Real-Time Tuning

I tried a number of things that are supposed to improve real-time performance, as described in this forum post.

But not much changed. This series of jitter-histograms shows little or no changes:

0 1 2 3 4

The things I tried are roughly

  1. measure first latency histogram 0.png
  2. uninstall the package irqbalance using synaptic. reboot.
  3. measure 1.png
  4. in /etc/default/grub modify GRUB_CMDLINE_LINUX_DEFAULT="isolcpus=1 acpi_irq_nobalance noirqbalance"  (Aside: why are the files in /etc/grub.d/ made so incredibly hard to read? Someone should re-write them in Python!). Run sudo update-grub. reboot.
  5. measure 2.png
  6. Add irq-affinity.conf to /etc/init/
  7. Add set-irq-affinity and watchirqs to /usr/local/sbin. reboot
  8. measure 3.png
  9. Try to tweak BIOS settings. Turn off power-saving features, etc.
  10. measure 4.png

The output of watchirqs looks like this:

watchirqs_before_boot watchirqs_last

The scripts mentioned above: irqstuff

LinuxCNC on Ubuntu 12.04LTS

Recent developments has made it possible to run LinuxCNC on the latest LTS release of Ubuntu. This is experimental work, so not recommended for controlling a real machine just yet. The main obstacle for moving LinuxCNC from 10.04LTS to a more recent distribution has been the RTAI real-time kernel, which has not been kept up-to-date with development of the normal Linux kernel. Fortunately there are alternatives such as Xenomai or RT_PREEMPT.

Here is a step-by-step description of the install/build process, if you want to experiment with this.

  1. Download and install a normal 32-bit 12.04LTS Ubuntu (ubuntu-12.04.1-desktop-i386.iso). Note that the 64-bit version is not supported for the steps that follow further down. I could not get Ubuntu's startup-disk-creator to work, so I used unetbootin to write the ISO-file to a USB-stick.
  2. It's possible to compile the xenomai-kernel from scratch, along with the runtime etc., but I used pre-compiled deb-packages by Michael Haberler from here:
  3. Install the xenomai kernel:
    sudo dpkg -i linux-headers-3.2.21-xenomai+_0.1_i386.deb
    sudo dpkg -i linux-image-3.2.21-xenomai+_0.1_i386.deb
  4. make sure it will show up as a GRUB-entry when booting:
    sudo update-initramfs -c -k 3.2.21-xenomai+
    sudo update-grub
  5. reboot. uname -r should now show: 3.2.21-xenomai+
  6. now install the xenomai runtime:
    sudo dpkg -i libxenomai1_2.6.1_i386.deb
    sudo dpkg -i libxenomai-dev_2.6.1_i386.deb
    sudo dpkg -i xenomai-runtime_2.6.1_i386.deb

This installs the xenomai system on top of which a recently available version of LinuxCNC can be built. There are probably many ways to now obtain the tools/dependencies that are required. I used the following:

  1. sudo apt-get install synaptic
    sudo apt-get install git
  2. Now using synaptic, install the following packages (I found these are required for a minimal linuxcnc build):
  3. Get Michael's version of LinuxCNC that can be compiled for Xenomai:
    git clone git:// emc2-dev
    cd emc2-dev
    git branch --track rtos origin/rtos-integration-preview1
    git checkout rtos
  4. Configure and build for Xenomai:
    cd src
    ./configure --with-threads=xenomai-user --enable-run-in-place
    sudo make setuid
  5. Test:
    . ./scripts/rip-environment

This new version of LinuxCNC can be built without a real-time kernel (previously called "simulator" or "sim") or with any of the real-time kernel alternatives: RTAI, Xenomai, RT_PREEMPT. It should be possible to compare real-time performance in the form of latency-numbers with different hardware and kernels.