Process Management in CMSC 412: PCBs, Queues, Forking, and Process Termination - Prof. Jef, Study notes of Operating Systems

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Announcements

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Program #1– Is on the web

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Reading– Chapter 4– Chapter 6 (for Tuesday)

Process Control Block

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Stores all of the information about a process

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PCB contains– process state: new, ready, etc.– processor registers– Memory Management Information

• page tables, and limit registers for segments
  • CPU scheduling information
    • process priority• pointers to process queues
      • Accounting information
        • time used (and limits)• files used• program owner
          • I/O status information
            • list of open files• pending I/O operations

Queues of Processes

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Store processes in queues based on state

P

P

ReadyQueue

P

P

DiskQueue

P

P

NetworkQueue

forking a new process

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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)

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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

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add it to the ready queue

Termination (cont.) - UNIX example

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Kernel– frees memory used by the process– moved process control block to the terminated queue

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Terminated process– signals parent of its death (SIGCHILD)– is called a zombie in UNIX– remains around waiting to be reclaimed

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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

Threads

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processes can be a heavy (expensive) object

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threads are like processes but generally a collectionof threads will share– memory (except stack)– open files (and buffered data)– signals

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can be user or system level– user level: kernel sees one process

• easy to implement by users- I/O management is difficult- in an multi-processor can’t get parallelism

• system level: kernel schedules threads

Dispatcher

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The inner most part of the OS that runs processes

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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

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Sometimes called the short term scheduler– but does more than schedule

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Switching between processes is called contextswitching

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One of the most time critical parts of the OS

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Almost never can be written completely in a highlevel language