Operating Systems COMP 4850/CISG 5550

1. Operating SystemsCOMP 4850/CISG 5550 Processes Introduction to Threads Dr. James Money

2. Processes • A process is the abstraction of a running program. This includes the code, associated memory, among other items. • Most operating systems allow more than one process to be runnable at a given time.

3. Process Model • A process is an executing program • We mentally think of the process having a virtual CPU. • However, in reality, the CPU switches between active processes in a rapid fashion. • This switching is called multiprogramming.

4. Process Model

5. Process Creation • We need a way to create and terminate a process • There are a number of ways a process is created.

6. Process Creation • Processes are created when: • System initialization • Execution of a process creation system call by a running process • A user request to create a new process • Initiation of a batch job

7. Types of processes • There are two types of processes: • Foreground – interact with user • Background – run a particular function. • These are called daemons. • Typically these handle mail, web pages, remote file requests, etc.

8. Creating Processes • UNIX: • fork() • Followed by exec() • Windows • CreateProcess • Separate address space for each process

9. Process Termination • Eventually a processes ends or terminates due to one of the following: • Normal, voluntary exit • Error exit, voluntary • Fatal error, involuntary • Killed by another process, involuntary

10. Process Hierarchies • When processes create new processes, you get a hierarchy in UNIX, which also called a process group. • One process creates a new one, which is called the child process. • The process that create the new process is called the parent process. • Windows has no hierarchy.

11. Process States • Many times processes interact with other process • Example: • cat chapter1 chapter2 chapter3|grep tree • Grep may run before cat starts it’s output • This state is called blocked.

12. Process States • There are three states • Running – using the CPU • Ready – runnable, but stopped • Blocked – waiting for an external event to occur

13. Process States • Examples of transitions between states:

14. Process States • This model results in a scheduler, which handles which process is executing a given instant. • We think of the scheduler being the base of the operating system • The scheduler handles interrupts and scheduling.

15. Implementing Processes • To implement, we use a process table, with one entry per process. • Each entry is a called a process control block.

16. Implementing Processes • Each PCB has • Process state • Program counter • Stack pointer • Memory allocations • Open files • Scheduling information • Other info needed for running

17. PCBs

18. Handling Interrupts

19. Threads • Normally, each process has one thread of execution, or shortened, a thread. • Many times it is desirable to have multiple threads • Sometimes called lightweight processes. • We use the term multithreading to refer to the fact multiple threads are running in a single process.

20. Thread Model

21. Thread Model • Different threads != different processes • No memory protection in threads, so they can wipe other threads memory values • Threads have unique: • Program counters • Registers • Stack • State information

22. Thread Model

23. Thread Model • Threads have three states like processes, plus a final one: • Running • Blocked • Ready • And terminated • Threads have independent stacks

24. Thread Model

25. Thread Functions • thread_create() – create a new thread from a function pointer • thread_exit() – exit a thread • thread_wait() – wait for a thread to finish • thread_yield() – voluntarily give up CPU to other threads