It sets different priorities for each thread. The generate threads run at PRIO_MIN. The other threads rarely take any CPU and run at normal.
#define THREAD_PRIORITY_LOWEST PRIO_MIN
#define THREAD_PRIORITY_BELOW_NORMAL 2
#define THREAD_PRIORITY_NORMAL 0
The priorities converted from Windows priorities were probably from a table like this:
"The following table shows the mapping between nice values and Win32 priorities. Refer to the Win32 documentation for SetThreadPriority() for more information on Win32 priority issues.
nice value Win32 Priority
-20 to -16 THREAD_PRIORITY_HIGHEST
-15 to -6 THREAD_PRIORITY_ABOVE_NORMAL
-5 to +4 THREAD_PRIORITY_NORMAL
+5 to +14 THREAD_PRIORITY_BELOW_NORMAL
+15 to +19 THREAD_PRIORITY_LOWEST"
If you have better values, suggestions welcome.
Also, there was some advice on the web that PRIO_PROCESS is used on Linux because threads are processes. If that's not true, maybe it accounts for unexpectedly setting the priority of the whole app.
// threads are processes on linux, so PRIO_PROCESS affects just the one thread
setpriority(PRIO_PROCESS, getpid(), nPriority);