How to tweak the performance of BeagleBone Black (with realtime)

2021-04-26T15:39:54.175000Z

How to tweak real time performance on a BeagleBone Black?

In the documentation of the Add-on "CODESYS Control for BeableBone SL", under point "Preparing the PLC", there is the hint to deactivate "Real-Time group scheduling". There is a code segment:

CONFIG_RT_GROUP_SCHED=n

but I don't know were to put (very weak doku here). The only place I found it, was in the kernel configuration files, found in /boot/config-<kernel name="">. But these file must not be edited.</kernel>

In the default kernel configuration files (ti, non-rt, e.g. "config-4.19.94-ti-r42") there is an entry like this:

CONFIG_RT_GROUP_SCHED=y

In other kernel configuration files (bone, non-rt, e.g. "config-4.19.165-bone59" there is an entry like this:

# CONFIG_RT_GROUP_SCHED is not set

And in the kernel configuration files of the pre-compiled realtime kernels, there is no entry like this at all.

I followed the next hint to uninstall the package "libpam-systemd" by shell command "sudo apt-get remove libpam-systemd".

At least this recommendation is only for BeagleBone Black and not for Raspberry Pi. (confused)

Next point was to install a precompiled realtime kernel (e.g. 4.19.165-bone-rt-r59) like described at https://elinux.org/EBC_Exercise_36_PREEMPT_RT.

I did the main program in a freewheeling task with priority 16 (low linux default priority), and the time relevant program in a separate task with priority 0 (highest realtime priority) and interval 500 µs.

In spite of definitely sawing a drop in jitter after doing this, I want to know if there are any other tweaks to enhance the performance.

On the other side, I found a post which claims (Edwin cited) to have these entries in the CODESYSControl_User.cfg file, when no realtime is needed. But I found these entries in the actual CODESYSControl.cfg file (in directory /etc/):

[CmpSchedule]
ProcessorLoad.Enable=1
ProcessorLoad.Maximum=95
ProcessorLoad.Interval=5000
DisableOmittedCycleWatchdog=1

Source: https://forge.codesys.com/forge/talk/Runtime/thread/e82256d0d8/#e81e

This information confuses me and implicates, that other values are necessary when realtime is needed.
Questions:
- Why these entries without realtime?
- Which entries with realtime?

With the library "CmpSchedule" and the function "SchedGetProcessorLoad" I can get the information about the CPU usage of the (realtime) kernel threads (60 % right now). But the total CPU usage out of "htop" is about 75 % (because of the additional normal threads). Is there a possibility to also get the CPU usage of non kernel/realtime tasks? Or the overall CPU usage? Or the CPU load averages?

I was a bit confused when it looked like the CPU is clocked with possibly higher frequency, if the value of parameter "_diMaxSpeed" from the SPI_master is set higher (than expected to be):
I've read that the BeagleBone Black should only have a SPI capability of 500 kHz. A lower values resulted in a CPU usage overload via watchdog. (After all it's a result of CPU seems to get slower.) So I decided to check higher values and saw the cycle time getting faster, till a threshold is reached. Now the SPI master works fine with 2 MHz for "SPI max speed" parameter. (Don't understand that after all)

Recent posts to How to tweak the performance of BeagleBone Black (with realtime)

How to tweak the performance of BeagleBone Black (with realtime)