Operating Systems

2.5: Process Scheduling

Scheduling processes on the processor is often called ``process scheduling'' or simply ``scheduling''.

The objectives of a good scheduling policy include

• Fairness.
• Efficiency.
• Low response time (important for interactive jobs).
• Low turnaround time (important for batch jobs).
• High throughput [the above are from Tanenbaum].
• More ``important'' processes are favored.
• Interactive processes are favored.
• Repeatability. Dartmouth (DTSS) ``wasted cycles'' and limited logins for repeatability.
• Fair across projects.
  • ``Cheating'' in unix by using multiple processes.
  • TOPS-10.
  • Fair share research project.
• Degrade gracefully under load.

Recall the basic diagram describing process states

For now we are discussing short-term scheduling, i.e., the arcs connecting running <--> ready.

Medium term scheduling is discussed later.

Preemption

It is important to distinguish preemptive from non-preemptive scheduling algorithms.

• Preemption means the operating system moves a process from running to ready without the process requesting it.
• Without preemption, the system implements ``run to completion (or yield or block)''.
• The ``preempt'' arc in the diagram.
• We do not consider yield (a solid arrow from running to ready).
• Preemption needs a clock interrupt (or equivalent).
• Preemption is needed to guarantee fairness.
• Found in all modern general purpose operating systems.
• Even non preemptive systems can be multiprogrammed (e.g., when processes block for I/O).

Deadline scheduling

This is used for real time systems. The objective of the scheduler is to find a schedule for all the tasks (there are a fixed set of tasks) so that each meets its deadline. The run time of each task is known in advance.

Actually it is more complicated.

• Periodic tasks
• What if we can't schedule all task so that each meets its deadline (i.e., what should be the penalty function)?
• What if the run-time is not constant but has a known probability distribution?

We do not cover deadline scheduling in this course.

The name game

There is an amazing inconsistency in naming the different (short-term) scheduling algorithms. Over the years I have used primarily 4 books: In chronological order they are Finkel, Deitel, Silberschatz, and Tanenbaum. The table just below illustrates the name game for these four books. After the table we discuss each scheduling policy in turn.

Finkel  Deitel  Silbershatz Tanenbaum
-------------------------------------
FCFS    FIFO    FCFS        FCFS
RR      RR      RR          RR
PS      **      PS          PS
SRR     **      SRR         **    not in tanenbaum
SPN     SJF     SJF         SJF
PSPN    SRT     PSJF/SRTF   --    unnamed in tanenbaum
HPRN    HRN     **          **    not in tanenbaum
**      **      MLQ         **    only in silbershatz
FB      MLFQ    MLFQ        MQ

Remark: For an alternate organization of the scheduling algorithms (due to Eric Freudenthal and presented by him Fall 2002) click here.

First Come First Served (FCFS, FIFO, FCFS, --)

If the OS ``doesn't'' schedule, it still needs to store the list of ready processes in some manner. If it is a queue you get FCFS. If it is a stack (strange), you get LCFS. Perhaps you could get some sort of random policy as well.

• Only FCFS is considered.
• Non-preemptive.
• The simplist scheduling policy.
• The most efficient usage of cpu since the scheduler is very fast.

Round Robin (RR, RR, RR, RR)

• An important preemptive policy.
• Essentially the preemptive version of FCFS.
• The key parameter is the quantum size q.
• When a process is put into the running state a timer is set to q.
• If the timer goes off and the process is still running, the OS preempts the process.
  • This process is moved to the ready state (the preempt arc in the diagram), where it is placed at the rear of the ready list (a queue).
  • The process at the front of the ready list is removed from the ready list and run (i.e., moves to state running).
• When a process is created, it is placed at the rear of the ready list.
• As q gets large, RR approaches FCFS
• As q gets small, RR approaches PS (Processor Sharing, described next)
• What value of q should we choose?
  • Trade-off
  • Small q makes system more responsive.
  • Large q makes system more efficient since less process switching.

Homework: 26, 35, 38.

Homework: Give an argument favoring a large quantum; give an argument favoring a small quantum.

Process	CPU Time	Creation Time
P1	20	0
P2	3	3
P3	2	5

Homework: (Remind me to discuss this last one in class next time): Consider the set of processes in the table below. When does each process finish if RR scheduling is used with q=1, if q=2, if q=3, if q=100. First assume (unrealistically) that context switch time is zero. Then assume it is .1. Each process performs no I/O (i.e., no process ever blocks). All times are in milliseconds. The CPU time is the total time required for the process (excluding any context switch time). The creation time is the time when the process is created. So P1 is created when the problem begins and P3 is created 5 milliseconds later. If two processes have equal priority (in RR this means if thy both enter the ready state at the same cycle), we give priority (in RR this means place first on the queue) to the process with the earliest creation time. If they also have the same creation time, then we give priority to the process with the lower number.