Operating system 31 multiple processor scheduling
- 1. Operating System 31 Multiple-Processor Scheduling Prof Neeraj Bhargava Vaibhav Khanna Department of Computer Science School of Engineering and Systems Sciences Maharshi Dayanand Saraswati University Ajmer
- 2. Multiple-Processor Scheduling • CPU scheduling more complex when multiple CPUs are available • Homogeneous processors within a multiprocessor • Asymmetric 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 – Currently, most common • Processor affinity – process has affinity for processor on which it is currently running – soft affinity – hard affinity – Variations including processor sets
- 3. NUMA and CPU Scheduling Note that memory-placement algorithms can also consider affinity
- 4. Multiple-Processor Scheduling – Load Balancing • If SMP, need to keep all CPUs loaded for efficiency • Load balancing attempts to keep workload evenly distributed • Push migration – periodic task checks load on each processor, and if found pushes task from overloaded CPU to other CPUs • Pull migration – idle processors pulls waiting task from busy processor
- 5. Multicore Processors • Recent trend to place multiple processor cores on same physical chip • Faster and consumes less power • Multiple threads per core also growing – Takes advantage of memory stall to make progress on another thread while memory retrieve happens
- 7. Real-Time CPU Scheduling • Can present obvious challenges • Soft real-time systems – no guarantee as to when critical real-time process will be scheduled • Hard real-time systems – task must be serviced by its deadline • Two types of latencies affect performance 1. Interrupt latency – time from arrival of interrupt to start of routine that services interrupt 2. Dispatch latency – time for schedule to take current process off CPU and switch to another
- 8. Real-Time CPU Scheduling (Cont.) • Conflict phase of dispatch latency: 1. Preemption of any process running in kernel mode 2. Release by low- priority process of resources needed by high- priority processes
- 9. Priority-based Scheduling • For real-time scheduling, scheduler must support preemptive, priority-based scheduling – But only guarantees soft real-time • For hard real-time must also provide ability to meet deadlines • Processes have new characteristics: periodic ones require CPU at constant intervals – Has processing time t, deadline d, period p – 0 ≤ t ≤ d ≤ p – Rate of periodic task is 1/p
- 10. Virtualization and Scheduling • Virtualization software schedules multiple guests onto CPU(s) • Each guest doing its own scheduling – Not knowing it doesn’t own the CPUs – Can result in poor response time – Can effect time-of-day clocks in guests • Can undo good scheduling algorithm efforts of guests
- 11. Rate Montonic Scheduling • A priority is assigned based on the inverse of its period • Shorter periods = higher priority; • Longer periods = lower priority • P1 is assigned a higher priority than P2.
- 12. Missed Deadlines with Rate Monotonic Scheduling
- 13. Earliest Deadline First Scheduling (EDF) • Priorities are assigned according to deadlines: the earlier the deadline, the higher the priority; the later the deadline, the lower the priority
- 14. Proportional Share Scheduling • T shares are allocated among all processes in the system • An application receives N shares where N < T • This ensures each application will receive N / T of the total processor time
- 15. Assignment • Explain Multiple-Processor Scheduling – Load Balancing