Le 20/04/2011 12:47, Johannes Bauer a Ãcrit :
Thats good news, thank you Remy!
Hi Johannes,
I was able to run cyclictest
Great news!
Can you post your latest hrtimer port for your HW? It could be useful
for anyone who will want to do the same thing in the future...
\\\"cyclictest -t1 -p 80 -n -i 10000 -l 1000\\\" an thw maximum latencies were about
650us. However the intervall timers \\\"cyclictest -t1 -p 80 -i 10000 -l 1000\\\"
result in abou 3000us maximum latencies.
Is it common that interval timers have higher latencies and could this values be
reasonable for a 72MHz arm7tdmi (ARMv4T).
You may read this paper (page 2) from OSADL:
https://www.osadl.org/fileadmin/dam/press/Industrial-Ethernet-Book-2009-05.pdf
It gives an empirical law for max latency under PREEMPT-RT:
"A number of commercially available Linux distributions (e.g. Red Hatâs
MRG) already contain the PREEMPT_RT patches in production quality. The
worst-case rule-of-thumb latency is in the range of 10^5 multiplied by
the CPU cycle duration with these systems without special optimisation.
For example, a 1GHz processor has a cycle duration of one nanosecond
which results in a worst-case latency of about 100Îs. This is considered
sufficient for the vast majority of real-time projects. Using latency
optimisation, however, the worst-case latency can be reduced by up to
three times."
For a 72 MHz CPU clock, it gives: 1388 Âs
I've done a PREEMPT-RT port for NIOS 2 with MMU (with hrtimer port too).
With cyclictest, I've measured for a low performance design with 50 MHz
NIOS 2 HW a latency up to 1400 Âs and for another more powerful design
with 125 MHz NIOS 2 HW a latency up to 650 Âs.
Your 650 Âs value sounds reasonable.
Cheers;
Pat.
Anyone has any values to compare or can tell me if those values are reasonable?
Kind Regards
Johannes Bauer
----- Original von: Remy Bohmer
Hi,
2011/4/15 Johannes Bauer<hannes_bauer@xxxxxx>:
Hi everybody!
I downloaded the newest uClinux dist a while ago and compiled it with 2.6.33
kernel and the 2.6.33.7.2-rt30 patch applied.
Then i by hand implemeted an old patch for arm lpc22xx machine (ARMv4T), since
i am running a lpc2478 board from embedded artists.
I had to implement a clockevent/source timer to add hrtimer support. I
compiled it all with uClibc since it as a NOMMU system.
Now i have some questions:
Is it possible to run the preempt-rt patch with the uClinux distribution and
uClibc?
The preempt-RT patch runs just fine with a uClibc userspace, I have no
idea what it will do in a NOMMU system (never tried it)
I read a few times that if doesnt support PI mutexes, but arent those RT
mutexes part of the kernel?
The kernel itself has PI-mutexes support, _internally_within_ the
kernel; regardless what type of userspace C-library is being used.
However, for _userspace_ PI-mutexes, you will need a C-library that
supports it (search for example for PTHREAD_PRIO_INHERIT in the
library code). 2 options:
* Glibc since version 2.5 (actually 2.4.90 and higher)
* uClibc master. You might have some luck since there is now NPTL
support in uClibc master. So, get the master tree of uClibc and see if
it works for you.
(http://git.uclibc.org/uClibc/snapshot/uClibc-master.tar.gz)
I have not tested it myself though, but if it works, please share the
good news.
Have Fun!
Remy
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