LECTURE 3:CPU SCHEDULING
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1 LECTURE 3:CPU SCHEDULING 1
2 Outline Basic Concepts Scheduling Criteria Scheduling Algorithms Multiple-Processor Scheduling Real-Time CPU Scheduling Operating Systems Examples Algorithm Evaluation 2
3 Objectives To introduce CPU scheduling, which is the basis for multiprogrammed operating systems To describe various CPU-scheduling algorithms To discuss evaluation criteria for selecting a CPU-scheduling algorithm for a particular system To examine the scheduling algorithms of several operating systems 3
4 Basic Concepts Maximum CPU utilization obtained with multiprogramming CPU I/O Burst Cycle Process execution consists of a cycle of CPU execution and I/O wait CPU burst followed by I/O burst CPU burst distribution is of main concern 4
5 Histogram of CPU-burst Times 5
6 CPU Scheduler Short-term scheduler selects from among the processes in ready queue, and allocates the CPU to one of them Queue may be ordered in various ways 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 Scheduling under 1 and 4 is nonpreemptive All other scheduling is preemptive Consider access to shared data Consider preemption while in kernel mode Consider interrupts occurring during crucial OS activities 6
7 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 7
8 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) 8
9 Scheduling Algorithm Optimization Criteria Max CPU utilization Max throughput Min turnaround time Min waiting time Min response time 9
10 First- Come, First-Served (FCFS) Scheduling Process Burst 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: P P P Waiting time for P 1 = 0; P 2 = 24; P 3 = 27 Average waiting time: ( )/3 = 17 10
11 FCFS Scheduling (Cont.) Suppose that the processes arrive in the order: P 2, P 3, P 1 The Gantt chart for the schedule is: P 2 P 3 P 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 Consider one CPU-bound and many I/O-bound processes 11
12 SJF Example ProcessArrival Time Burst Time P P P P SJF scheduling chart P 4 P 1 P 3 P Average waiting time = ( ) / 4 = 7 12
13 Determining Length of Next CPU Burst Can only estimate the length should be similar to the previous one Then pick process with shortest predicted next CPU burst Can be done by using the length of previous CPU bursts, using exponential averaging 1. t n actual length of n CPU burst 2. predicted value for the next CPU n 1 3., Define : th burst Commonly, α set to ½ Preemptive version called shortest-remaining-time-first 13
14 Prediction of the Length of the Next CPU Burst 14
15 Examples of Exponential Averaging =0 =1 n+1 = n Recent history does not count n+1 = t n Only the actual last CPU burst counts If we expand the formula, we get: n+1 = t n +(1 - ) t n (1 - ) j t n -j + +(1 - ) n +1 0 Since both and (1 - ) are less than or equal to 1, each successive term has less weight than its predecessor 15
16 Example of Shortest-remaining-time-first Now we add the concepts of varying arrival times and preemption to the analysis ProcessAarri Arrival TimeT Burst Time P P P P Preemptive SJF Gantt Chart P 1 P 2 P 4 P 1 P Average waiting time = [(10-1)+(1-1)+(17-2)+5-3)]/4 = 26/4 = 6.5 msec 16
17 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 priority scheduling where priority is the inverse of predicted next CPU burst time Problem Starvation low priority processes may never execute Solution Aging as time progresses increase the priority of the process 17
18 Example of Priority Scheduling ProcessA arri Burst TimeT Priority P P P P P Priority scheduling Gantt Chart P 1 P 2 P 1 P 3 P Average waiting time = 8.2 msec 18
19 Turnaround Time Varies With The Time Quantum 80% of CPU bursts should be shorter than q 19
20 Multilevel Queue Ready queue is partitioned into separate queues, eg: foreground (interactive) background (batch) Process permanently in a given queue 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 20% to background in FCFS 20
21 Multilevel Queue Scheduling 21
22 Multilevel Queue Ready queue is partitioned into separate queues, eg: foreground (interactive) background (batch) Process permanently in a given queue 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 20% to background in FCFS 22
23 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 23
24 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 which is served FCFS 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 is again served FCFS and receives 16 additional milliseconds If it still does not complete, it is preempted and moved to queue Q 2 24
25 Multiple-Processor Scheduling CPU scheduling more complex when multiple CPUs are available Homogeneous processors within a multiprocessor Asymmetric multiprocessing only one processor (Master server) accesses the system data structures, alleviating the need for data sharing All scheduling decions, I/O processing and other systems activities handled by a single processor (Master server) Symmetric multiprocessing (SMP) each processor is selfscheduling, 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 25
26 Multiple-Processor Scheduling CPU scheduling more complex when multiple CPUs are available Homogeneous processors within a multiprocessor Asymmetric multiprocessing only one processor (Master server) accesses the system data structures, alleviating the need for data sharing All scheduling decions, I/O processing and other systems activities handled by a single processor (Master server) Symmetric multiprocessing (SMP) each processor is selfscheduling, 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 26
27 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 27
28 THREADS Despite of the fact that a thread must execute in process, the process and its associated threads are different concept. Processes are used to group resources together and threads are the entities scheduled for execution on the CPU. A thread is a single sequence stream within in a process. Because threads have some of the properties of processes, they are sometimes called lightweight processes. In a process, threads allow multiple executions of streams. In many respect, threads are popular way to improve application through parallelism. 28
29 THREADS(Cont.) The CPU switches rapidly back and forth among the threads giving illusion that the threads are running in parallel. Like a traditional process i.e., process with one thread, a thread can be in any of several states (Running, Blocked, Ready or Terminated). Each thread has its own stack. Since thread will generally call different procedures and thus a different execution history. This is why thread needs its own stack. An operating system that has thread facility, the basic unit of CPU utilization is a thread. A thread has or consists of a program counter (PC), a register set, and a stack space. Threads are not independent of one other like processes as a result threads shares with other threads their code section, data section, OS resources also known as task, such as open files and signals. 29
30 THREADS(Cont.) The CPU switches rapidly back and forth among the threads giving illusion that the threads are running in parallel. Like a traditional process i.e., process with one thread, a thread can be in any of several states (Running, Blocked, Ready or Terminated). Each thread has its own stack. Since thread will generally call different procedures and thus a different execution history. This is why thread needs its own stack. An operating system that has thread facility, the basic unit of CPU utilization is a thread. A thread has or consists of a program counter (PC), a register set, and a stack space. Threads are not independent of one other like processes as a result threads shares with other threads their code section, data section, OS resources also known as task, such as open files and signals. 30
31 THREADS(Cont.) Many software packages are multi-threaded Web browser: one thread display images, another thread retrieves data from the network Word processor: threads for displaying graphics, reading keystrokes from the user, performing spelling and grammar checking in the background Web server: instead of creating a process when a request is received, which is time consuming and resource intensive, server creates a thread to service the request A thread is sometimes called a lightweight process It is comprised over a thread ID, program counter, a register set and a stack It shares with other threads belonging to the same process its code section, data section and other OS resources (e.g., open files) A process that has multiples threads can do more than one task at a time 31
32 Process Vs Threads As we mentioned earlier that in many respect threads operate in the same way as that of processes. Similarities Like processes threads share CPU and only one thread active (running) at a time. Like processes, threads within a processe, execute sequentially. Like processes, thread can create children. And like process, if one thread is blocked, another thread can run. Differences Unlike processes, threads are not independent of one another. Unlike processes, all threads can access every address in the task. Unlike processes, thread are design to assist one other. Note that processes might or might not assist one another because processes may originate from different users. 32
33 Advantages of Threads over Multiple Processes Context Switching Threads are very inexpensive to create and destroy, and they are inexpensive to represent. For example, they require space to store, the PC, the SP, and the general-purpose registers, but they do not require space to share memory information, Information about open files of I/O devices in use, etc. With so little context, it is much faster to switch between threads. In other words, it is relatively easier for a context switch using threads. Sharing Treads allow the sharing of a lot resources that cannot be shared in process, for example, sharing code section, data section, Operating System resources like open file etc. 33
34 Disadvantages of Threads over Multiple Processes Blocking The major disadvantage if that if the kernel is single threaded, a system call of one thread will block the whole process and CPU may be idle during the blocking period. Security Since there is, an extensive sharing among threads there is a potential problem of security. It is quite possible that one thread over writes the stack of another thread (or damaged shared data) although it is very unlikely since threads are meant to cooperate on a single task. 34
35 Applications that Benefit from Threads A proxy server satisfying the requests for a number of computers on a LAN would be benefited by a multi-threaded process. I In general, any program that has to do more than one task at a time could benefit from multitasking. For example, a program that reads input, process it, and outputs could have three threads, one for each task. 35
36 Linux Scheduling Prior to kernel version 2.5, ran variation of standard UNIX scheduling algorithm Version 2.5 moved to constant order O(1) scheduling time Preemptive, priority based Two priority ranges: time-sharing and real-time Real-time range from 0 to 99 and nice value from 100 to 140 Map into global priority with numerically lower values indicating higher priority Higher priority gets larger q Task run-able as long as time left in time slice (active) If no time left (expired), not run-able until all other tasks use their slices All run-able tasks tracked in per-cpu runqueue data structure Two priority arrays (active, expired) Tasks indexed by priority When no more active, arrays are exchanged Worked well, but poor response times for interactive processes 36
37 MS Windows Scheduling Windows uses priority-based preemptive scheduling Highest-priority thread runs next Dispatcher is scheduler Thread runs until (1) blocks, (2) uses time slice, (3) preempted by higher-priority thread Real-time threads can preempt non-real-time 32-level priority scheme Variable class is 1-15, real-time class is Priority 0 is memory-management thread Queue for each priority If no run-able thread, runs idle thread 37
38 Solaris Scheduling Priority-based scheduling Six classes available Time sharing (default) (TS) Interactive (IA) Real time (RT) System (SYS) Fair Share (FSS) Fixed priority (FP) Given thread can be in one class at a time Each class has its own scheduling algorithm Time sharing is multi-level feedback queue Loadable table configurable by sysadmin 38
39 Deterministic Evaluation For each algorithm, calculate minimum average waiting time Simple and fast, but requires exact numbers for input, applies only to those inputs FCS is 28ms: Non-preemptive SFJ is 13ms: RR is 23ms: 39
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