Module Unix.Scheduler

module Policy : sig ... end
val set : pid:Core_kernel.Pid.t option -> policy:Policy.t -> priority:int -> unit

See man sched_setscheduler.

The priority supplied here is *not* the nice value of a process. It is the "static" priority (1 .. 99) used in conjunction with real-time processes. If you want to set the nice value of a normal process, use Linux_ext.setpriority or Core_unix.nice.