• Dates: 1/30-2/6.
• Reading: Ch6.

Scheduling Concepts: Why?

• why scheduling
  • view from task side:
    CPU vs. I/O bursts
    meet demand without much delay
    "time" performance
  • resource side:
    single CPU and multiple I/O devices
    efficient resource utilization
  • focus: CPU scheduling (why?)
• effect of scheduling
• better for both sides

• characteristics of jobs/bursts:
• I/O bursts, relatively long
  • waiting for I/O to finish
  • parallelism in devices and control
  • simple queue
• CPU bursts
  • typical distribution: Fig 6.2 (p.153)
  • exponential (what is it like?)
  • hyper-exponential (?)
  • minimal task
  • few long ones
  • implications on scheduling algorithms
  • measurement/prediction of burst length?
    => implications on scheduling algorithms

Scheduling Activities and Criteria

• characteristics of process scheduling:
• CPU scheduling, short-term scheduling
  • queuing discipline and capacity
  • priority: explicit vs implicit
  • preemptive?
  • handling CPU bursts?
  • duration: ready, select, dispatch, running
  • selection: scheduling algorithm
  • dispatching: switch context as well as mode, pick up from where left off
• job mix => long term scheduling
• other situations, medium term scheduling before

• scheduling criteria (<- effect?)
  • both sides: user/task side + system/resource side
  • performance measures
  • utilization measures
• commonly used criteria
  • CPU utilization (and other utilization)
  • throughput
  • turnaround time
  • waiting time
  • response time
• commonly used measures
  • time: direct measure
  • utilization measured over time
  • throughput measured over time per process
• system performance:
  • use above criteria/measures
  • minimize time
  • maximize throughput and utilization
  • average vs worst case measure
• Gantt charts
  to illustrate algorithms and
  to determine performance
• evaluation techniques, more later

Basic Scheduling Algorithms

• commonly used algorithms
  • FIFO or FCFS
  • SJF
  • round robin (RR)
  • other
• FIFO example
• main advantage: fairness and simplicity
• main problem with FIFO: performance
• dealing with problem => other scheduling algorithms

• SJF: shortest job first
  • dealing with FIFO performance problem
  • motivation and example
  • optimality: sketch of proof
  • non-preemptive, no added context switching
  • difficulty: "shortest"? how to determine?
  • estimate and prediction methods
  • based on time-series (AR, and MA)
  • "exponential" model,
    weights on last observation and estimates
    formula: p.159
    sample effect: Fig 6.3, p.160
  • starvation possibility

• RR: round robin
  • time quantum (tq)
  • balance (fast) performance and fairness
  • FIFO with preemption
  • example
  • key decision: tq selection
    • two small: context switching cost up
    • two large: poor performance
    • decision: most done without context switching
    • special case, fast context switching, small tq
      processor sharing: 1/n of the processor
      example: multiple set of registers
      implementation connection to threads

Other algorithms and implementation concerns

• general prioritized and/or pre-emptive scheduling
  • concept and example
  • setting the priority:
    internal: computation on resource, type, job mix, etc.
    external: importance, $, account, etc.
  • starvation problem: indefinite blocking
  • aging solution
  • preemptive SJF: SRTF

• Other scheduling algorithms and mixed queues
  • mixing FIFO with priority
    example in Fig 6.6 (p.166)
  • aging implementation in above case
  • generalize: feedback queues
    example in Fig 6.7 (p.168)
  • general mixed queues
    • selection within individual queues
    • selection among the queues
    • swapping
    • feedback scheme
  • added overhead and complexity

• other scheduling
  • multiprocessor scheduling
    similar to multi-queue scheduling with added complexity
  • real-time scheduling
    time-bound
  • thread scheduling
    same basic concept
    examples in textbook

Algorithm Evaluation

• evaluation techniques
  • deterministic: most of the above examples
    should be able to do for hw/exams
    other three: basic concept only
  • queuing theory: statistical models (of stochastic processes)
  • simulation: system and traffic profiles used in simulation
    example Fig. 6.9, p.176
  • benchmarking: real system, standard benchmarks
  • actual measurement: real system, real workload/traffic
• common purpose: performance evaluation results
• general study: performance evaluation