Job Scheduling

1. Objective

  • maximum CPU utilization obtained with multiprogramming
  • CPU-I/O Burst Cycle- Process execution consist of a cycle of CPU execution and I/O wait
2. CPU Scheduler
  • Selects from among the processes in ready queue, and allocates the CPU to one of them
    • queues may be ordered in various ways
  • CPU scheduling decisions may take place when a process
    • switches from running to waiting sate
    • switches from running to ready state
    • switches from waiting to ready state
    • terminate
  • scheduling under 1 and 4 is non-preemptive
  • all other scheduling is preemptive
    • consider access to shared data
    • consider preemption while in kernel mode
    • consider interrupts occurring during crucial OS activicites
3. Dispatcher
  • Dispacher module gives control of the CPU to the process selected by the short-term scheduler; this involves
    • switching context
    • switching to user mode
    • jumping to the proper location in the user program to restart that program
  • Dispatch latency
    • time it takes for the dispatcher to stop one process and start another running
4. Scheduling Criteria
  • CPU utilization
    • keep the CPU as busy as possible
  • Throughput 
    • # of processes that complete their execution per time unit
  • Turnaround time
    • amount of time to execute a particular process
  • Waiting time
    • amount of time a process has been waiting in the ready queue

5. First-Come. First-Server (FCFS) Scheduling

6. Shortest-Job-First (SJF) Scheduling

  • associate with each process the length of its next CPU burst
    • use these lengths to schedule the process with the shortest time
  • SJF is optimal
    • gives minimum average waiting time for a given set of processes
      • the difficult is knowing the length of the next CPU request
7. Determine the length of next CPU burst
  • can only estimate the length
    • should be similar to the previous one
    • then pick process with shortest predicted next CPU burst
  • can be done by using the length of previous CPU bursts, using exponential averaging
    • t_n = average length of the n-th CPU burst
    • tau_{n+1} = predicted value of the next CPU burst
    • alpha, 0 leq alpha leq (1-alpha) tau_n
  • commonly, alpha set to 1/2
  • preemptive version called shortest-remaining-time-first
8. Example of Exponential Averaging
  • alpha = 0
    • tau_{n+1} = tau_n
    • recent history does not count
  • alpha = 1
    • tau_{n+1} = n tau_n
    • only the actual last CPU burst counts
  • If we expand the formula, we get
    • tau_{n+1} = alpha tau_n + (1-alpha) tau_{n-1} + ...
    •                     = (1-alpha)^j alpha tau_{n-j} + ...
    •                     = (1-alpha)^{n+1} tau_0
  • since both alpha and 1-alpha are less than or equal to 1, each successive term has less weight than its predecessor
9. Example of Shortest Remaining-time-first

10. Priority Scheduling
  • A priority number (integer) is associated with each process
  • The CPU is allocated to the process with the highest priority (smaller integer = highest priority)
    • preemptive
    • non-preemptive
  • SJF is priority scheduling where priority is the inverse of predicted next CPU burst time
  • Problem = starvation 
    • low priority processes may never execute
  • Solution = aging
    • as time progresses, increase the priority of the process
11. Round Robin
  • Each process gets a small unit of CPU time (time quantum q), usually 10-100 milliseconds.
    • after this time has elapsed, the process is preempted and added to the end of the ready queue
  • If there are n processes in the ready queue and the time quantum is q, then each process gets 1/n of the CPU time is chunks of at most q time units at once. 
    • No process waits more than (n-1)q time units.
  • Time interrupts every quantum to schedule next process
  • Performance
    • q large : FIFO
    • q small: q must be large with respect to context switch, otherwise, overhead is too high
  • Example of Round robin with time Quantum =4 
12. Multilevel Queue
  • Ready queue is partitioned into separate queues, e..g,
    • forground (iterative)
    • background (batch)
  • Process permanently in a given queue
  • Each queue has its own scheduling algorithm
    • forground-RR
    • background-FCFR
  • Scheduling must be done between the queues
    • fixed priority scheduling: i.e., serve all from foreground then from background
      • possibility starvation
    • time slices: each queue gets a certain amount of CPU time which it can schedule amongst its processes, i.e., 80% to foreground in RR
    • 20% to background in FCFS

13. Multi-level feedback queue
  • a process can move between the various queues;
  • aging can be implemented this way
  • multi-level-feedback-queue scheduler defined by the following parameters
    • number of queues
    • scheduling algorithms for each queue
    • method used to determine when to upgrade a process
    • method used to determine when to demote a process
    • method used to determine which queue a process will enter when that process needs service
  • Example of multilevel feedback queue
    • three queues
      • Q_0: time quantum 8 milliseconds
      • Q_1: time quantum 16 milliseconds
      • Q_2; FCFS
    • scheduling
      • a new job enters queue Q_0 which is served FCFS
        • when it gains CPU, job received 8 milliseconds
        • if it does not finish in 8 milliseconds, job is moved to queue Q_1
    • at Q_1 job is again served FCFS and receives 16 additional milliseconds
      • if it still does not complete, it is preempted and moved to queue Q_2
14. Thread Scheduling
  • Distinction between user-level and kernel-level threads
  • when threads supported, threads scheduled, not processes
  • many-to-one and many-to-many methods, thread library schedules user-level threads to run on LWP
    • known as process-contention scope (PCS) since scheduling competition is within the process
    • typically thread scheduled onto available CPU is system contention scope (SCS)
      • competition among all threads in the system
15. Multi-Processor Scheduling
  • CPU scheduling more complex when multiple CPUs are available
  • Homogeneous processors within a multiprocessor
  • Asymetric multiprocessing
    • only one processor accesses the system data structures, alleviating the need for data sharing
  • Symmetric multiprocessing (SMP)
    • each processor is self-scheduling, all processes in common ready queue
    • or each has its own private queue of ready processes
  • Processor Affinity
    • process has affinity for processor on which it is currently running
      • soft affinity
      • hard affinity
      • variation including processor sets
16. Multiplecore Processor
  • recent trend to place multiple processor cores on the same physical chip
  • faster and consumes lees power 
  • multiple threads per core also growing
    • takes advantage of memory stall to make progress on another thread while memory retrieve happens

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