Reducing Jitter
07 Oct 2017 18:41 #100055
by Escain
Reducing Jitter was created by Escain
Hello 
I am currently trying to get LinuxCNC working on a Raspberry Pi. It's about a few weeks that I am trying, so I guess is the moment to ask, but my expertise is still limited: forgive me if I ask some newbie stuff.
So, what is the current status:
First, I installed a fresh Raspian Linux on the machine.
Second, I Cross-compiled an RT kernel following the page here . After that, using `uname` print the correct new kernel with PREEMPT in it.I did some tests with "cyclictest" and results, even if not very good, are still bellow 150us maximum and average 20us
I configured the kernel to start with following options:
After, I cloned LinuxCNC and following this guide
What with LinuxCNC?
Running the test: "latency-histogram" I get terrible results with maximum jitter over 700-1400us
It seem to me that there is no preemption usage, but I am unable to check it.
Also, trying to add CPU affinity to the two latency-histogram threads does not seem to help.
Conclusion and question
I fully understand that the RaspberryPi cheap computer will not provides 5us jitter control, but at the moment it seem the CPU is able of more than what I got.
My question/s are:
How to ensure LinuxCNC is using Preemptive kernel features?
If using Preemptive, why is the jitter so high?
Any hint of how to improve and bound this jitter?
Notes:
LinuxCNC version used: 2.8.0~pre1, from today repository status.
I am currently trying to get LinuxCNC working on a Raspberry Pi. It's about a few weeks that I am trying, so I guess is the moment to ask, but my expertise is still limited: forgive me if I ask some newbie stuff.
So, what is the current status:
First, I installed a fresh Raspian Linux on the machine.
Second, I Cross-compiled an RT kernel following the page here . After that, using `uname` print the correct new kernel with PREEMPT in it.
Linux raspberrypi 4.9.50-rt37-v7+ #2 SMP PREEMPT RT Wed Sep 20 21:44:16 CEST 2017 armv7l GNU/LinuxI configured the kernel to start with following options:
sdhci_bcm2708.enable_llm=0 dwc_otg.fiq_fsm_enable=0 dwc_otg.fiq_enable=0 dwc_otg.nak_holdoff=0 isolcpus=0,1After, I cloned LinuxCNC and following this guide
git clone git://github.com/linuxcnc/linuxcnc.git linuxcnc-dev
sudo apt-get install dpkg-dev
cd linuxcnc-dev/debian
./configure uspace
cd ..
dpkg-checkbuilddeps
apt-get install <all list>
apt-get install bwidget
apt-get install libtk-img
apt-get install tclx
cd src
./configure --with-realtime=uspace | tee ../configure.log
make | tee ../make.log
sudo make setuid | tee ../makesetuid.log
source ../scripts/rip-environment
runtiestsWhat with LinuxCNC?
Running the test: "latency-histogram" I get terrible results with maximum jitter over 700-1400us
It seem to me that there is no preemption usage, but I am unable to check it.
Also, trying to add CPU affinity to the two latency-histogram threads does not seem to help.
Conclusion and question
I fully understand that the RaspberryPi cheap computer will not provides 5us jitter control, but at the moment it seem the CPU is able of more than what I got.
My question/s are:
How to ensure LinuxCNC is using Preemptive kernel features?
If using Preemptive, why is the jitter so high?
Any hint of how to improve and bound this jitter?
Notes:
LinuxCNC version used: 2.8.0~pre1, from today repository status.
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