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ITFN 2601 Introduction to Operating Systems Lecture 4/5 Scheduling Agenda Scheduling Batch Interactive Real-Time Threads Processes A process is an executing Program Multiprogramming Consist of Program, input, output and a state Process Creation System Initialization System call by running process User request to create new process Initiation of batch job Process Termination Normal Exit Error Exit Fatal Error Killed by another process Process States Running Ready Blocked Threads Lightweight processes Threads handle all execution activities A thread is a program counter, a stack, and a set of registers Thread creation is relatively cheap in terms of CPU costs Thread Usage Programming model becomes simpler Easy to create and destroy Speeds up applications Useful on systems with multiple CPUs User-level threads Get the time of only one process to execute User-level threads are managed by runtime library routines linked into each application so that thread management operations require no kernel intervention. User-level threads are also flexible; they can be customized to the needs of the language or user without kernel modification User-level threads execute within the context of traditional processes The Cost and Benefits of User-Level Threads Thread operations do not have to cross protection boundary. Parameters do not need to be copied. Kernel implements a single policy or pays overhead of managing multiple policies. Applications can link in the correct thread management policy for their needs. Performance is inherently better in user-level threads. User-level threads that issue blocking calls to the kernel will block an entire kernel thread User-level Thread Problems How to block system calls Page Faults Thread must give up the CPU Threads in the Kernal Avoids the system integrations problems exhibited by user-level threads, because the kernel directly schedules each applications threads onto physical processors Performance has been typical an order of magnitude worse than the best-case performance of user-level threads Employ user-level threads, which have good performance and correct behavior provided the application is uniprogrammed and does no I/O, or employ kernel threads, which have worse performance but are not as restricted. The Cost and Benefit of Kernel-Level Threads Kernel threads are expensive! Kernel does not understand application behavior. Deschedule a thread holding a spin lock. Thread priority inversion. May run out of kernel threads to handle all the user threads. "Correct" Kernel Level Support Scheduler Activations Threads are needed for parallel applications. User-level and kernel-level threads both have problems. User-level threads offer good performance, but does not handle I/O well. Kernel-level threads are expensive, but correct. Scheduler Activations SAs notify user-level schedulers of changes in kernel scheduling decisions. SAs provide kernel space for threads that block in the kernel. Create one activation for each virtual processor. Kernel creates SAs to upcall into applications, notifying them of scheduling events. Scheduler Activations (cont) Key difference between SAs and kernel threads When an SA blocks, the application is notified by a different SA. The blocking SA's thread is marked blocked and the old SA is freed. The new SA can now be scheduled. The number of SAs under control of the application never changes (unless requested/told explicitly). Kernel level state is passed to thread system on upcall, so that registers of the blocking thread are accessible to the user-level scheduler. Popup Threads Thread is created spontaneously to handle an incoming request. Incoming message mapped into thread's address space Advantages over traditional request: no waiting on work (no context needs to be saved) creating new thread is cheaper than restoring old thread (no context is saved) Definition of Critical Sections The overlapping portion of each process, where the shared variables are being accessed. Mutual Exclusion --- if Pi is executing in one of its critical sections, noPj , i ≠ j , is executing in its critical sections Race Conditions Race conditions generally involve one or more processes accessing a shared resource (such a file or variable), where this multiple access has not been properly controlled Race conditions appear in three situations: implicit calls to schedule from within a function blocking operations access to data shared by interrupt code and system calls. Critical Regions No two processes may be simultaneously inside their critical regions No assumptions may be made about speeds or the number of CPUs No process running outside its critical region may block another process No process should have to wait forever to enter its critical region Mutual Exclusion Busy Waiting or Spin Lock Priority inversion Producer-Consumer Problem Scheduling Process Conditions Processor Bound I/O Bound Scheduling how? Pre-emptive Non-pre-emptive Scheduling: When New Process is Created Parent process Child process Process Exits When a process Blocks I/O Interrupt occurs Clock Interrupts Non preemptive Preemptive Objectives of a Good Scheduling Policy Fairness Efficiency Low response time (important for interactive jobs) Low turnaround time (important for batch jobs) High throughput Scheduling Throughput. The amount of useful work accomplished per unit time. This depends, of course, on what constitutes ``useful work.'' One common measure of throughput is jobs/minute (or second, or hour, depending on the kinds of job). Utilization. For each device, the utilization of a device is the fraction of time the device is busy. A good scheduling algorithm keeps all the devices (CPU's, disk drives, etc.) busy most of the time. Scheduling Turnaround. The length of time between when the job arrives in the system and when it finally finishes. Response Time. The length of time between when the job arrives in the system and when it starts to produce output. For interactive jobs, response time might be more important than turnaround. Waiting Time. The amount of time the job is ready (runnable but not running). This is a better measure of scheduling quality than turnaround, since the scheduler has no control of the amount of time the process spends computing or blocked waiting for I/O. Preemption Needs a clock interrupt (or equivalent) Needed to guarantee fairness Found in all modern general purpose operating systems Without preemption, the system implements ``run to completion (or yield)'' Semaphores • Semaphores are used to block a process from entering a `critical section' of its machine code, if this critical section accesses a shared resource (e.g a memory location) which another program is currently accessing A process cannot atomically test the state of the semaphore, and block itself if the semaphore is owned by another process. However, the operating system can do this work, as it can ensure that the running process is not pre-empted while the test, and possible block, are performed. This is why the operations on semaphores are typically implemented as system calls. First-Come-First-Served The simplest possible scheduling discipline is called First-come, firstserved (FCFS). The ready list is a simple queue (first-in/first-out). The scheduler simply runs the first job on the queue until it blocks, then it runs the new first job, and so on. When a job becomes ready, it is simply added to the end of the queue FCFS Main advantage of FCFS is that it is easy to write and understand No starvation If one process gets into an infinite loop, it will run forever and shut out all the others. FCFS tends to excessively favor long bursts. CPU-bound processes Shortest-job-first (SJF) Whenever the CPU has to choose a burst to run, it chooses the shortest one Non-preemptive policy preemptive version of the SJF, called shortest-remaining-time-first (SRTF). Starvation is possible Three-Level Scheduling Admission Scheduler – which jobs to admit to the system Memory Scheduler – Which processes are kept in memory and which on disk CPU Scheduler – Pick ready process to run Round-Robin Round-robin (RR). RR keeps all the burst in a queue and runs the first one, like FCFS. But after a length of time q (called a quantum), if the current burst hasn't completed, it is moved to the tail of the queue and the next burst is started. Round Robbin An important preemptive policy Essentially the preemptive version of FCFS The key parameter is the quantum size q When a process is put into the running state a timer is set to q. If the timer goes off and the process is still running, the OS preempts the process. This process is moved to the ready state (the preempt arc in the diagram. The next job in the ready list (normally a queue) is selected to run Round Robbin As q gets large, RR approaches FCFS As q gets small, RR approaches PS What q should we choose Tradeoff Small q makes system more responsive Large q makes system more efficient since less switching Priority Scheduling Always to run the highest priority burst preemptive or non-preemptive Priorities can be assigned externally to processes based on their importance Assigned (and changed) dynamically Other Interactive Scheduling Multiple Queues Shortest Process Next Guaranteed Scheduling Lottery Scheduling Fair-Share Scheduling Scheduler Goals Generic Goals Fairness of processor allocation Enforcement of Scheduling Policies Balance of utilization Batch-based Goals Maximize throughput of jobs Minimize turnaround on jobs Scheduler Goals II Interactive System Goals Minimize response time for user I/O User expectations should be met Real-time System Goals Deadlines must be met for Process Completion System Performance must be predictable Scheduling Algorithms (Batch) FIFO (First In First Out) NON-PREEMPTIVE Fairest Low throughput High Turnaround Shortest First NON-PREEMPTIVE High Throughput Low Turnaround Unfair for Large Jobs Scheduling Algorithms (Batch, cont) Shortest Remaining - PREEMPTIVE High Turnaround on Long Jobs Unfair for Large Jobs Multi-Scheduling (CPU or Memory Limited) HIGH Turnaround (disk swaps) Throughput highly variable, probably low Fairness highly variable Scheduling Algorithms (Interactive) Round Robin - PREEMPTIVE Fairest overall Response time variable but finite Priority Scheduling - PREEMPTIVE Fair “More Fair” for users with higher priorities Response time inverse to priority Windows/Unix typically implement this Round Robin, Example Scheduling Algorithms (Real-Time) Small Jobs High Priority Periodic/Aperiodic Schedulable? Iff the sum of the ratios CPU Time to Period time is less than one Sum(CPU/Period) <= 1 Static/Dynamic? Summary Scheduler responsible for many goals Scheduling algorithms complex Know your math!
