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CSE451 Scheduling Spring 2001

This lecture discusses the importance of scheduling algorithms in operating systems and explores different strategies such as first-come-first-served, shortest job first, priority scheduling, round-robin, and multi-level queue. It also covers the impact of multiple processors on scheduling decisions.

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CSE451 Scheduling Spring 2001

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  1. CSE451 Scheduling Spring 2001 Gary Kimura Lecture #6 April 6, 2001

  2. Today • Finish comparing user and kernel threads • Whether it be processes or threads that are the basic execution unit the OS needs to some way of scheduling each process time on the CPU • Why? • To make better utilization of the system

  3. Goals for Multiprogramming • In a multiprogramming system, we try to increase utilization and throughput by overlapping I/O and CPU activities. • This requires several os policy decisions: • Determine the multiprogramming level -- the number of jobs loaded in primary memory • Decide what job is to run next to guarantee good service • These decisions are long-term and short-term scheduling decisions, respectively. • Short-term scheduling executes more frequently, changes of multiprogramming level are more costly.

  4. Scheduling • The scheduler is a main component in the OS kernel • It chooses processes/threads to run from the ready queue. • The scheduling algorithm determines how jobs are scheduled. • In general, the scheduler runs: • When a process/thread switches from running to waiting • When an interrupt occurs • When a process/thread is created or terminated • In a preemptive system, the scheduler can interrupt a process/thread that is running. • In a non-preemptive system, the scheduler waits for a running process/thread to explicitly block

  5. Preemptive and Non-preemptive Systems • In a non-preemptive system the OS will not stop a running jobs until the job either exists or does an explicit wait • In a preemptive system the OS can potentially stop a job midstream in its execution in order to run another job • Quite often a timer going off and the current jobs time-slice or quantum being exhausted will cause preemption

  6. Preemptive and Non-preemptive System (continued) • I cannot over emphasize the need to understand the difference between preemptive and non-preemptive systems • Preemptive systems also come in various degrees • Preemptive user but non-preemptive kernel • Preemptive user and kernel • This affects your choice of scheduling algorithm, OS complexity, and system performance

  7. Scheduling Algorithms Criteria • There are many possible criteria for evaluating a scheduling algorithm: • CPU utilization • Throughput • Turnaround time • Waiting time • Response time • In an interactive system, predictability may be more important than a low average but high variance. • One OS goal is to give applications the illusion they are running unhindered by other jobs sharing the CPU and memory

  8. Various Scheduling Algorithms • First-come First-served • Shortest Job First • Priority scheduling • Round Robin • Multi-level queue • We’ll examine each in turn

  9. Scheduling AlgorithmsFirst-Come First-Served • First-come First-served (FCFC) (FIFO) • Jobs are scheduled in order of arrival to ready Q • Typically non-preemptive • Problem: • Average waiting time can be large if small jobs wait behind long ones • May lead to poor overlap of I/O and CPU time B Job A C B C Job A

  10. Scheduling AlgorithmsShortest Job First • Shortest Job First (SJF) • Choose the job with the smallest (expected) CPU burst • Provability optimal min. average waiting time • Problem: • Impossible to know size of CPU burst (but can try to predict from previous activity) • Can be either preemptive or non-preemptive • Preemptive SJF is called shortest remaining time first

  11. Scheduling AlgorithmsPriority Scheduling • Priority Scheduling • Choose next job based on priority • For SJF, priority = expected CPU burst • Can be either preemptive or non-preemptive • Problem: • Starvation: jobs can wait indefinitely • Solution to starvation • Age processes: increase priority as a function of waiting time

  12. Scheduling AlgorithmsRound Robin • Round Robin • Used for timesharing in particular • Ready queue is treated as a circular queue (FIFO) • Each process is given a time slice called a quantum • It is run for the quantum or until it blocks • Problem: • Frequent context switch overhead

  13. Scheduling AlgorithmsMulti-level Queues • Multi-level Queues • Probably the most common method used • Implement multiple ready Queues based on the job priority • Multiple queues allow us to rank each job’s scheduling priority relative to other jobs in the system • Windows NT/2000 has 32 priority levels • Each running job is given a time slice or quantum • After each time slice the next job of highest priority is given a chance to run • Jobs can migrate up and down the priority levels based on various activities

  14. Multiple Processors • Multiple processors (i.e., more than one CPU on the system) changes some of the scheduling strategies • There are both heterogeneous systems and homogeneous systems • There is also the notion of asymmetric multiprocessing and symmetric multiprocessing • SMP is great but when we look at synchronization and locks we’ll understand why it is difficult to actually pull off.

  15. Next time • With so much potentially going on in the system how do we synchronize all of it?

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