Process Scheduling

• The following calls manipulate parameters that set the scheduling algorithm and priorities associated with a process.

  #include <sched.h>
  int sched_setscheduler(pid_t pid, int policy,
  const struct sched_param *p);
  int sched_getscheduler(pid_t pid);
  struct sched_param {
  ...
  int sched_priority;
  ...
  };
  #include <unistd.h>
  int nice(int inc);
  #include <sys/time.h>
  #include <sys/resource.h>
  int getpriority(int which, int who);
  int setpriority(int which, int who, int prio);
  #include <sched.h>
  int sched_get_priority_max(int policy);
  int sched_get_priority_min(int policy);
  

• A process with a higher static priority will always preempt a process with a lower static priority.
• For the traditional scheduling algorithm, processes within static priority 0 will be allocated time based on their dynamic priority (nice value).
• The system calls sched_setscheduler and sched_getscheduler are used to set or get, respectively, the scheduling policy and parameters (set only) associated with a particular process.
• The scheduling policy, policy, is one of
  • SCHED_OTHER (the default universal time-sharing scheduler policy used by most processes ),
  • SCHED_FIFO,
  • SCHED_RR.
• The latter two specify special policies for time critical applications and will preempt processes using SCHED_OTHER.
• A SCHED_FIFO process can only be preempted by a higher priority process, but a SCHED_RR process will be preempted if necessary to share time with other processes at the same priority.
• Linux schedules the parent and child processes independently; there's no guarantee of which one will run first, or how long it will run before Linux interrupts it and lets the other process (or some other process on the system) run.
• In particular; none, part, or all of the ls command may run in the child process before the parent completes.
• You may specify that a process is less important and should be given a lower priority by assigning it a higher niceness value.
• By default, every process has a niceness of zero.
• A higher niceness value means that the process is given a lesser execution priority; conversely, a process with a lower (that is, negative) niceness gets more execution time.

  $ nice -n 10 sort input.txt > output.txt
  

• You can use the renice command to change the niceness of a running process from the command line.
• Only a process with root privilege can run a process with a negative niceness value or reduce the niceness value of a running process.