Real-time properties of Linux¶
Regular Linux is not a real-time operating system. Profinet have rather strict timing constraints. For example a Simatic PLC (with default settings) will send an alarm if no incoming Profinet frame is received for 7-8 ms. So it is important that Linux is responsive enough.
There are a few methods that can be used to improve the Linux responsiveness. The most important is to use FIFO scheduling and to isolate the application on a separate CPU core.
Use USE_SCHED_FIFO option¶
Select the FIFO Linux kernel scheduling option. This is done by passing
-DUSE_SCHED_FIFO=ON command line argument to cmake.
Run the application on a separate processor core¶
It is possible to tell the Linux kernel not to put any processes on a specific processor core. This assumes that you have more than one core in your CPU. Check it using:
By setting the
isolcpus=2 Linux boot command, the kernel will not put any
processes on CPU core number 2 automatically. This option is typically set from
the boot loader.
The best way to check which CPU cores that are currently isolated:
To see which kernel command line options that have been used:
Put your Profinet application on the isolated CPU core. It is done using:
taskset -c 2 pn_dev
-c 2 tells which CPU core to use.
By applying the real-time patches (PREEMPT_RT) the real-time properties can be improved.
For more details, see:
It is important to write you application so that it can use the benefits of the real-time patches. This includes running in a separate thread and setting the priorities properly.
For the real-time patches to have an effect on p-net, set the
Increase application cycle time¶
For testing, you can increase the cycle time from the PLC in order to reduce the time-out problems. Also the allowed number of missed frames can be increased in the PLC settings.