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Template 1 ch5
1.
Chapter 5: CPU
Scheduling Operating System Concepts – 8th Edition, SLU SCIS Silberschatz, Galvin and Gagne ©2009
2.
Chapter 5: CPU
Scheduling • Basic Concepts • Scheduling Criteria • Scheduling Algorithms • Thread Scheduling • Multiple-Processor Scheduling • Operating Systems Examples • Algorithm Evaluation Operating System Concepts – 8th Edition, SLU SCIS 5.2 Silberschatz, Galvin and Gagne ©2009
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 Operating System Concepts – 8th Edition, SLU SCIS 5.3 Silberschatz, Galvin and Gagne ©2009
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 distribution Operating System Concepts – 8th Edition, SLU SCIS 5.4 Silberschatz, Galvin and Gagne ©2009
5.
Histogram of CPU-burst
Times Operating System Concepts – 8th Edition, SLU SCIS 5.5 Silberschatz, Galvin and Gagne ©2009
6.
Alternating Sequence of
CPU And I/O Bursts Operating System Concepts – 8th Edition, SLU SCIS 5.6 Silberschatz, Galvin and Gagne ©2009
7.
CPU Scheduler
• Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them • 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 Operating System Concepts – 8th Edition, SLU SCIS 5.7 Silberschatz, Galvin and Gagne ©2009
8.
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 Operating System Concepts – 8th Edition, SLU SCIS 5.8 Silberschatz, Galvin and Gagne ©2009
9.
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 timesharing environment) Operating System Concepts – 8th Edition, SLU SCIS 5.9 Silberschatz, Galvin and Gagne ©2009
10.
Scheduling Algorithm
Optimization Criteria • Max CPU utilization • Max throughput • Min turnaround time • Min waiting time • Min response time Operating System Concepts – 8th Edition, SLU SCIS 5.10 Silberschatz, Galvin and Gagne ©2009
11.
First-Come, First-Served (FCFS)
Scheduling • Suppose that the processes arrive in the order: P1 , P2 , P3 The Gantt Chart for the schedule is: • Waiting time for P1 = 0; P2 = 24; P3 = 27 • Average waiting time: (0 + 24 + 27)/3 = 17 Operating System Concepts – 8th Edition, SLU SCIS 5.11 Silberschatz, Galvin and Gagne ©2009
12.
First-Come, First-Served (FCFS)
Scheduling Suppose that the processes arrive in the order P2 , P3 , P1 • The Gantt chart for the schedule is: • Waiting time for P1 = 6; P2 = 0; P3 = 3 • Average waiting time: (6 + 0 + 3)/3 = 3 •Much better than previous case • Convoy effect short process behind long process Operating System Concepts – 8th Edition, SLU SCIS 5.12 Silberschatz, Galvin and Gagne ©2009
13.
Shortest-Job-First (SJF) Scheduling
• Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time • SJF is optimal – gives minimum average waiting time for a given set of processes  The difficulty is knowing the length of the next CPU request Operating System Concepts – 8th Edition, SLU SCIS 5.13 Silberschatz, Galvin and Gagne ©2009
14.
Example of SJF Operating
System Concepts – 8th Edition, SLU SCIS 5.14 Silberschatz, Galvin and Gagne ©2009
15.
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 Operating System Concepts – 8th Edition, SLU SCIS 5.15 Silberschatz, Galvin and Gagne ©2009
16.
Prediction of the
Length of the Next CPU Burst Operating System Concepts – 8th Edition, SLU SCIS 5.16 Silberschatz, Galvin and Gagne ©2009
17.
Examples of Exponential
Averaging • α =0  τ τ n+1 = n  Recent history does not count • α =1  τn+1 = α tn  Only the actual last CPU burst counts • If we expand the formula, we get: τn+1 = α tn+(1 - α)α tn -1 + … +(1 - α )j α tn -j + … +(1 - α )n +1 τ0 • Since both α and (1 - α) are less than or equal to 1, each successive term has less weight than its predecessor Operating System Concepts – 8th Edition, SLU SCIS 5.17 Silberschatz, Galvin and Gagne ©2009
18.
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 Operating System Concepts – 8th Edition, SLU SCIS 5.18 Silberschatz, Galvin and Gagne ©2009
19.
Round Robin (RR)
• Each process gets a small unit of CPU time (time quantum), usually 10-100 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. • Performance  q large ⇒ FIFO  q small ⇒ q must be large with respect to context switch, otherwise overhead is too high Operating System Concepts – 8th Edition, SLU SCIS 5.19 Silberschatz, Galvin and Gagne ©2009
20.
Example of RR
with Time Quantum = 4 Operating System Concepts – 8th Edition, SLU SCIS 5.20 Silberschatz, Galvin and Gagne ©2009
21.
Time Quantum and
Context Switch Operating System Concepts – 8th Edition, SLU SCIS 5.21 Silberschatz, Galvin and Gagne ©2009
22.
Turnaround Time Varies
With The Time Quantum Operating System Concepts – 8th Edition, SLU SCIS 5.22 Silberschatz, Galvin and Gagne ©2009
23.
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  20% to background in FCFS Operating System Concepts – 8th Edition, SLU SCIS 5.23 Silberschatz, Galvin and Gagne ©2009
24.
Multilevel Queue Scheduling Operating
System Concepts – 8th Edition, SLU SCIS 5.24 Silberschatz, Galvin and Gagne ©2009
25.
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 Operating System Concepts – 8th Edition, SLU SCIS 5.25 Silberschatz, Galvin and Gagne ©2009
26.
Example of Multilevel
Feedback Queue • Three queues:  Q0 – RR with time quantum 8 milliseconds  Q1 – RR time quantum 16 milliseconds  Q2 – FCFS • Scheduling  A new job enters queue Q0 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 Q1.  At Q1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q2. Operating System Concepts – 8th Edition, SLU SCIS 5.26 Silberschatz, Galvin and Gagne ©2009
27.
Multilevel Feedback Queues Operating
System Concepts – 8th Edition, SLU SCIS 5.27 Silberschatz, Galvin and Gagne ©2009
28.
Thread Scheduling
• Distinction between user-level and kernel-level threads • Many-to-one and many-to-many models, thread library schedules user-level threads to run on LWP  Known as process-contention scope (PCS) since scheduling competition is within the process • Kernel thread scheduled onto available CPU is system-contention scope (SCS) – competition among all threads in system Operating System Concepts – 8th Edition, SLU SCIS 5.28 Silberschatz, Galvin and Gagne ©2009
29.
Pthread Scheduling
• API allows specifying either PCS or SCS during thread creation  PTHREAD SCOPE PROCESS schedules threads using PCS scheduling  PTHREAD SCOPE SYSTEM schedules threads using SCS scheduling. Operating System Concepts – 8th Edition, SLU SCIS 5.29 Silberschatz, Galvin and Gagne ©2009
30.
Pthread Scheduling API Operating
System Concepts – 8th Edition, SLU SCIS 5.30 Silberschatz, Galvin and Gagne ©2009
31.
Pthread Scheduling API Operating
System Concepts – 8th Edition, SLU SCIS 5.31 Silberschatz, Galvin and Gagne ©2009
32.
Multiple-Processor Scheduling
• CPU scheduling more complex when multiple CPUs are available • Homogeneous processors within a multiprocessor • Asymmetric multiprocessing – only one processor accesses the system data structures, alleviating the need for data sharing • Symmetric multiprocessing (SMP) – each processor is self- scheduling, all processes in common ready queue, or each has its own private queue of ready processes • Processor affinity – process has affinity for processor on which it is currently running  soft affinity  hard affinity Operating System Concepts – 8th Edition, SLU SCIS 5.32 Silberschatz, Galvin and Gagne ©2009
33.
NUMA and CPU
Scheduling Operating System Concepts – 8th Edition, SLU SCIS 5.33 Silberschatz, Galvin and Gagne ©2009
34.
Multicore Processors
• Recent trend to place multiple processor cores on same physical chip • Faster and consume less power • Multiple threads per core also growing • Takes advantage of memory stall to make progress on another thread while memory retrieve happens Operating System Concepts – 8th Edition, SLU SCIS 5.34 Silberschatz, Galvin and Gagne ©2009
35.
Multithreaded Multicore System Operating
System Concepts – 8th Edition, SLU SCIS 5.35 Silberschatz, Galvin and Gagne ©2009
36.
Operating System Examples
• Solaris scheduling • Windows XP scheduling • Linux scheduling Operating System Concepts – 8th Edition, SLU SCIS 5.36 Silberschatz, Galvin and Gagne ©2009
37.
Solaris Dispatch Table Operating
System Concepts – 8th Edition, SLU SCIS 5.37 Silberschatz, Galvin and Gagne ©2009
38.
Solaris Scheduling Operating System
Concepts – 8th Edition, SLU SCIS 5.38 Silberschatz, Galvin and Gagne ©2009
39.
Windows XP Priorities Operating
System Concepts – 8th Edition, SLU SCIS 5.39 Silberschatz, Galvin and Gagne ©2009
40.
Linux Scheduling
• Constant order O(1) scheduling time • Two priority ranges: time-sharing and real-time • Real-time range from 0 to 99 and nice value from 100 to 140 • (figure 5.15) Operating System Concepts – 8th Edition, SLU SCIS 5.40 Silberschatz, Galvin and Gagne ©2009
41.
Priorities and Time-slice
length Operating System Concepts – 8th Edition, SLU SCIS 5.41 Silberschatz, Galvin and Gagne ©2009
42.
List of Tasks
Indexed According to Priorities Operating System Concepts – 8th Edition, SLU SCIS 5.42 Silberschatz, Galvin and Gagne ©2009
43.
Algorithm Evaluation
• Deterministic modeling – takes a particular predetermined workload and defines the performance of each algorithm for that workload • Queueing models • Implementation Operating System Concepts – 8th Edition, SLU SCIS 5.42 Silberschatz, Galvin and Gagne ©2009
44.
Evaluation of CPU
schedulers by Simulation Operating System Concepts – 8th Edition, SLU SCIS 5.44 Silberschatz, Galvin and Gagne ©2009
45.
End of Chapter
5 Operating System Concepts – 8th Edition, SLU SCIS Silberschatz, Galvin and Gagne ©2009
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