mark
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