Shan Gao Fall 2007 Department of Computer Science Georgia State University

Chapter 5: CPU Scheduling  Basic Concepts  Scheduling Criteria  Scheduling Algorithms

Basic Concepts  Process scheduling vs Thread scheduling  CPU  One of the most important resources  CPU–I/O Burst Cycle  Process execution consists of a cycle of CPU execution and I/O wait  Maximum CPU utilization obtained with multiprogramming  Keep CPU busy  CPU burst distribution

Alternating Sequence of CPU And I/O Bursts

Histogram of CPU-burst Times

CPU Scheduler  Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them  Ready queue  CPU scheduling decisions may take place when a process: 1.Switches from running to waiting state 2.Switches from running to ready state 3.Switches from waiting to ready 4.Terminates

 Nonpreemptive  Once the cpu is allocated to a process, the process keeps the cpu until it releases cpu either by terminating or by switching to the waiting state.  Preemptive  A running process could be suspended by another process.  Process Sync problem  Interrupt issues  Kernel, device drivers…

Dispatcher  Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves:  switching context  switching to user mode  jumping to the proper location in the user program to restart that program  Dispatch latency  Time it takes for the dispatcher to stop one process and start another running  Usually very short and written in C and assembly language

Scheduling Criteria  CPU utilization  keep the CPU as busy as possible  Throughput  # of processes that complete their execution per time unit  Turnaround time  amount of time to execute a particular process  Waiting time  amount of time a process has been waiting in the ready queue  Response time  amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment)

Optimization Criteria  Max CPU utilization  Max throughput  Min turnaround time  Min waiting time  Min response time

First-Come, First-Served (FCFS) Scheduling ProcessBurst Time P 1 24 P 2 3 P 3 3  Suppose that the processes arrive in the order: P 1, P 2, P 3 The Gantt Chart for the schedule is:  Waiting time for P 1 = 0; P 2 = 24; P 3 = 27  Average waiting time: ( )/3 = 17 P1P1 P2P2 P3P

FCFS Scheduling (Cont.) Suppose that the processes arrive in the order P 2, P 3, P 1  The Gantt chart for the schedule is:  Waiting time for P 1 = 6; P 2 = 0 ; P 3 = 3  Average waiting time: ( )/3 = 3  Much better than previous case  Convoy effect short process behind long process P1P1 P3P3 P2P

Shortest-Job-First (SJR) Scheduling  Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time  Two schemes:  nonpreemptive – once CPU given to the process it cannot be preempted until completes its CPU burst  preemptive – if a new process arrives with CPU burst (next) length less than remaining time of current executing process, preempt. This scheme is know as the Shortest-Remaining-Time-First (SRTF)  SJF is optimal – gives minimum average waiting time for a given set of processes  Used frequently in long-term scheduling

Example of Non-Preemptive SJF ProcessArrival TimeBurst Time P P P P  SJF (non-preemptive)  Average waiting time = ( )/4 = 4 P1P1 P3P3 P2P P4P4 812

Example of Preemptive SJF ProcessArrival TimeBurst Time P P P P  SJF (preemptive)  Average waiting time = ( )/4 = 3 P1P1 P3P3 P2P P4P4 57 P2P2 P1P1 16

Determining Length of Next CPU Burst  Can only estimate the length  Can be done by using the length of previous CPU bursts, using exponential averaging

Prediction of the Length of the Next CPU Burst

Priority Scheduling  A priority number (integer) is associated with each process  The CPU is allocated to the process with the highest priority (smallest integer  highest priority)  Preemptive  nonpreemptive  SJF is a priority scheduling where priority is the predicted next CPU burst time  Problem  Starvation  low priority processes may never execute  Solution  Aging  as time progresses increase the priority of the process

Round Robin (RR)  Each process gets a small unit of CPU time (time quantum), usually milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue.  If there are n processes in the ready queue and the time quantum is q, then each process gets 1/n of the CPU time in chunks of at most q time units at once. No process waits more than (n-1)q time units.  Processor sharing: each of n processes has its own processor running at 1/n the speed of the real processor  Performance  q large  FIFO  q small  q must be large with respect to context switch, otherwise overhead is too high

Example of RR with Time Quantum = 20 ProcessBurst Time P 1 53 P 2 17 P 3 68 P 4 24  The Gantt chart is:  Typically, higher average turnaround than SJF, but better response P1P1 P2P2 P3P3 P4P4 P1P1 P3P3 P4P4 P1P1 P3P3 P3P

Time Quantum and Context Switch Time

Turnaround Time Varies With The Time Quantum

Multilevel Queue  Ready queue is partitioned into separate queues: foreground (interactive) background (batch)  Each queue has its own scheduling algorithm  Foreground: RR  Background: FCFS  Scheduling must be done between the queues  Fixed priority scheduling; (i.e., serve all from foreground then from background). Possibility of starvation.  Time slice – each queue gets a certain amount of CPU time which it can schedule amongst its processes; i.e., 80% to foreground in RR and 20% to background in FCFS

Multilevel Queue Scheduling

Multilevel Feedback Queue  A process can move between the various queues; aging can be implemented this way  Multilevel-feedback-queue scheduler defined by the following parameters:  number of queues  scheduling algorithms for each queue  method used to determine when to upgrade a process  method used to determine when to demote a process  method used to determine which queue a process will enter when that process needs service

Example of Multilevel Feedback Queue  Three queues:  Q 0 – RR with time quantum 8 milliseconds  Q 1 – RR time quantum 16 milliseconds  Q 2 – FCFS  Scheduling  A new job enters queue Q 0. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q 1.  At Q 1 job receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q 2.

Multilevel Feedback Queues

Multiple-Processor Scheduling  Homogeneous processors within a multiprocessor system  Asymmetric multiprocessing  The master server  SMP  Self-scheduling  CPU scheduling more complex when multiple CPUs are available  Synchronization  Processor affinity  Load balance, process migration(push & pull)

 Symmetric multithreading (SMT)  Logical processor  Physical processor  Hyperthreading technology  Hardware support

Thread Scheduling  Process-contention scope (PCS)  Competition takes place among threads belonging to the same process  OS  Process  thread  How the threads library decides which thread to put onto an available LWP  System-contention scope (SCS)  Competition takes place among all threads in the system  OS  thread  How the kernel decides which kernel thread to run next  One-to-one model only uses SCS

Pthread Scheduling API #include #define NUM THREADS 5 int main(int argc, char *argv[]) { int i; pthread t tid[NUM THREADS]; pthread attr t attr; pthread attr init(&attr); /* get the default attributes */ /* set the scheduling algorithm to PROCESS or SYSTEM */ pthread attr setscope(&attr, PTHREAD_SCOPE_SYSTEM); /* set the scheduling policy - FIFO, RT, or OTHER */ pthread attr setschedpolicy(&attr, SCHED_OTHER); /* create the threads */ for (i = 0; i < NUM THREADS; i++) pthread create(&tid[i],&attr,runner,NULL);

Pthread Scheduling API /* now join on each thread */ for (i = 0; i < NUM THREADS; i++) pthread join(tid[i], NULL); } /* Each thread will begin control in this function */ void *runner(void *param) { printf("I am a thread\n"); pthread exit(0); }

Operating System Examples  Solaris scheduling  Windows XP scheduling  Linux scheduling

Solaris  4 classes of scheduling  Within each class there are different priorities and different scheduling algorithms

Solaris Dispatch Table

Windows XP  Dispatcher schedule a thread when:  Preempted by a higher-priority thread  Termination  Time quantum ends  Call a blocking system call  A queue for each priority  REALTIME_PRIORITY_CLASS (not variable)  HIGH_PRIORITY_CLASS  ABOVE_NORMAL_PRIORITY_CLASS  NORMAL_PRIORITY_CLASS  BELOW_NORMAL_PRIORITY_CLASS  IDLE_PRIORITY_CLASS

 Base priority: NORMAL  Never lower than this base priority  NORMAL_PRIORITY_CLASS  Unless its parent is IDLE_PRIORITY_CLASS or specified during creation  The amount of boost depends on what the thread was waiting for  Keyboard input  Disk operation  Foreground processes, background processes  3 times

Linux Scheduling  Support SMP, processor affinity, load balancing  A preemptive, priority-based algorithm with two separate priority ranges:  0-99 (real-time)   Higher priority : longer time quanta  Lower priority : shorter time quanta

The Relationship Between Priorities and Time-slice length

 Each CPU has one runqueue  active  All tasks with time remaining in their time slices  expired  All expired tasks  Each CPU schedules itself independently  A task is moved to expired when exhausted its time quantum  Meanwhile, recalculate its new priority and assign corresponding time slice  When active array is empty, the two arrays are exchanged  All tasks have been assigned new priorities and corresponding time slices

List of Tasks Indexed According to Prorities

 Dynamic priority  More interactive: longer sleep times  Higher priority  Less interactive: shorter sleep times  Lower priority  Tasks with shorter sleep times are often more CPU-bound and thus will have their priority lowered.

Algorithm Evaluation  Deterministic modeling  takes a particular predetermined workload and defines the performance of each algorithm for that workload  Require exact numbers for input, and its answer only apply to those cases  Example: All arrive at time 0 ProcessBurst Time P1 10 P2 29 P3 3 P4 7 P5 12

 FCFS ( )/5=28  SJF ( )/5=13  RR ( )/5=23

Queuing Models  Little’s formula  n = λ * W  n: the avg queue length  W: the avg waiting time in the queue  λ: the avg arrival rate for new processes in the queue  Only approximations of real systems  The accuracy of the results may be questionable

Simulations  Program a model of the computer system  Trace tapes  Higher accuracy

Implementation  Implement the algorithm, put it in the OS and see how it works  The only completely accurate way  High cost  Coding  Modify OS  Users’ reactions to a constantly changing OS  Changing environment  Tunable scheduling algorithms