Operating Systems

Operating Systems Operating Systems Unit 5: Processor scheduling Java Linux Windows XP

Process Lifecycle COP 5994 - Operating Systems

Processor scheduling policy • Decides which process runs at given time • scheduling goals • Maximize processor utilization • Minimize response-time to user • Also: • Maximize throughput • Complete process by given deadline COP 5994 - Operating Systems

Concept: CPU And I/O Bursts COP 5994 - Operating Systems

Histogram of CPU-burst Times COP 5994 - Operating Systems

Concept: Preemption • Preemptive scheduling • Process can be removed from current processor • improved response times • important for interactive environments • (preempted process remains in memory) • Non-preemptive scheduling • Process runs to completion or yield • unimportant processes can block important ones indefinitely COP 5994 - Operating Systems

Concept: Priority • Static priorities • Priority assigned to a process does not change • Easy to implement, low overhead • Not responsive to changes in environment • Dynamic priorities • Responsive to change • Promote smooth interactivity • Incur more overhead than static priorities • Justified by increased responsiveness COP 5994 - Operating Systems

Scheduling Criteria • CPU utilization • keep the CPU as busy as possible • Throughput • # of processes that complete per time unit • Turnaround time • amount of time to execute a process • Waiting time • time a process has been waiting in the ready queue • Response time • time from request until first response is produced COP 5994 - Operating Systems

Scheduling Objectives Summary • Several goals common to most schedulers • Fairness • Predictability • Scalability COP 5994 - Operating Systems

Scheduling Criteria • Consider run-time behavior of process • Compute vs. I/O wait • Process categories: • Processor-bound • I/O-bound • Batch processes • no user interaction • Interactive processes • frequent user interaction COP 5994 - Operating Systems

Scheduling Algorithms • Decide when and for how long a process runs • Make choices about • Preemptibility • Priority • Running time • Run-time-to-completion • Fairness COP 5994 - Operating Systems

First-In-First-Out Scheduling • Simplest scheme • Processes dispatched according to arrival time • Non-preemptive • Rarely used as primary scheduling algorithm COP 5994 - Operating Systems

Round-Robin Scheduling • Based on FIFO • Processes run only for a limited time slice or quantum • Must be preemptible • Context switch overhead must be minimized • Often used as part of more complex algorithms COP 5994 - Operating Systems

RR Factor: time quantum size • Determines response time to interactive requests • Very large quantum size • Processes run for long periods • Degenerates to FIFO • Very small quantum size • System spends most time context switching • Compromise: • Long enough for normal processes to issue I/O request • Batch processes still get majority of processor time COP 5994 - Operating Systems

Shortest-Process-First Scheduling • process with smallest time to finish goes first • Lower average wait time than FIFO • Reduces the number of waiting processes • Potentially large variance in wait times • Non-preemptive • Results in slow response times to new processes • Relies on estimates of time-to-completion • Can be inaccurate or falsified • Unsuitable for use in modern interactive systems COP 5994 - Operating Systems

waiting-time + service-time priority = service-time Highest-Response-Ratio-Next Scheduling • Improves upon SPF scheduling • also considers priority • Considers how long process has been waiting • Prevents indefinite postponement • Still non-preemptive COP 5994 - Operating Systems

Shortest-Remaining-Time Scheduling • Preemptive version of SPF • Shorter arriving process preempts running process • Problems: • Very large variance of response times: • long processes wait even longer than under SPF • Context-switching overhead can become significant • Short incoming process can preempt a running process that is near completion COP 5994 - Operating Systems

Scheduling Realities • Different processes have different needs • Short I/O-bound interactive processes should generally run before processor-bound batch processes • Behavior patterns not immediately obvious to the scheduler COP 5994 - Operating Systems

Example: Selfish RR Scheduling • process has priority • 2 queues: • Holding vs. Active • process enters into holding queue • Increases priority as process ages • moves to active queue when it reaches priority of other processes in active queue • RR scheduling in active queue COP 5994 - Operating Systems

Multilevel Feedback Queues COP 5994 - Operating Systems

Multilevel Feedback Queues • Arriving processes enter the highest-level queue with higher priority • Long processes descend into lower levels • Gives short and I/O-bound processes higher priority • Long processes will run when short and I/O-bound processes terminate • Process entering a higher-level queue preempt running process • Each queue has own scheduler • Typical: FIFO or RR COP 5994 - Operating Systems

Fair Share Scheduling COP 5994 - Operating Systems

Deadline Scheduling • Process must complete by specific time • Used when results would be useless if not delivered on-time • Difficult to implement • Must plan resource requirements in advance • Incurs significant overhead • Service provided to other processes can degrade COP 5994 - Operating Systems

Real-time Scheduling • Processes have timing constraints • Soft real-time system • Does not guarantee that timing constraints will be met • For example, multimedia playback • Hard real-time system • Timing constraints will always be met • modes of operation: • periodic execution of processes • response to external event • catastrophic result if constraint is not met COP 5994 - Operating Systems

Static Real-Time Scheduling • Does not adjust priorities over time • Low overhead • Suitable for systems where conditions rarely change • used for hard real-time systems • Rate-monotonic (RM) scheduling • priority-based RR scheduling • Process priority increases monotonically with the frequency with which it must execute • Deadline RM scheduling • priority can be adjusted for deadline COP 5994 - Operating Systems

Dynamic Real-Time Scheduling • Adjusts priorities to changing conditions • has overhead: must ensure no missed deadlines • Priorities are based on processes’ deadlines • Earliest-deadline-first (EDF) • Preemptive, always dispatch the process with the earliest deadline • Minimum-laxity-first • Similar to EDF, but bases priority on laxity • L = Deadline – (currentTime + timeToComplete) scenario: L = 0 scenario: L negative ? COP 5994 - Operating Systems

Java Thread Scheduling • threads have priority • set by programmer • queue per priority level • round-robin scheduling • arriving thread of higher thread preempts • implemented via • time slice • yield operation COP 5994 - Operating Systems

Java Thread Scheduling COP 5994 - Operating Systems

Linux Task Scheduling • Linux 2.6: • O(1) scheduler • scheduling operations run in constant time • improve scalability because execution time independent of number of tasks in the system • system timer interrupt every 1ms (IA32) • to update kernel data structure COP 5994 - Operating Systems

Linux Task Priority • task has static priority • -20 to 19 (lower number is higher priority) • scheduling is based on effective priority • adjusted for execution behavior • ratio of sleep vs. run time • task that sleeps more is given higher priority COP 5994 - Operating Systems

Preemptive Scheduler • variable time slice between 10 and 200ms • higher priority gets larger time slice • each task runs until either: • its time slice expires • a higher priority process becomes runnable • the process blocks • tasks placed in run queue levels for each priority • priority array refers to each level sub-queue • RR scheduling for priority level sub-queue COP 5994 - Operating Systems

Scheduler priority array COP 5994 - Operating Systems

Indefinite Postponement • each task executes once per epoch • epoch duration limited by starvation limit • default: 10 * tasks-in-run-queue seconds • when starvation limit is reached • tasks run that have not executed in the epoch • epoch ends, new epoch starts • guarantees that low-priority tasks will run eventually COP 5994 - Operating Systems

Linux Multiprocessor scheduling • one run queue per processor • tasks tend to stay on a processor • advantage: processor cache • scheduler performs dynamic load balancing • attempts to migrate only cache-cold tasks COP 5994 - Operating Systems

Linux Real-time Scheduling • Soft real-time scheduler • scheduling classes • SCHED_FIFO • FIFO real-time tasks • SCHED_RR • RR real-time tasks • SCHED_OTHER • as before, non real-time tasks COP 5994 - Operating Systems

Windows XP Thread Scheduling • thread dispatcher is part of micro kernel • 32 Priority levels • 0 = lowest priority • 31 = highest priority • 16 to 31: real-time threads • 0: zero-page thread for idle-time tasks • 32 ready queues • RR for highest-priority ready queue COP 5994 - Operating Systems

class level Windows XP Priorities • priority class + priority level = thread priority COP 5994 - Operating Systems

Thread Priority changes • Only for dynamic threads (1-15) • Priority boosts: • Exiting the wait state • Window receives user input • Has not executed for a long time • Priority reduction • Cannot go below base priority • Reduced by one if thread executes for entire quantum • Priority returns to previous level after one quantum if thread boosted because had not executed for a long time COP 5994 - Operating Systems

Windows XP Multiprocessor Scheduling • Dispatcher considers: • affinity mask • processors which may run thread • less important for SMP • last processor • processor on which thread executed last time it ran • attempt to preserve processor cache • ideal processor • Maximize parallelism • related threads, different ideal processors COP 5994 - Operating Systems

Agenda for next week: • Chapter 9 • Memory management • Read ahead ! COP 5994 - Operating Systems

Operating Systems