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[David Godsey-2]

So I have been combing the internet for information on realtime FreeBSD,
but the information is limited.  So there must be some more information
out there, but I do not know where to look.  Can someone direct me please?


A positive attitude may not solve all your problems, but it will annoy
enough people to make it worth the effort.

If there are no stupid questions, then what kind of questions do stupid
people ask? Do they get smart just in time to ask questions?

David Godsey

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[Arne "Wörner"]

--- David Godsey <freebsd@...> wrote:
> So I have been combing the internet for information on realtime FreeBSD,
> but the information is limited.  So there must be some more information
> out there, but I do not know where to look.  Can someone direct me please?
>
Hmm... I do not have practical experience in real time operations (just my
mostly theoretical studies of task scheduling and control engineering), so I
can only hope my contribution is not too useless/stupid... :-)

I say, have u read the man page rtprio(2) and polling(4)?

I would guess, that u might want to set kern.clockrate to something high
(1000..2000) (e. g. via /boot/loader.conf's kern.hz).

Furthermore I would recommend, that u do some tests/research/experiments in ur
specific environment (remember: real time constraints can be very different (e.
g. a simple electronic calculator is ok, when u just want to compute the taxes
in a shop or in a restaurant, but when u want to compute the course of Cpt.
Picard's Enterprise (StarTrek) u would need something faster (or it might get
struck by some kind of lightning or get stuck in a sun))). In this case here it
is the question, if u can tolerate fluctuations in scheduling accuracy
(response time) due to voluntary/unpredictable/dynamically-scheduled kernel
activity (I think polling(4) could just help (a little) with network devices,
so that hard disc activity can still hinder real time tasks).

>From the point of view of digital control engineering operating systems like
*BSD are no real real-time operating systems... Real real-time operating
systems are obviously able to guarantee a very high scheduling accuracy...

I just did this litte experiment with (avg. about 1.3Mbit/sec, peak about
16Mbit/sec) disc traffic at the same time and a clock rate of 1000Hz:

> rtprio
rtprio: normal priority
> repeat 10 time sleep .2
0.007u 0.015s 0:00.22 4.5%      12+396k 0+0io 0pf+0w
0.000u 0.030s 0:00.23 13.0%     182+1332k 0+0io 0pf+0w
0.000u 0.032s 0:00.23 13.0%     181+1288k 0+0io 0pf+0w
0.010u 0.021s 0:00.23 13.0%     276+2064k 0+0io 0pf+0w
0.006u 0.026s 0:00.24 8.3%      144+1230k 0+0io 0pf+0w
0.000u 0.029s 0:00.23 8.6%      136+966k 0+0io 0pf+0w
0.000u 0.032s 0:00.24 12.5%     185+1420k 0+0io 0pf+0w
0.021u 0.010s 0:00.24 12.5%     182+1332k 0+0io 0pf+0w
0.006u 0.025s 0:00.28 7.1%      278+2130k 0+0io 0pf+0w
0.008u 0.026s 0:00.24 8.3%      276+2064k 0+0io 0pf+0w
neo# rtprio 30 -$$
neo# rtprio
rtprio: realtime priority 30
neo# repeat 10 time sleep .3
0.000u 0.025s 0:00.33 6.0%      138+1018k 0+0io 0pf+0w
0.000u 0.028s 0:00.33 6.0%      276+2036k 0+0io 0pf+0w
0.000u 0.027s 0:00.33 6.0%      140+1084k 0+0io 0pf+0w
0.000u 0.028s 0:00.33 6.0%      138+1018k 0+0io 0pf+0w
0.006u 0.024s 0:00.32 6.2%      412+2988k 0+0io 0pf+0w
0.000u 0.026s 0:00.32 6.2%      138+1018k 0+0io 0pf+0w
0.006u 0.020s 0:00.47 4.2%      274+1970k 0+0io 0pf+0w
0.006u 0.019s 0:00.32 3.1%      552+4072k 0+0io 0pf+0w
0.000u 0.013s 0:00.31 3.2%      276+2036k 0+0io 0pf+0w
0.000u 0.014s 0:00.31 3.2%      276+2036k 0+0io 0pf+0w

-Arne



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