I am using some QThread based worker threads in KDE Neon 18.04 (based on Ubuntu 18.04, Kernel 4.15.0-46-generic). The worker threads interfered with my desktop applications so I decided to reduce their priority.
The Qt documentation of QThread::start(priority) says:
The effect of the priority parameter is dependent on the operating system's scheduling policy. In particular, the priority will be ignored on systems that do not support thread priorities (such as on Linux, see http://linux.die.net/man/2/sched_setscheduler for more details).
After reading the above documentation I expected priorities would have no effect on my Linux system. Still I gave it a try. And guess what - it worked perfectly.
So, why does the Qt documentation state there would be no thread priorities on Linux? And why does it work anyway?
Depending on which flavour of Linux/Unix/*Nix you use the scheduler may or may not support it. As far as I'm aware the majority of Qt's priority levels are supported on most Linux systems now, but not all of the priority levels. I suspect the documentation says it's unsupported so they don't need to list every combination of OS variant and scheduler variant that do support priority levels and which levels are supported.
You can validate it has created it with the correct priority by using htop or top and processing with awk: https://unix.stackexchange.com/questions/19301/what-is-a-command-to-find-priority-of-process-in-linux