The Oracle Solaris kernel has three ranges of dispatching priority. The highest-priority range (100 to 159) corresponds to the Realtime (RT) scheduling class. The middle-priority range (60 to 99) corresponds to the system (SYS) scheduling class. The system class cannot be applied to a user process. The lowest-priority range (0 to 59) is shared by the timesharing (TS), interactive (IA), fair-share (FSS), and fixed priority (FX) scheduling classes.
A scheduling class is maintained for each LWP. When a process is created, the initial LWP inherits the scheduling class and priority of the creating LWP in the parent process. As more threads are created, their associated LWPs also inherit this scheduling class and priority.
Threads have the scheduling class and priority of their underlying LWPs. Each LWP in a process can have a unique scheduling class and priority that are visible to the kernel.
Thread priorities regulate contention for synchronization objects. By default, LWPs are in the timesharing class. For compute-bound multithreading, thread priorities are not very useful. For multithreaded applications that use the MT libraries to do synchronization frequently, thread priorities are more meaningful.
The scheduling class is set by priocntl(2). How you specify the first two arguments determines whether only the calling LWP or all the LWPs of one or more processes are affected. The third argument of priocntl() is the command, which can be one of the following commands.
PC_GETCID - Get the class ID and class attributes for a specific class.
PC_GETCLINFO - Get the class name and class attributes for a specific class.
PC_GETPARMS - Get the class identifier and the class-specific scheduling parameters of a process, an LWP with a process, or a group of processes.
PC_SETPARMS - Set the class identifier and the class-specific scheduling parameters of a process, an LWP with a process, or a group of processes.
The user-level priority of an LWP is its priority within its class, not its dispatch priority. This does not change over time except by the application of the priocntl() system call. The kernel determines the dispatch priority of an LWP based on its scheduling class, its priority within that class, and possibly other factors such as its recently-used CPU time.