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  1. Processes Dr. Yingwu Zhu

  2. Process Concept • Process – a program in execution • What is not a process? -- program on a disk - a process is an active object, but a program is just a file • A process includes: • program counter • Text section: program code • Stack: local variables, function params, return addresses • data section: global variables • Heap (optional): dynamically allocated memory

  3. Process State • Processes switch between different states based on internal and external events • Each process is in exactly one state at a time • As a process executes, it changes state (Typical States of Processes (varies with OS)) • new: The process is being created • running: Instructions are being executed (only one process per processor may be running) • waiting: The process is waiting for some event (e.g., I/O, signals) to occur • ready: The process is waiting to be assigned to a processor • terminated: The process has finished execution

  4. Diagram of Process State

  5. CPU Switch From Process to Process

  6. Process Control Block (PCB) -- PCB Stores all of the information about a process Information associated with each process • Process state • Program counter • CPU registers: accumulators, index registers, stack pointers, etc. • CPU scheduling information: priority, etc. • Memory-management information: base/limit, page tables, or segment tables • Accounting information: CPU, etc • I/O status information: a list of I/O devices allocated, a list of open files, etc.

  7. Process Control Block (PCB)

  8. P1 P2 P3 P5 P2 P4 Waiting Ready Waiting Term Ready New Maintaining PCBs • Keep track of the different processes in the system • Collection of PCBs is called a process table • How to store the process table? • Option 1: • Problems with Option 1: • hard to find processes • how to fairly select a process

  9. Solution: Process Scheduling Queues • Store processes in queues based on state • Processes migrate among the various queues • Job queue – set of all processes in the system • Ready queue – set of all processes residing in main memory, ready and waiting to execute • Device queues – set of processes waiting for an I/O device

  10. Ready Queue And Various I/O Device Queues

  11. Representation of Process Scheduling

  12. Schedulers • Long-term scheduler (or job scheduler) • selects which processes should be brought into the memory; controls degree of multiprogramming (# of processes in memory) • Short-term scheduler (or CPU scheduler) • selects which process should be executed next and allocates CPU

  13. Addition of Medium-Term Scheduling Swap in/out processes memory, adaptive to memory status and process status

  14. Schedulers (Cont.) • Short-term scheduler is invoked very frequently (milliseconds)  (must be fast) • Long-term scheduler is invoked very infrequently (seconds, minutes)  (may be slow) • The long-term scheduler controls the degree of multiprogramming • Processes can be described as either: • I/O-bound process – spends more time doing I/O than computations, many short CPU bursts • CPU-bound process – spends more time doing computations; few very long CPU bursts

  15. Short-Term Scheduler • Responsible for: • saving state into PCB when switching to a new process • selecting a process to run (from the ready queue) • loading state of another process • One of the most time critical parts of the OS

  16. Selecting a Process to Run • called scheduling • can simply pick the first item in the queue • called round-robin scheduling • is round-robin scheduling fair? • can use more complex schemes • we will study these in the future • use alarm interrupts to switch between processes • when time is up, a process is put back on the end of the ready queue • frequency of these interrupts is an important parameter • typically 3-10ms on modern systems • need to balance overhead of switching vs. responsiveness

  17. Context Switch • When CPU switches to another process, the system must save the state of the old process and load the saved state for the new process • Context-switch time is overhead; the system does NO useful work while switching • Time dependent on hardware support • Depends on • Memory speed, #-of-registers to copy, special instructions (single instruction to load/save all registers) • A few milliseconds

  18. Process Creation • Parent processes create children processes, which, in turn create other processes, forming a tree of processes • Resource sharing • Parent and children share all resources • Children share subset of parent’s resources • Parent and child share no resources • Execution • Parent and children execute concurrently • Parent waits until children terminate

  19. Process Creation (Cont.) • Address space • Child duplicate of parent • Child has a program loaded into it • UNIX examples • fork system call creates new process • exec system call used after a fork to replace the process’ memory space with a new program

  20. Forking a New Process • create a PCB for the new process • copy most entries from the parent • clear accounting fields • buffered pending I/O • allocate a pid (process id for the new process) • allocate memory for it • could require copying all of the parents segments • however, text segment usually doesn’t change so that could be shared • might be able to use memory mapping hardware to help • will talk more about this in the memory management part of the class • add it to the ready queue

  21. C Program Forking Separate Process int main() { pid_t pid; /* fork another process */ pid = fork(); if (pid < 0) { /* error occurred */ fprintf(stderr, "Fork Failed"); exit(-1); } else if (pid == 0) { /* child process */ execlp("/bin/ls", "ls", NULL); } else { /* parent process */ /* parent will wait for the child to complete */ wait (NULL); printf ("Child Complete"); exit(0); } }

  22. Process Creation

  23. Process Termination • Process executes last statement and asks the operating system to delete it (exit) • Output data from child to parent (via wait) • Process’ resources are deallocated by operating system • Parent may terminate execution of children processes (abort) • Child has exceeded allocated resources • Task assigned to child is no longer required • If parent is exiting  orphan process • Some operating system do not allow child to continue if its parent terminates • All children terminated - cascading termination (VMS) • in UNIX becomes child of the root process

  24. Process Termination - UNIX example • Kernel • frees memory used by the process • moved PCB to the terminated queue • Terminated process • signals parent of its death (SIGCHILD) • is called a zombie in UNIX • remains around waiting to be reclaimed • parent process • wait system call retrieves info about the dead process • exit status • accounting information • signal handler is generally called the reaper • since its job is to collect the dead processes

  25. Linux case: controlling processes

  26. Linux: Process Attributes • The process ID or PID: a unique identification number used to refer to the process. • The parent process ID or PPID: the number of the process (PID) that started this process. • Nice number: the degree of friendliness of this process toward other processes (not to be confused with process priority, which is calculated based on this nice number and recent CPU usage of the process). • Terminal or TTY: terminal to which the process is connected. • User name of the real and effective user (RUID and EUID): the owner of the process. The real owner is the user issuing the command, the effective user is the one determining access to system resources. RUID and EUID are usually the same, and the process has the same access rights the issuing user would have. • Real and effective group owner (RGID and EGID): The real group owner of a process is the primary group of the user who started the process. The effective group owner is usually the same, except when SGID access mode has been applied to a file. • Cmd: ps -af

  27. ps -ef | grep username • This displays all processes owned by a particular user • pstree

  28. Case Study: Unix Processes • See Supl. Materials online

  29. Exercise • How many processes for the segment of code int main() { for (int m = 0; m<3; m++) pid_t x = fork(); //assume success all the times return 0; }