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Creating and testing an RTAI patched linux kernel
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LICENSE Initial commit Jun 14, 2018 added musl to docu Jun 28, 2019 added alive script Jun 19, 2018 uninstall kernel also uninstalls rtai Jul 12, 2019


Building and testing RTAI-patched linux kernels.

  • bash-script for building and testing RTAI-patched linux kernels
  • python script for analyzing, displaying, and summarizing test reports produced by the script.
  • script producing some output on your console to indicate that the machine is still alive.
  • cpulatency: kernel module for setting CPU latencies to zero via the PM-QoS kernel interface.
  • poll: kernel module intended to poll on a single CPU to keep it in C0 state (not working yet).


For a summary of test results see RTAI tests

Install an RTAI-patched linux kernel, RTAI, and comedi

The script executes all the commands needed to download and build an RTAI-patched linux kernel, the newlib or musl library (needed for math support), and the RTAI and comedi kernel modules.


./ help

for an overview.

The script is so far only used and tested on debian/ubuntu based systems and is likely to fail on other Linux distributions. Feel free to adapt the script to your Linux distribution; in particular check the setup_* functions.


When you use for the first time, then follow the following instructions:

  1. Change into the directory with the script.

  2. Write the default configuration file

    ./ config
  3. Check the settings by running

    ./ info settings

    In particular the RTAI source, i.e. the value assinged to the RTAI_DIR variable.

    Also check whether you want to use newlib or musl for math support by setting one of MAKE_NEWLIB and MAKE_MUSL to true.

    If you need comedi, then make sure MAKE_COMEDI is set to true.

    If you want to change the RTAI source, then open the configuration file makertaikernel.cfg in your favourite text editor and select an RTAI source by modifying the value assinged to the RTAI_DIR variable. See comments in the configuration file for options.

  4. Run

    sudo ./ init

    to make sure you have /var/log/messages (needed for RTAI switch tests), get a visible boot menu, can pass kernel parameter to the RTAI kernel, have comedi devices assigned to the iocard group, and have the RTAI sources downloaded.

    This command ends with showing you the RTAI kernel patches available for your machine.

Building the first kernel

Then you can go on by building the RTAI-patched kernel for the first time:

  1. Select a Linux kernel and the corresponding RTAI patch from the displayed list.

    Modify the variables LINUX_KERNEL, RTAI_PATCH, and KERNEL_PATH variables in the makertaikernel.cfg configuration file to match the kernel version, rtai patch you want to use, and the directory where to build the linux kernel (you need up to 2 GB space there for a kernel with most device drivers disabled (e.g. via localmodconf) or 5-10GB for a full kernel).

    Best is to choose a kernel that is available as a package for your linux distribution (see next point).

    Recheck for available RTAI patches and linux kernels:

    sudo ./ info rtai
  2. You should start out with a kernel configuration of a kernel image from your linux distribution closest to the kernel version you selected (same major.minor kernel version). First, the provided kernel configuration was tested and works, and second, we can then deselect all unused kernel modules, which speeds up compile time considerably (from one hour down to 10 minutes). It definitely pays off to invest some time in the beginning to start out with a matching working kernel.

    On a Debian-based system install a matching kernel by executing

    sudo apt-get install linux-image-4.4.0-79

    Modify the name of the kernel image to the kernel version you want, use the tab-key for autocompletion. If autocompletion does not work run

    apt-cache search -n linux-image-XXX | awk -F ' - ' '{if ( $1 !~ "-dbg" ) print $1}'

    where XXX is replaced by the kernel version you are looking for. Then boot into this kernel. Either restart your computer and select the kernel from the grub boot menu or check the grub menu with

    sudo ./ info grub

    and reboot directly into this kernel by using

    ./ reboot 3

    Replace 3 by the index of the appropriate menu entry. The latter approach has the advantage that it doesn't matter to miss the boot menu.

    Starting out with a kernel matching the one from the selected RTAI patch ensures that will use this kernel's configuration and deselect all kernel modules that are not used (localmodconfig). This dramatically shortens the compile time (from about one full hour down to 5-15min).

    If you use with the -l switch or the version of the running kernel does not match the one of the selected kernel (major or minor version number differ), localmodconfig is not applied, resulting in a much larger kernel (takes much more time to compile).

    If your distribtion does not supply a kernel with a matching version, then you should first build a kernel without the rtai patch by running

    sudo ./ download kernel
    sudo ./ -n plain buildplain

    Boot into this kernel

    sudo ./ -n plain reboot

    Finally run

    sudo ./ clean kernel

    to make sure that the RTAi patches are aplied in the next step.

  3. Once you booted into the kernel on which you want to base your RTAI kernel run

    sudo ./

    This will download the required sources (takes a while...) and build the kernel using the kernel configuration of the running kernel (takes even more time...).

    With the -c flag you can provide a different kernel configuration on which the kernel configuration should be based (see help for details).

  4. You will get the menu for configuring the kernel. You need to change a few settings to get a running RTAI-patched kernel. See the next section Basic kernel configuration for instructions.

  5. Reboot into the new kernel by executing

    ./ reboot
  6. Go on with checking, testing, and improving your RTAI kernel as described in the following sections.

Basic kernel configuration

For making the Linux kernel work with RTAI you should check the following settings in the kernel configuration dialog.

This list is updated for RTAI 5.1. For other RTAI versions read /usr/local/src/rtai/README.CONF_RMRKS !

  • "General setup":

    • Disable "Enable sytem-call auditing support" (AUDITSYSCALL)
    • Important: set "Stack Protector buffer overflow detection" (at the bottom of the menu) to "Regular" (CC_STACKPROTECTOR_REGULAR) - or even "None" (CC_STACKPROTECTOR_NONE) if the latency test crashes.
  • "Procesor type an features":

    • Disable "Support x2apic" if present (X86_X2APIC)
    • Maximum number of CPUs (NR_CPUS), optional, if you want to disable hyperthreading and to reduce the output of RTAI tests in the kernel messages.
    • Disable "Multi-core scheduler support" (SCHED_MC) (in order to be able to disable CPU frequency scaling, for kernel >= 4.14)
    • Enable "Interrupt pipeline" (IPIPE)
  • "Power management and ACPI options":

    • In "ACPI (Advanced Configuration and Power Interface) Support":
      • Disable "Processor" (ACPI_PROCESSOR)
    • Disable "CPU Frequency scaling" (CPU_FREQ) (sometimes you need to disbale this first in order to be able to disable ACPI_PROCESSOR)
    • In "CPU Idle":
      • Disable "CPU idle PM support" (CPU_IDLE)
  • "Device Drivers":

    • In "Staging drivers":
      • Deselect "Data acquisition support (comedi)" (COMEDI)
  • "Kernel hacking":

    • In "Compile-time checks and compiler options":
      • Disable "Compile the kernel with debug info" (DEBUG_INFO) Disabling debugging information makes the kernel much smaller. So unless you know that you need it disable it.
    • Disable "Tracers" (FTRACE)

Leave the configuration dialog by pressing "Exit" until you are asked "Save kernel config?". Select "Yes".

Then the new kernel is being compiled - be patient.

Getting a running RTAI-patched kernel

Sometimes the RTAI-patched linux kernel fails already at boot-up.

  • In case booting into the RTAI-patched kernel failed, reboot into your standard kernel (select it via the grub menu) and build a kernel without the RTAI patch:

    sudo ./ buildplain

    This uses the kernel configuration of your running kernel. First do not change it, then if this kernel runs, modify the the kernel configuration step-by-step as described in the previous section Basic kernel configuration to figure out the culprit.

  • If you did not start out with a kernel version matching the version of your RTAI-patched kernel, and you are going to change the kernel configuration, then you should run

    sudo ./ -c mod

    in order to deselect all unused kernel modules from compilation. This speeds up the following kernel builds dramatically!

  • Your compiler could be too new. Try an older distribution with older versions of compiler, linker, etc.

  • Once you successfully booted into the RTAI-patched kernel (this is usually the case) then you can go on with testing and improving your RTAI kernel as described in sections Testing the RTAI-patched kernel and Improve the RTAI-patched kernel below.

Checking RTAI modules

If you managed to boot in to the RTAI-patched linux kernel, then check whether the RTAI modules can be loaded without problems. For this run

sudo ./ test none

This will load (insmod) the rtai_hal, rtai_sched, rtai_math, and comedi kernel modules. Afterwards, the modules are unloaded (rmmod) again.

If you only want to check loading rtai_hal then call

sudo ./ test hal none

Equivalently, you can use the sched and math option.

If this fails:

  • Make sure you have the STACKPROTECTOR set to "None" or "Regular" (see Basic kernel configuration)
  • Check the output of
    for some hints.

Once the RTAI modules load flawlessly you can proceed with checking the RTAI performance as described in the next section.

Testing the RTAI-patched kernel

Testing and improving your RTAI patched kernel is crucial for a good real-time performance!

The script can also be used for testing with the advantage that it writes the test results and the kernel configuration into files.

The script can also run whole test batches automatically where the RTAI-patched kernel is tested with various kernel parameters and kernels with new configurations are compiled as specified in a batch file (see Test batches below).


./ help test

for a summary of test options that are described in more detail in the following sections.

Run the kernel tests from the RTAI test suite by calling

sudo ./ test

The script will first ask for a short description of your kernel configuration and parameter. This string is then used for naming the files for the test results.

This runs the kernel latency, switch, and preempt tests.

Each of the tests need to be manually terminated by pressing CTRL-C.

All three tests are available for kernel, kernel threads, and user space test suites. They can be selected in the following way:

sudo ./ test kern     # runs the kern tests
sudo ./ test kthreads # runs the kernel threads tests
sudo ./ test user     # runs the user tests
sudo ./ test all      # runs all tests

How to read and interpret test results is described in section Test results.

Run tests under load

To really check out the performance of the kernel you should run the tests under heavy load. This can be easily controlled by adding one or several of the following keywords to the test options:

  • cpu: run computations on each core.
  • io : do some file reading and writing.
  • mem: do some memory access.
  • net: produce network traffic.
  • full: all of the above. For example
./ test cpu net

will run the kern tests with cpu and network load.

Run tests on a specific CPU core

In particular when checking the isolcpus kernel parameter you may want to run the latency test on a specific CPU. You can specify the CPU id on which you want to run the tests with cpu=<id>, where <id> is the CPU id, first CPU is 0. For example:

./ test cpu=2

will run the kern tests on the third CPU.

Requesting zero CPU latency

To be done ...

With rtai-5.2 on an Intel i7:

  • With ACPI_PROCESSOR, CPU_FREQ, and CPU_IDLE enabled, you still get the same great results if idle=poll kernel parameter is used.
  • Without X86_INTEL_PSTATE just writing a zero to /dev/cpu_dma_latency can replace idle=poll.

Automatized testing

For automatic termination of the tests (no CTRL-C required) provide the duration for the latency test as a simple number (in seconds):

sudo ./ test 60       # run the kern latency test for 60 seconds

For preventing any user interaction you can also provide the test description after the "auto" keyword (here "basic"):

sudo ./ test 60 auto basic

Test batches

For a completely automized series of tests of various kernel parameters and kernel configurations you can prepare a file with the necessary instructions (see below) and pass it to the sript with the batch option:

sudo ./ test batch <test-batch-file>

This will successively reboot into the RTAI kernel with the kernel parameter set to the ones specified by the KERNEL_PARAM variable and as specified in , and runs the tests as specified by the test key words that have been described above (without the "auto" command).

For example,

sudo ./ test sched kern kthreads 240 batch <test-batch-file>

would test loading of the rtai_hal and rtai_sched modules, run both the kern and kthreads tests, and abort the latency tests after 240 seconds for each configuration specified in the file .

Special lines in cause reboots into the default kernel and building an RTAI-patched kernel with a new configuration.

Format of test-batch files

In a batch file

  • everything behind a hash ('#') is a comment that is completely ignored

  • empty lines are ignored

  • a line of the format

    <descr> : <load/cpu> : <params>

    describes a configuration to be tested:

    • <descr> is a one-word string describing the kernel parameter (a description of the load settings is added automatically to the description)
    • <load/cpu> defines the load processes to be started before testing (cpu io mem net full, see above) and/or the CPUs on which the test should be run (cpu=, see above)
    • <param> is a list of kernel parameter to be used.
  • a line of the format

    <descr> : CONFIG : <file>

    specifies a new kernel configuration stored in <file>, that is compiled after booting into the default kernel. <descr> describes the kernel configuration; it is used for naming successive tests.

    Actually, <file> can be everything the -c otion is accepting. This will mostly be actual kernel configuration files, for example from the /boot/ directory, the config-* files saved along with the test results, or configuration files generated and saved by prepare. In particular

    <descr> : CONFIG : backup

    compiles a kernel with the configuration of the kernel at the beginning of the tests. This is particularly usefull as the last line of a batch file.

  • The first line of the batch file can be just

    <descr> : CONFIG :

    this sets <descr> as the description of the already existing RTAI kernel for the following tests.

Example batch file:

  noacpi : CONFIG :
  idlenohz : : idle=poll nohz=off
  nodyntics : CONFIG : config-nodyntics
  idleisol : cpu io : idle=poll isolcpus=0,1
  isol2 : io cpu=2 : isolcpus=2


./ prepare

to quickly generate various kernel configurations that you can use in a batch file.

Note: Running a test batch makes your computer practially unuseable, because it will repeatedly reboot.

If you reboot or restart your computer during a running test batch (because a test hangs), the test batch stops itself automatically. If you want to abort a running test batch then log in and run

sudo killall      # to kill the already scheduled reboot restore testbatch # to really stop the test batch

Test results

The result of the tests are written into files named latencies-HOST-RTAI-KERNEL-NNN-DATE-NAME-QUALITY in the current directory. HOST is replaced by the hostname of your machine, RTAI by your choice of an RTAI source (the value of the RTAI_DIR variable, e.g. magma, rtai-5.1, etc.), KERNEL is the kernel version (the value of the LINUX_KERNEL variable), NNN is replaced by a consecutive number, DATE is the current date, NAME and QUALITY are strings describing your kernel configuration and the test performance that is retrieved from the latency test.

View the test results with a text editor or with

less -S latencies-aeshna-rtai-5.1-4.4.115-004-2018-03-29-regularnohz-good

Along with the test results the configuration of the tested kenel is saved in the config-* files of the same name. This file can later be used in a test batch (see below) or to recreate this particular kernel configuration by means of

sudo ./ -c <config-file> reconfigure

(simply leave the kernel menu without changing anything).

The first few lines in a latencies-* file summarize the test results. This summary is used for test reports (see below Test reports):

Test summary (in nanoseconds):

RTH| general                                           | kern latencies                           | kern switches      | kern preempt                   | kthreads latencies                       | kthreads switches  | kthreads preempt               | user latencies                           | user switches      | user preempt                   | kernel
RTH| description                             | progress|  ovlmax|  avgmax|     std|     n| maxover|  susp|   sem|   rpc|       max|   jitfast|   jitslow|  ovlmax|  avgmax|     std|     n| maxover|  susp|   sem|   rpc|       max|   jitfast|   jitslow|  ovlmax|  avgmax|     std|     n| maxover|  susp|   sem|   rpc|       max|   jitfast|   jitslow| configuration
RTD| smp4smtmulticore-idle-highres-plain     | hsmk    |   17156|    2838|     824|   599|       0|   217|   240|   284|      2482|      5852|      5491|       -|       -|       -|     -|       -|     -|     -|     -|         -|         -|         -|       -|       -|       -|     -|       -|     -|     -|     -|         -|         -|         -| config-4.4.115-rtai-5.1-aeshna-025-2018-04-17-smp4smtmulticore-idle-highres-plain-ok

This is followed by a summary of the load that was applied during the tests:

Load: 8.03 7.34 4.25
  cpu: stress -c 2
  io : stress --hdd-bytes 128M -d 2
  mem: stress -m 2
  net: ping -f localhost

Then the output of the RTAI tests is listed.

This is followed by a list of loaded modules

loaded modules (lsmod):
  Module                  Size  Used by
  btrfs                 819200  0 
  xor                    24576  1 btrfs
  raid6_pq              102400  1 btrfs
  ufs                    69632  0 

and the output of /proc/interrupts:

interrupts (/proc/interrupts):
             CPU0       CPU1       CPU2       CPU3       
    0:         26          0          0          0   IO-APIC   2-edge      timer
    1:         10          0          0          0   IO-APIC   1-edge      i8042
    8:          1          0          0          0   IO-APIC   8-edge      rtc0
    9:        169          0          0          0   IO-APIC   9-fasteoi   acpi
   12:       1949          0          0          0   IO-APIC  12-edge      i8042
   16:        254          0          0          0   IO-APIC  16-fasteoi   ehci_hcd:usb1, mmc0
   19:         13          0          0          0   IO-APIC  19-fasteoi 
   23:         34          0          0          0   IO-APIC  23-fasteoi   ehci_hcd:usb2
   24:          0          0          0          0   PCI-MSI 327680-edge      xhci_hcd
   25:          7          0        397          0   PCI-MSI 409600-edge      eth0
   26:      21558          0        919          0   PCI-MSI 512000-edge      0000:00:1f.2
   27:         74          0          0          0   PCI-MSI 32768-edge      i915
   28:         25          0          0          0   PCI-MSI 360448-edge      mei_me
   29:        371          0          0          0   PCI-MSI 442368-edge      snd_hda_intel
   30:      28553          0          0          0   PCI-MSI 1572864-edge      iwlwifi
  NMI:          0          0          0          0   Non-maskable interrupts
  LOC:     685217     666148     642070     652469   Local timer interrupts
  SPU:          0          0          0          0   Spurious interrupts

Ideally you do not want any interrupt on the CPU where the RTAI test runs. When playing around with NO_HZ look at the LOC interrupts (second last line in the snippet above).

Then various information about the system, the CPU, the grub menu, and the settings used by is printed.

Finally the output of rtai_info and dmesg during the tests is shown.

Test reports

You can generate a summary report from all your tested kernels by means of

./ report | less -S                    # report of all latencies-* files
./ report tests/latencies-* | less -S  # report of all latencies-* files in tests/


./ report --help

for further options.

In particular, the -g switch produces a graphical comparison of the latency histograms.

Interpreting test results

Read, for more information on the RTAI tests.

Latency tests

The first test that is run is the latency test. The output looks like this:

RTAI Testsuite - KERNEL latency (all data in nanoseconds)
RTH|    lat min|    ovl min|    lat avg|    lat max|    ovl max|   overruns
RTD|         60|         60|        141|       1040|       1040|          0
RTD|         61|         60|        126|        372|       1040|          0
RTD|         66|         60|        127|        364|       1040|          0
RTD|         66|         60|        128|        748|       1040|          0
RTD|         92|         60|        129|        719|       1040|          0
  • Overuns are bad - you want a configuration without overuns!

    Note that the "overruns" column shows the total overruns counted from the start of the latency test. So if the overruns do not increase any more this might be tolerable.

  • "lat max" minus "lat min" should definitely be smaller than your period (default of the latency test is 100 000 ns)

  • As a rough rule of thumb, "lat max" minus "lat min"

    • less than 1000 ns is awesome
    • less than 5 000 ns is good
    • less than 10 000 ns is kind of ok
    • longer than 10 000 ns is bad on an idle machine (only running the latency test and nothing else). Under load (see below) these numbers will usually increase by several microsenconds (1000 ns).

In order to reduce latency jitter you need to improve your kernel configuration (see Improve the RTAI-patched kernel).

Switch test

Then the switch test is run:

The output looks like this:

Apr  7 16:08:45 knifefish kernel: [  178.940333] 
Apr  7 16:08:45 knifefish kernel: [  178.940333] Wait for it ...
Apr  7 16:08:45 knifefish kernel: [  178.959986] 
Apr  7 16:08:45 knifefish kernel: [  178.959986] 
Apr  7 16:08:45 knifefish kernel: [  178.959986] FOR 10 TASKS: TIME 5 (ms), SUSP/RES SWITCHES 40000, SWITCH TIME (INCLUDING FULL FP SUPPORT) 145 (ns)
Apr  7 16:08:45 knifefish kernel: [  178.959988] 
Apr  7 16:08:45 knifefish kernel: [  178.959988] FOR 10 TASKS: TIME 6 (ms), SEM SIG/WAIT SWITCHES 40000, SWITCH TIME (INCLUDING FULL FP SUPPORT) 158 (ns)
Apr  7 16:08:45 knifefish kernel: [  178.959990] 
Apr  7 16:08:45 knifefish kernel: [  178.959990] FOR 10 TASKS: TIME 7 (ms), RPC/RCV-RET SWITCHES 40000, SWITCH TIME (INCLUDING FULL FP SUPPORT) 186 (ns)

The reported switching times should be well below 1000ns.

Preemption test

Finally the preempt test is run.

Improve the RTAI-patched kernel

Usually, the RTAI performance can be easily improved via a few kernel parameter. The following sections guide you through a mostly automatized procedure that assists you in finding the best setting.

Disable CPU power saving modes

Most importantly, for CPUs like Intel i3/i5/i7 CPUs, disabling CPU power saving modes improves real-time performance dramatically. Before you try anything else do:

  • Use the kernel parameter idle=poll to disable C-state transitions completely by setting in the makertaikernel.cfg configuration file:
    This is usually sufficient for a good RTAI performance.

Basic kernel parameter

Go on with checking the following kernel parameter:

  • tsc=reliable
  • tsc=noirqtime
  • highres=off
  • nohz=off
  • elevator=noop
  • nosoftlockup=0

You get these kernel parameters for a test batch file by running

./ test batch basics

then run

./ test batch testbasics.mrk

Check the test results with

./ report | less -S

Choose the one kernel parameter with the best mean jitter and/or best max jitter (if there is one).

When choosing the best kernel parameter, keep in mind that the reported jitters could vary by about the reported standard deviation if you run the tests again. So only jitters that are better by at least one standrad deviation in comparison to another one are really better. Check the differences of the "plain" test that is run twice by testbasics.mrk. Alternatively, run the tests much longer to get more precise results.

Add this kernel parameter to the KERNEL_PARAM variable in the configuration file and comment it out in the testbasics.mrk file.

In case nosoftlockup=0 is successfull:

Isolate CPUs for real time tasks

For further improving the RTAI performance, you might want to reserve at least one CPU for RTAI on a multi-core machine. You can isolate CPUs by using the kernel parameter


(for isolating the cores no. 2 and 3). Additional parameters are

isolcpus=2,3 nohz_full=2,3 rcu_nocbs=2,3

See the file /usr/local/src/rtai/README.ISOLCPUS for more details.

You should check each of your CPUs. In particular running and isolating an RTAI task on the first CPU may result in worse performance than on the other CPUs.

In addition to isolating a CPU you also need to make sure that the RTAI tests are run on that CPU. For this you need to modify the test sources. With the script this can be easily achieved by passing the cpu=<CPUID> option:

sudo ./ test cpu=2

will run the tests on CPU 2 (the third CPU).

Running RTAI on isolated CPUs may reduce maximum jitter on an idle machine. Under load, isolation can improve mean and maximum latencies considerably.

You get these kernel parameters for a test batch file by running

./ test batch isolcpus

It will test RTAI performance for each of your CPUs with and without load. Edit the file to adapt it to the number of CPUs you have on your machine.

Note: You cannot isolate the CPU on which the system boots (usually cpu 0).

Note: If inserting rtai_hal crashes when using the isolcpus kernel parameter, disable CONFIG_X86_X2APIC (in "Procesor type an features" - "Support x2apic") and compile the kernel again.

Once you figured out which core is best go on with the test batch you get via

./ test batch nohzrcu

Set the right CPU id in this test batch. Read Documentation/timers/NO_HZ.txt for details.

Linux Torvalds says:

Recheck basic kernel parameter

With idle=poll, a basic kernel parameter set, and the best cpu isolation figured out, go on with rechecking the basic kernel parameter with and without load:

./ test cpu=1 full batch testbasics.mrk
./ test cpu=1 batch testbasics.mrk

Remember the best setting from this test batch.


  • disable all DMA transfers by setting kernel parameter libata.dma=0.
  • If you still have IDE harddrives try ide-core.nodma.
  • README.CONF_RMRKS: LINUX use of DMA can add latency, especially when it is supported in burst mode.

You get a batch file for testing these kernel parameters by running

./ test batch dma

Final testing

Choose a few parameter combinations that turned out to be most successfull. Write them into a test batch file. Run it with much longer test times (e.g. 2000 seconds, or much more if you like):

./ test 2000 batch testfinal.mrk

Further improving the RTAI-patched kernel

If your test results are still not satisfactory, then you need to pass kernel parameters to the RTAI kernel or reconfigure the kernel, probably disabling some devices, compile and install the kernel again, and compile and install rtai again. The main culprits are power saving modes, frequency scaling, interrupts, some devices and their drivers. Which ones are bad usually depends on your specific machine.

Read the file /usr/local/src/rtai/README.CONF_RMRKS for some hints. The notes below cite the README.CONF_RMRKS of RTAI-5.1.

How to modify your system

There are four levels, at which you can modify your system:

  1. The running kernel, e.g. by using the /sys interface of the kernel.
  2. On boot by passing parameter to the kernel via the boot loader.
  3. When compiling the kernel by setting a new kernel configuration.
  4. In the BIOS.

BIOS settings will affect all the kernels you run on your computer. Better is if you can achieve the same effect via a kernel configuration that will be specific for your kernel. But for this you need to compile a new kernel. Even better is if there is a kernel parameter that allows you to set the configuration at boot time. This only requires a reboot. Use these three options to find a configuration with the lowest latency jitters.

Some configurations can be achieved even in the running kernel. For conveniently using them one should write a little wrapper around the real time application (e.g. RELACS) the sets these parameters only when running the application, maybe even specific for the CPU on which the real-time task runs.

Simply check your BIOS whether there is anything of interest (e.g. hyperthreading, powersaving, force all fans to run at full speed) that you might want to try.

Modify the kernel configuration

The kernel configuration is changed in the menu that you get when building a new kernel. That is when running

sudo ./ reconfigure


sudo ./ prepare

for generating kernel configurations to be used in a test batch.

If you do not want to override your RTAI-patched kernel when reconfiguring you can give it a new name via the -n switch. You also need to supply this setting to the reboot and test action:

sudo ./ -n 2 reconfigure
sudo ./ -n 2 reboot
sudo ./ -n 2 test

Note that reconfigure will first uninstall an already existing kernel with the same name.

You can recreate a kernel or use this kernel's configuration for further modifications by specifying the kernel configuration file saved by test using the -c option

sudo ./ -c config-3.14.17-rtai-4.1-002-basic-good reconfigure

The KERNEL_MENU variable in the makertaikernel.cfg configuration file lets you choose which menu type you get.

Set kernel parameter

Kernel parameter can be passed directly to the reboot command:

./ reboot param1=xxx param2=yyy

boots the RTAI-patched kernel and passes the kernel parameter "param1=xxx param2=yyy" to it. See

./ help reboot

for further options and details.

Kernel parameter that you want to set all the time while testing can be specified via the KERNEL_PARAM variable in the makertaikernel.cfg configuration file. Also describe these parameter in the KERNEL_PARAM_DESCR variable by a single word, so that the test results can be appropriately named.

Setting kernel parameter via the KERNEL_PARAM variable is in particular usefull when running test batches.

For applying the kernel parameter permanently add them to the GRUB_CMDLINE_RTAI variable in /etc/defaults/grub and run update-grub.

There are several interesting kernel parameter that influence the real-time performance. See the file Documentation/kernel-parameters.txt in your linux kernel source tree (usually in /usr/src) for a general documentation of all available kernel parameter.

The following is a collection of various hints on possibly influential configuration parameter from all levels.

CPU power management and frequency scaling


  • Power management, see CONFIG_CPU_FREQ and CONFIG_CPU_IDLE below; on portables battery management too.
  • Recent Intel SpeedStepping and Boosting.
  • Disable CPU_FREQ.
  • Disable CPU_IDLE and INTEL_IDLE, or boot with "intel_idle.max_cstate=0". If you want to be sure to have a never sleeping CPU execute, at the lowest priority, your own, per cpu, idle task, i.e. one just doing "while(1);".
  • Disable APM and CONFIG_ACPI_PROCESSOR, but not everything related to power management. Take also into account that without ACPI enabled you might not see more than a single CPU. .


./ info cpu

for information on frequency scaling and idle states of your CPUs.

The output might look like this:

CPU topology, frequencies, and idle states (/sys/devices/system/cpu/*):
CPU topology                   CPU frequency scaling                CPU idle states (enabled fraction%)
logical  socket  core  online  freq/MHz      governor  transitions  POLL     C1-IVB   C1E-IVB  C3-IVB   C6-IVB   C7-IVB 
  cpu0        0     0       1     2.901      ondemand       278047  0  0.0%  0  3.5%  0  6.9%  0  5.8%  0  0.0%  0 83.6%
  cpu1        0     0       1     1.800      ondemand       303763  0  0.0%  0  6.1%  0  7.2%  0  3.6%  0  0.0%  0 82.9%
  cpu2        0     1       1     1.200      ondemand       251421  0  0.0%  0  3.5%  0  6.6%  0  4.3%  0  0.0%  0 85.4%
  cpu3        0     1       1     1.800      ondemand       297520  0  0.0%  0  6.1%  0  7.1%  0  3.6%  0  0.0%  0 83.1%


CPU (/proc/cpuinfo):
  model name        :  Intel(R) Core(TM) i7-3520M CPU @ 2.90GHz
  number of CPUs    : 4
  max CPU frequency : 2.901 MHz
  CPU family        : 6
  machine (uname -m): x86_64
  memory (free -h)  : 7.5G RAM

Here, CPU frequency scaling is active ("ondemand" governor and different frequencies in the 5-th column, that are below the maximum possible CPU frequency at "max CPU frequency" below). Also, CPU powersaving is enabled because there are "CPU idle states" available ("POLL", "C1-IVB", etc.).

Disable powermanagement and frequency scaling completely

You are on the safe side when you configure the ACPI properties of your kernel as follows (as you did according to Basic kernel configuration):

In "Power management and ACPI options":

  • In "ACPI (Advanced Configuration and Power Interface) Support":
    • Disable "Processor" (ACPI_PROCESSOR)
  • Disable "CPU Frequency scaling" (CPU_FREQ)
  • In "CPU Idle":
    • Disable "CPU idle PM support" (CPU_IDLE)

This disables all OS controlled powersavings on your CPU. But you certainly need to add the idle=poll kernel parameter to keep the CPU in C0 state at maximum frequency.

Disable frequency scaling only

Instead you can try to only disable in "Power management and ACPI options":

  • Disable "CPU Frequency scaling" (CPU_FREQ)

In addition, make sure your CPU stays in C0 idle state by passing idle=poll to the kernel parameter to keep the CPUs in C0 state via an polling idle loop, or write a zero to the /dev/cpu_dma_latency file (the DynamicClampAnalogInput plugin of RELACS can do that). This also keeps the CPU frequency at its maximum but makes the system run hot.

Kernel parameter intel_pstate=disable disables intel_pstate as the default scaling driver. intel_pstate bypasses the cpufreq frequency governors (CONFIG_X86_INTEL_PSTATE).

performance governor sets CPU frequency to the highest available one.

Further aspects of power saving options

Also check the BIOS for disabling CPU power management.

All the other kernel parameter that control CPU idle states are usually not sufficient:

  • idle=halt idle cpus enter at maximum the C1 state, higher C-states are disabled. CPU frequency stays close at maximum.
  • intel_idle.max_cstate=1 this leaves us with TWO C-states (POLL and C1)!
  • intel_idle.max_cstate=0 this disables the intel_idle driver and switches to acpi_idle with several C-states
  • processor.max_cstate=1 on its own has no effect when the intel_idle driver is active
  • intel_idle.max_cstate=0 processor.max_cstate=0 sames as processor.max_cstate=1
  • intel_idle.max_cstate=0 processor.max_cstate=1 disables C-states higher than C1 like idle=halt but cpu frequency might go lower.
  • intel_idle.max_cstate=0 processor.max_cstate=2 this leaves you with three C-states (POLL, C1, C2)
  • intel_pstate=disable seems to disable frequency scaling ???

C-states can be nicely monitored with the i7z or powertop programs (as root). They also show CPU core temperature (they should be well below 100 degrees celsius). For these programs to work, make sure that you have the following options enabled in the kernel configuration:

In "Processor type and features":

  • Enable "/dev/cpu/ * /msr - Model-specific register support"
  • Enable "/dev/cpu/ * /cpuid - CPU information support"

Check frequency scaling of CPUs with cpufreq-info from the cpufrequtils package.


./ test batch cstates

you can generate a test-batch file to check these kernel parameter.

For more information on CPU power management and frequency scaling read in the Documentation/ folder in the kernel source

  • cpuidle/sysfs.txt
  • cpu-freq/cpufreq-stats.txt
  • cpu-freq/boost.txt
  • cpu-freq/user-guide.txt
  • cpu-freq/governors.txt
  • cpu-freq/intel-pstate.txt
  • power/pm_qos_interface.txt

More infos:



  • Under SMP set the number of CPUs equal to the real ones and have it matched in RTAI, no hyperthreading intended.
  • Even if RTAI can work with hyperthreading enabled, such an option is deprecated as a possible cause of latency; in any case try and verify if it is acceptable, with your hardware and for your applications.

So, check whether you have hyperthreading. Run

./ info cpus

The top of the output looks like this (Intel i7-4770):

CPU topology, frequencies, and idle states (/sys/devices/system/cpu/*):
cpu topology                   cpu frequency scaling
logical  socket  core  online  freq/MHz      governor
  cpu0        0     0       1     0.800      ondemand
  cpu1        0     1       1     0.800      ondemand
  cpu2        0     2       1     0.800      ondemand
  cpu3        0     3       1     0.800      ondemand
  cpu4        0     0       1     0.800      ondemand
  cpu5        0     1       1     1.000      ondemand
  cpu6        0     2       1     1.200      ondemand
  cpu7        0     3       1     1.400      ondemand

If in the "core" column the numbers appear twice or more often (like in the example shown), then you run in hyperthreading mode (as is the case in the example).

By reducing the number of CPUs to four, you can eliminate hyperthreading (see below).

If the topology looks like this (Intel i7-3520M):

CPU topology, frequencies, and idle states (/sys/devices/system/cpu/*):
cpu topology                   cpu frequency scaling
logical  socket  core  online  freq/MHz      governor
  cpu0        0     0       1     2.700      ondemand
  cpu1        0     0       1     2.901      ondemand
  cpu2        0     1       1     2.901      ondemand
  cpu3        0     1       1     2.901      ondemand

the trick to simply reduce the number of cpus to the number of actual cores does not work. You need to switch off hyperthreading in the BIOS.

This is an example of a machine without hyperthreading (Intel i5-3570):

CPU topology, frequencies, and idle states (/sys/devices/system/cpu/*):
cpu topology                   cpu frequency scaling
logical  socket  core  online  freq/MHz      governor
  cpu0        0     0       1     3.400     powersave
  cpu1        0     1       1     3.144     powersave
  cpu2        0     2       1     2.863     powersave
  cpu3        0     3       1     3.260     powersave

In this case you do not need to do anything.

This is what you need to do when configuring the kernel:

  • In "Processor type and features":
    • In case you have a uniprocessor system, deselect "Symmetric multi-processing support" (SMP)
    • Set the "Maximum numbers of CPUs" (NR_CPUS) to the number of physical cores you have in your machine. The automatically sets the same number of CPUs for the RTAI configuration.

You can also do this via the kernel parameter:

  • nr_cpus=<n> Maximum number of processors that an SMP kernel could support. n >= 1 limits the kernel to supporting 'n' processors. Later in runtime you can not use hotplug cpu feature to put more cpu back to online. Just like you compile the kernel NR_CPUS=n.

Alternatively, you could switch off individual CPUs:

echo 0 > /sys/devices/system/cpu/cpuX/online

When I tried this, however, the computer crashed when inserting rtai_hal (even if CONFIG_RTAI_CPUS is adapted to the lower CPU count).

For more information see:

The related kernel configs in the "Processor type and features" submenu

  • "SMT (Hyperthreading) scheduler support"
  • "Multi-core scheduler support"

do not influence RTAI performance.

Cached memory disruption


  • Cached memory disruption can add significant latencies, till the cache becomes hot again, experienced first hand after a far jump in the code and data in a digital controller. . ???

Select processor family


  • If unsure on the CPU to choose, care of setting one featuring the Time Stamp Clock (TSC), which means no 486 and "false" i586, since generic INTEL i586 compatibles often do not have a TSC, while true INTEL ones do have it.

Check your processor by running

./ info cpus

The last part looks like this:

CPU (/proc/cpuinfo):
  model name    : Intel(R) Core(TM) i7-3520M CPU @ 2.90GHz
  number of cpus: 4
  cpu family: 6
  cpuidle driver: intel_idle
  machine (uname -m): x86_64
  memory (free -h)  : 7.5G RAM

Select your processor in the kernel configuration:

  • "Processor type and features":
    • "Processor family" In the kernel menu check "help" to find out, which processor family you have to select. Processors with "cpu family : 6" are "Core 2/newer Xeon".

Low-latency kernel configuration

So-called low-latency kernels have the following two settings:

  • "Processor type and features":
    • Select "Preemption Model (Preemptible kernel (Low-Latency Desktop))" (PREEMPT)
    • Set "Timer frequency" to 1000Hz (CONFIG_HZ_1000=y and CONFIG_HZ=1000) This has, however, no effect on the RTAI performance! The two parameter control how responsive Linux processes are, but not how quickly RTAI can intersept the Linux kernel.

Disable device drivers you do not need

A good strategy is to disable as many as possible device drivers. See


for listing all the currently loaded kernel modules. Or

lsmod -k

for a list of PCI devices on your system and their associated kernel modules.

When configuring the kernel you can hit '/', enter a search term (the module name) and you get a list of matching configuration parameters.

More information on how select/deselect device drivers for RTAI kernel can be found at

Devices to consider are:

  • Disable DRM:

    • Device Drivers:
      • Graphics support:
        • Disable "Direct Rendering Manager"
  • Check video cards and graphic acceleration


    • Some peripherals, e.g. video cards, may stall CPUs attempting to access IO space. Verify "what ifs" related to graphic acceleration, likely better if disabled. Consider also if X term usage is really needed. If possible avoid it, especially in production work.
    • Any initialization of the device drivers, or anything related to the hardware, may lead to high latencies, e.g., but not always. doing "startx &" while a real time application is running. Once it is started there should be no major problems. If the truoble persists and you really need X, concurrently with your RTAI tasks, try disabling hardware graphic acceleration. The best latencies usually come with no graphic application running.
  • Disable 'Error Detection and Correction (EDAC) units' (

    • Device Drivers:
      • Disable "EDAC (Error Detection And Correction) reporting"
  • Device Drivers:

    • Keep "Real Time Clock"
    • "Generic Dynamic Voltage and Frequency Scaling (DVFS) support" does not matter
    • sound might be ok
  • You probably do not need bluetooth or WLAN! (but as long as you do not use it it might not hurt...):

    • Device Drivers:
      • Network device support:
        • Disable "Wireless LAN"
    • Networking support:
      • Disable "Bluetooth subsystem support"
      • Disable "Wireless"
  • USB:

    • According to README.CONF_RMRKS from rtai-5.0.1: "Do not disable USB, but just any legacy support, possibly in the BIOS also. Once upon a time old USB was a source of high RTAI latencies. Now that should be legacy support."
    • If you get the error "usb: device not accepting address" add noapic to the kernel parameter.

Some more hints for kernel configuration parameters

Here is a list of some kernel configuration parameter that you might try to improve your real-time perfomance (low latencies):

  • In "Device drivers" keep "Multiple devices driver support (RAID and LVM)" and in there keep "Device mapper support" (somehow needed for grub).

  • In "Processor type and features": disabling the following seems to have an effect:

    • Disable "Supervisor Mode Access Prevention"
    • Disable "EFI runtime service support"
    • Disable "Enable seccomp to safely compute untrusted bytecode"
  • NUMA (disabling seems to improve maximum latencies):

    • General setup:
      • Disabel "Memory placement aware NUMA scheduler"
    • Processor type and features:
      • Disable "Numa Memory Allocation and Scheduler Support"
  • Maybe also try:


Kernel parameter

Here is a list of some more potentially influential kernel parameter:

Advanced configuration and power interface (ACPI):

  • acpi=off # often very effective, but weired system behavior
  • acpi=noirq
  • pci=noacpi
  • pci=nomsi With disabled acpi your rtai-patched linux kernel might not properly halt or reboot. Try reboot=triple as a kernel parameter. See /usr/src/linux/Documentation/x86/x86_64/boot-options.txt for more options for the reboot parameter.

You get these kernel parameters for a test batch file by running

./ test batch acpi

Advanced programmable interrupt controller (APIC):

  • noapic
  • nolapic , usually not a good idea, becaus RTAI uses the lapic timer.
  • lapic

You get these kernel parameters for a test batch file by running

./ test batch apic

After restart, check for the number of CPUs - they might be reduced if you disabled too much ACPI:

./ info cpu

Disable SMI interrupts

Finally, there are the evil SMIs. They might periodically produce some long latencies. See /usr/local/src/rtai/base/arch/x86/calibration/README.SMI and README.SMISPV for details.

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