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5. Process and thread scheduling

5. Process and thread scheduling. 5.1 Organization of Schedulers Embedded and Autonomous Schedulers 5.2 Scheduling Methods A Framework for Scheduling Common Scheduling Algorithms Comparison of Methods 5.3 Priority Inversion. Process and Thread Scheduling.

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5. Process and thread scheduling

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  1. 5. Process and thread scheduling 5.1 Organization of Schedulers • Embedded and Autonomous Schedulers 5.2 Scheduling Methods • A Framework for Scheduling • Common Scheduling Algorithms • Comparison of Methods 5.3 Priority Inversion

  2. Process and Thread Scheduling • Scheduling occurs at two levels: • Process/job scheduling • Long term scheduling (seconds, minutes, …) • Move process to Ready List (RL) after creation(When and in which order?) • Dispatching • Short term scheduling (milliseconds) • Select process from Ready List to run • We use the term scheduling to refer to both

  3. Organization of Schedulers • Embedded • Called as function at end of kernel call • Runs as part of calling process • Autonomous • Separate process • Multiprocessor: may have dedicated CPU • Single-processor:scheduler and other processes alternate (every quantum)

  4. An Embedded Scheduler Scheduler() { do { find highest priority process p with p.status==ready_a; find a free cpu; if (cpu != NIL) Allocate_CPU(p,cpu); } while (cpu != NIL); do { find highest priority process p with p.status==ready_a; find lowest priority process q with p.status==running; if (Priority(p) > Priority(q)) Preempt(p,q); } while (Priority(p) > Priority(q)); if (self->Status.Type!=running) Preempt(p,self); }

  5. Scheduling Methods • Priority determines who runs • Can be static or dynamic • Given by Priority function: P = Priority(p) • p are parameters • Arbitration rule to break ties • Random • Chronological (FIFO) • Cyclic (Round Robin = RR) • When is scheduler invoked?—Decision mode • Preemptive: scheduler called periodically(quantum-oriented) or when system state changes • Nonpreemptive: scheduler called when process terminates or blocks

  6. Priority function Parameters • Possible parameters: • Attained service time (a) • Real time in system (r) • Total service time (t) • Period (d) • Deadline (explicit or implied by period) • External priority (e) • System load (not process-specific)

  7. Scheduling algorithms Name Decision mode Priority Arbitration FIFO: nonpreemptiveP = r random SJF: nonpreemptiveP = –t chronological/random SRT: preemptive P = –(t–a) chronological/random RR: preemptive P = 0 cyclic ML: preemptive P = e cyclic nonpreemptiveP = e chronological • n fixed priority levels • level P is serviced when n through P+1 empty

  8. Scheduling algorithms • MLF (Multilevel Feedback) • Like ML, but prioritychanges dynamically • Every process enters at highest level n • Each level P prescribes maximum time tP • tPincreases as P decreases • Typically: tn= T (a constant) tP = 2  tP+1

  9. Scheduling algorithms • MLF priority function: • depends on level n, and attained time, a P = n– log2(a/T+1) • derivation is in the book • but note: • there is no need to ever compute P • priority is known automatically by position in queue • MLF is preemptive: CPU is taken away whenever process exhausts tP • Arbitration rule (at highest non-empty level): cyclic (at quantum) or chronological (at tP)

  10. Scheduling Algorithms RM (Rate Monotonic): • Intended for periodic (real-time) processes • Preemptive • Highest priority: shortest period: P = –d EDF (Earliest Deadline First): • Intended for periodic (real-time) processes • Preemptive • Highest priority: shortest time to next deadline: P = –(d – r % d) • derivation r  d number of completed periods r % d time in current period d – r % d time remaining in current period

  11. Comparison of Methods • FIFO, SJF, SRT: Primarily for batch systems • total length of computation is the same for all • FIFO simplest • SJF & SRT have better average turnaround times: (r1+r2+…+rn)/n • Example: • Average turnaround times: • FIFO: ((0+5)+(3+2))/2 = 5.0 • SRT: ((2+5)+(0+2))/2 = 4.5 • SJF: same as FIFO (p1 arrived first, non-preemptive)

  12. Comparison of Methods • Time-sharing systems • Response time is critical • Use RR or MLF with RR within each queue • Choice of quantum determines overhead • When q  , RR approaches FIFO • When q  0, context switch overhead  100% • When q is much greater than context switch overhead, n processes run concurrently at 1/n CPU speed

  13. Comparison of Methods • Real-time systems • Feasible schedule: All deadlines are met • CPU utilization is defined as: U=∑ ti/di for all pi • If schedule is feasible, U  1 (but not vice versa): • EDF always yields feasible schedule if U  1. • RM yields feasible schedule if U<0.7 (approximately), otherwise, it may fail

  14. Comparison of Methods • Example • p1has service time 1.5, period 4 • p2 has service time 3, period 5 • U=(1.5/4) +3/5=.975 < 1 • EDF succeeds butRM fails

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