1. Slide 6-1 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Processes and Threads 6

2. Slide 6-2 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Announcements Homework Set #2 due Thursday at 11 am Program Assignment #1 due Thursday Feb. 10 at 11 am –TA will introduce in recitation Wednesday Read chapters 6 and 7

3. Slide 6-3 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 What is a Process? A process is a program actively executing from main memory –has a Program Counter (PC) and execution state associated with it CPU registers keep state OS keeps process state in memory it’s alive! –has an address space associated with it a limited set of (virtual) addresses that can be accessed by the executing code Code Data Main Memory Program P1 binary CPU Execution Program Counter (PC) Registers ALU Fetch Code and Data Write Data Process Heap Stack

4. Slide 6-4 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 How is a Process Structured in Memory? Run-time memory image Essentially code, data, stack, and heap Code and data loaded from executable file Stack grows downward, heap grows upward User stack Heap Read/write.data,.bss Read-only.init,.text,.rodata Unallocated Run-time memory address 0 max address

5. Slide 6-5 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Multiple Processes Main Memory Code Data Process P1 Heap Stack Code Data Process P2 Heap Stack Process state, e.g. ready, running, or waiting accounting info, e.g. process ID Program Counter CPU registers CPU- scheduling info, e.g. priority Memory management info, e.g. base and limit registers, page tables I/O status info, e.g. list of open files Code More Data, Heap, Stack OS PCB for P2 PCB for P1

6. Slide 6-6 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Multiple Processes Main Memory Code Data Process P1 Heap Stack Code Data Process P2 Heap Stack Code More Data, Heap, Stack OS PCB for P2 PCB for P1 CPU Execution Program Counter (PC) ALU

7. Slide 6-7 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Context Switching Process Manager Interrupt Handler P1P1 P2P2 PnPn Executable Memory Initialization 1 2 3 4 5 7 Interrupt 8 9 6 Each time a process is switched out, its context must be saved, e.g. in the PCB Each time a process is switched in, its context is restored This usually requires copying of registers

8. Slide 6-8 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Threads A thread is a logical flow of execution that runs within the context of a process –has its own program counter (PC), register state, and stack –shares the memory address space with other threads in the same process, share the same code and data and resources (e.g. open files)

9. Slide 6-9 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Threads Why would you want multithreaded processes? –reduced context switch overhead In Solaris, context switching between processes is 5x slower than switching between threads –shared resources => less memory consumption => more threads can be supported, especially for a scalable system, e.g. Web server must handle thousands of connections –inter-thread communication is easier and faster than inter-process communication –thread also called a lightweight process

10. Slide 6-10 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Threads Process P1 is multithreaded Process P2 is single threaded The OS is multiprogram med If there is preemptive timeslicing, the system is multitasked Main Memory Code Data Process P1’s Address Space Heap Code Data Process P2 Heap Stack PC1 Reg. State Thread 1 Stack PC2 Reg. State Thread 2 Stack PC3 Reg. State Thread 3

11. Slide 6-11 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Processes &Threads Address Space Map Stack State Program Static data Resources Stack State Map

12. Slide 6-12 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Thread-Safe/Reentrant Code If two threads share and execute the same code, then the code needs to be thread-safe –the use of global variables is not thread safe –the use of static variables is not thread safe –the use of local variables is thread safe need to govern access to persistent data like global/static variables with locking and synchronization mechanisms reentrant is a special case of thread-safe: –reentrant code does not have any references to global variables –thread-safe code protects and synchronizes access to global variables

13. Slide 6-13 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 User-Space and Kernel Threads pthreads is a POSIX user space threading API –provides interface to create, delete threads in the same process –threads will synchronize with each other via this package –no need to involve the OS –implementations of pthreads API differ underneath the API Kernel threads are supported by the OS –kernel must be involved in switching threads –mapping of user-level threads to kernel threads is usually one-to- one

14. Slide 6-14 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Model of Process Execution Ready List Ready List Scheduler CPU Resource Manager Resource Manager Resources Preemption or voluntary yield AllocateRequest Done New Process job “Ready” “Running” “Blocked”

15. Slide 6-15 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 The Scheduler Ready Process Enqueuer Ready List Ready List Dispatcher Context Switcher Context Switcher Process Descriptor Process Descriptor CPU From Other States Running Process

16. Slide 6-16 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Invoking the Scheduler Need a mechanism to call the scheduler Voluntary call –Process blocks itself –Calls the scheduler Involuntary call –External force (interrupt) blocks the process –Calls the scheduler

17. Slide 6-17 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Voluntary CPU Sharing yield(p i.pc, p j.pc) { memory[p i.pc] = PC; PC = memory[p j.pc]; } p i can be “automatically” determined from the processor status registers yield(*, p j.pc) { memory[p i.pc] = PC; PC = memory[p j.pc]; }

18. Slide 6-18 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 More on Yield yield(*, p j.pc);... yield(*, p i.pc);... yield(*, p j.pc);... p i and p j can resume one another’s execution Suppose p j is the scheduler: // p_i yields to scheduler yield(*, p j.pc); // scheduler chooses p k yield(*, p k.pc); // p k yields to scheduler yield(*, p j.pc); // scheduler chooses...

19. Slide 6-19 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Voluntary Sharing Every process periodically yields to the scheduler Relies on correct process behavior –process can fail to yield: infinite loop either intentionally (while(1)) or due to logical error (while(!DONE)) Malicious Accidental –process can yield to soon: unfairness for the “nice” processes who give up the CPU, while others do not –process can fail to yield in time: another process urgently needs the CPU to read incoming data flowing into a bounded buffer, but doesn’t get the CPU in time to prevent the buffer from overflowing and dropping information Need a mechanism to override running process

20. Slide 6-20 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Involuntary CPU Sharing Interval timer –Device to produce a periodic interrupt –Programmable period IntervalTimer() { InterruptCount--; if(InterruptCount <= 0) { InterruptRequest = TRUE; InterruptCount = K; } SetInterval(programmableValue) { K = programmableValue: InterruptCount = K; }

21. Slide 6-21 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Involuntary CPU Sharing (cont) Interval timer device handler –Keeps an in-memory clock up-to-date (see Chap 4 lab exercise) –Invokes the scheduler IntervalTimerHandler() { Time++; // update the clock TimeToSchedule--; if(TimeToSchedule <= 0) { ; TimeToSchedule = TimeSlice; }

22. Slide 6-22 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Contemporary Scheduling Involuntary CPU sharing – timer interrupts –Time quantum determined by interval timer – usually fixed size for every process using the system –Sometimes called the time slice length

23. Slide 6-23 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Choosing a Process to Run Mechanism never changes Strategy = policy the dispatcher uses to select a process from the ready list Different policies for different requirements Ready Process Enqueue Ready List Ready List Dispatch Context Switch Context Switch Process Descriptor Process Descriptor CPU Running Process

24. Slide 6-24 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Policy Considerations Policy can control/influence: –CPU utilization –Average time a process waits for service –Average amount of time to complete a job Could strive for any of: –Equitability –Favor very short or long jobs –Meet priority requirements –Meet deadlines

25. Slide 6-25 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Optimal Scheduling Suppose the scheduler knows each process p i ’s service time,  p i  -- or it can estimate each  p i  : Policy can optimize on any criteria, e.g., –CPU utilization –Waiting time –Deadline To find an optimal schedule: –Have a finite, fixed # of p i –Know  p i  for each p i –Enumerate all schedules, then choose the best

26. Slide 6-26 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 However... The  (p i ) are almost certainly just estimates General algorithm to choose optimal schedule is O(n 2 ) Other processes may arrive while these processes are being serviced Usually, optimal schedule is only a theoretical benchmark – scheduling policies try to approximate an optimal schedule

27. Slide 6-27 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Talking About Scheduling... Let P = {p i | 0  i < n} = set of processes Let S(p i )  {running, ready, blocked} Let  (p i ) = Time process needs to be in running state (the service time) Let W(p i ) = Time p i is in ready state before first transition to running (wait time) Let T TRnd (p i ) = Time from p i first enter ready to last exit ready (turnaround time) Batch Throughput rate = inverse of avg T TRnd Timesharing response time = W(p i )

28. Slide 6-28 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Simplified Model Ready List Ready List Scheduler CPU Resource Manager Resource Manager Resources AllocateRequest Done New Process job “Ready” “Running” “Blocked” Simplified, but still provide analysis result Easy to analyze performance No issue of voluntary/involuntary sharing Preemption or voluntary yield

29. Slide 6-29 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Estimating CPU Utilization Ready List Ready List Scheduler CPU Done New Process System p i per second Each p i uses 1/  units of the CPU Let = the average rate at which processes are placed in the Ready List, arrival rate Let  = the average service rate  1/  = the average  (p i )

30. Slide 6-30 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Estimating CPU Utilization Ready List Ready List Scheduler CPU Done New Process Let = the average rate at which processes are placed in the Ready List, arrival rate Let  = the average service rate  1/  = the average  (p i ) Let  = the fraction of the time that the CPU is expected to be busy  = # p i that arrive per unit time * avg time each spends on CPU  = * 1/  = /  Notice must have <  (i.e.,  < 1) What if  approaches 1?

31. Slide 6-31 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Nonpreemptive Schedulers Ready List Ready List Scheduler CPU Done New Process Try to use the simplified scheduling model Only consider running and ready states Ignores time in blocked state: –“New process created when it enters ready state” –“Process is destroyed when it enters blocked state” –Really just looking at “small phases” of a process Blocked or preempted processes