Comp 310 Computer Systems and Organization

Transcription

1 Comp 310 Computer Systems and Organization Lecture #9 Process Management (CPU Scheduling) 1 Prof. Joseph Vybihal

2 Announcements Oct 16 Midterm exam (in class) In class review Oct 14 (½ class review) Ass#2 out 2

3 Basic OS Architecture (Course Table of Contents) Security Phase 4 User Interface Phase 1 Memory Manager Phase 3 Disk / Storage Manager Process Manager Phase 2 Network Manager Hardware Manager 3

4 Lecture Purpose Creating a fair CPU scheduler Understanding the properties of a process Taking advantage of these properties when engineering the OS. 4

5 Scheduling Criteria CPU Utilization How busy or idle is the cpu in %. Throughput Number of processes that terminate per time period. Turnaround time Averge time it takes for a process to terminate Waiting time Averge time process is not using the cpu Response time 5 How quickly the process is activated by an event

6 Part 1 6 Process Management

7 7 Basic Architecture

8 Basic Architecture Assume one CPU on computer (scalable to many) Many processes in RAM wanting CPU A Queue is used for turn taking Each node in queue is a PCB Queue runs in OS RAM space User s code not in PCB, but PCB points to it in user RAM space. Code sits in RAM. The following functions are needed: OS has a Loader to convert programs on disk into processes in RAM inserts a PCB on queue OS has a Terminator to delete process in RAM and free memory. OS has a Swapper that implements the process switch 8 OS has a Scheduler that operates the Queue

9 Even with simple OS a Task Switcher is needed... No quanta but task switch needed between: Process OS Device drivers Draw how this happens... 9

10 Process Properties 10

11 Properties of a Process: I/O Alternating bursts CPU Burst Concept Run process until I/O or System Call Task switch to OS or Driver Example: menu, word processor, browser, Return to program 11

12 Properties of a Process: Short What does this mean about fairness? Few processes run for a long time Due to I/O (maybe buffers all loaded in RAM) 12

13 13 The OS Scheduling Process

14 Generally Series of Queues & Interrupts From loader Pass off to scheduler At OS scheduler I/O Normal run-time operating environment This means running process but assumes multiple CPUs. Long I/O or other problem requiring it to wait a long time. 14

15 HOW IT STARTS: The Loader Function The command-line prompt processes user request to run a program. The command-line activates the Loader. Request entry OS PCB Store (double click) (queued until OS has time) (then find user space, insert PCB) User Space program disk RAM 15

16 Management Software Loader Short-term Scheduler The CPU Scheduler The Kernel Calls Dispatcher, terminator, Each light circle is a queue with a PCB Dispatcher Switching context Switching to user mode Jumping to proper location in user s program to restart Terminator Not a queue, it is actually on CPU Preemptive Scheduling Interrupting the normal execution of a program (context switch, I/O) 16

17 Process Termination A process enters the exiting state for one of the following reasons Normal completion A process executes a system call for termination (e.g. in Unix exit()) Abnormal termination Program errors Run time I/O User intervention 17

18 About Optimization 18

19 Optimizing the Quanta Smaller quanta increases context switches What does this mean about turnaround time? What does this mean about fairness? 19

20 Optimizing the Quanta How long it takes for a program to finish Given these in this order But is this fair? Adjusting quanta gives 20

21 Waiting for CPU? If quanta is uniformly the value q Then waiting for the CPU = (n 1) * q where n-1 is the number of processes ahead So what is a good quanta? How expensive is a context switch? Quanta = 90% exec + 10% switch Actually hard to deduce If majority of Pi load/run/finish in 90% then good? But maybe not fair 21

22 Want to Optimize the Scheduler on these Units CPU Utilization Throughput how many programs terminated per T? Measure depends on working environment Turnaround Time how long did the program take to run? T = execution + waiting Waiting Time CPU = waiting in ready queue / turnaround time The I/O wait is another queue Response Time from double-click to window appears Request to process to beginning of execution Optimization Criteria keep the CPU busy A CPU being idle is bad not getting its full usage 40% light load, 90% heavy load 22

23 Question How can we program the OS scheduler to dynamically adjust itself to reduce: Turn around time Waiting time (what are the side-effects?) 23

24 Standard scheduling and issues of fairness 24

25 Standard Scheduling Architecture RAM user Child Process User s space User s process 25

26 Basic Scheduling (FIFO Scheduling / Round Robin Queue) quanta I/O interrupt First-in is the First-out Queue The one out is put in the back of the line (round robin) 26

27 Fairness of FIFO? Process p1 p2 p3 Burst-time Arrival Order p1 p3 p2 Which is more fair? p2 p1 p3 Other Order A B Average waiting time? A = ( ) / 3 = 17 B = ( ) / 3 = 3 27

28 More Fair Queues? Direct CPU Usage Scheduling Shortest-Job-First Scheduling (SJF) Provable optimal Problem: how long does the next PCB need the CPU? Unknown Prediction = ά tn + (1 ά) Tn Where: t is last CPU burst, T is accumulated average bust Info stored in PCB Shortest-Remaining-Time-First (preemption of CPU) Priority Scheduling More than one level of queue, sort by SJF and Priority Other Effects to Scheduling (I/O, Interrupts, Delays, Priority) 28

29 Standard Queue Implementations that attempt to be more fair 29

30 Multilevel Priority Queue TO CPU 30

31 q=4 Wait until top Q done but get longer Quanta Multilevel Feedback Queue q=8 q = 16 Reset after I/O burst q=n 31

32 Balanced Resource Scheduling Fair Share Scheduler CPU Groupings by resource 32

33 Low Level Schedulers 33

34 Multi-CPU Scope Process Contention Scope A process competing for LWP assignment System Contention Scope A LWP competing for a pooled CPU 34

35 Cache Problems If a process has been running on CPU1 for a long time, its data will be in the cache of CPU1. If it is assigned later to another CPU, what effect does this have on execution? 35

36 Multi-Core CPU Problems One CPU but multi-cored pipelines One CPU multi-cored but multi-threaded Problems: Compute cycles and Memory stalls Coarse & fine grained multi-threading At stall or at instruction cycle Appearance as multiple CPUs to the OS 36

37 Part 2 37 At Home

38 Things to try out 1. Windows and Linux use a priority queue Use your OS and identify processes that have higher and lower priorities Try to locate where, in the OS, you can adjust the priority of a process How much freedom do you have in adjusting the priority? 38