I am working on a bulk image processing program which does not require user attention and need to choose a priority level for this process.
The superuser is able to set values from PRI_MIN (-20) to PRI_MAX (+20) and normal users can set from 0 to PRI_MAX. The default level is zero.
On Mac OS X 10.11, the man nice(1) documentation discusses priority levels and notes conformance to POSIX.2 (IEEE Std 1003.2-1992). However this documentation is not publicly available and, also, has been superseded.
Other than randomly picking a number between 0 and 20 inclusive, is there a prescribed, meaningful way to choose the priority level based on some POSIX or Mac standard?
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Answer
The batch processing priority is often OS specific and generally is not covered by POSIX.
Check the sched.h header, and the function sched_get_priority_min. This is a newer more advanced interface, compared to the nice.
Linux additionally supports SCHED_BATCH and SCHED_IDLE policies, while Mac OS X provides entirely different interface called Grand Central Dispatch with DISPATCH_QUEUE_PRIORITY_BACKGROUND.