ITFN 2601 Introduction to Operating Systems Lecture 4 Scheduling

Agenda Scheduling Batch Interactive Real-Time Threads

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 kind 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 (CPUs, 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 a job arrives in the system and when it starts to produce output. For interactive jobs, response time might be more important than turnaround. Wait 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 mount 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)”

Scheduling Algorithms (Batch) FIFO (First In First Out) Fairest Low throughput High Turnaround Shortest First High Throughput Low Turnaround Unfair for Large Jobs

Scheduling Algorithms (Batch, cont) Shortest Remaining 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

First-Come First-Served The simplest possible scheduling discipline is called First-come, first- served (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 SJN, called shortest remaining time first Starvation is possible

Scheduling Algorithms (Interactive) Round Robin Fairest overall Response time variable but finite Priority Scheduling Fair “More Fair” for users with higher priorities Response time inverse to priority

Round-Robin Round-robin (RR). RR keeps all the processes in a queue and runs the first one, like FCFS. After a length of time q (called quantum), if the current burst hasn’t completed, it is moved to the tail of the queue and the next process is started

Round Robin 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. The process is moved to the ready state The next job in the queue is selected to run

Round Robin Quantum can’t be too large Quantum can’t be too small What quantum should we choose Tradeoff Small q makes system more responsive Large q makes system more efficient since there’s less switching

Round Robin, Example

Priority Scheduling Always run the highest priority process 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

Scheduling Algorithms (Interactive, cont) Multi-Quantized Response time proportionate to quanta More bookkeeping Shortest Process Next Estimation of process length Unfair for large jobs Fast response for small jobs

Scheduling Algorithms (Interactive, cont) Guaranteed Scheduling Lottery Scheduling Alotted time proportional to Job Size/Importance Sharing Fair by user, not necessarily fair by job Responses become disproportionate

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?

Scheduler Mechanism Some processes are intrinsically more important at some times than others Time-dependent response High-priority request How can a process raise it’s scheduling priority?

Thread Scheduling User-level threads Process has a Thread Scheduler Same concept as Process Scheduler Conflicts with Process Scheduling Kernel Level Threads Kernel has Thread and Process Scheduler Two Quanta’s Thread Quanta Process Quanta

Summary Scheduler responsible for many goals Scheduling algorithms complex Know your math!