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The ‘process’ abstraction

The ‘process’ abstraction. An introductory exploration of ‘process management’ as conducted by Linux. ‘program’ versus ‘process’. A ‘program’ is a sequence of instructions It’s an algorithm, expressed in a computer language, but it does nothing on its own

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The ‘process’ abstraction

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  1. The ‘process’ abstraction An introductory exploration of ‘process management’ as conducted by Linux

  2. ‘program’ versus ‘process’ • A ‘program’ is a sequence of instructions • It’s an algorithm, expressed in a computer language, but it does nothing on its own • A ‘process’ is a succession of actions that are performed when a program executes • It’s a dynamically changing entity, which has an existence over time, but it requires a processor and it uses various resources

  3. Process management • Some operating systems were designed to manage only one process at a time (e.g. CP/M, PC-DOS) • But most modern systems are designed to manage multiple processes concurrently (e.g., Windows, UNIX/Linux) • Not surprisingly, these latter systems are much more complex: they must cope with issues of ‘cooperation’ and ‘competition’

  4. Tracking a ‘process’ • As a process-manager, the OS must keep track of each process that is currently in existence, to facilitate cooperation among them and to mediate competing demands • It uses some special data-structures to do this (with assistance from a timing-device) • The various processes ‘take turns’ using the processor’s time, and share access to the system’s peripheral hardware devices

  5. The process control block • The OS kernel creates a data-structure for each current process, known as a process control block or task record or ‘task_struct’ • In Linux, these structures are all arranged in a doubly-linked list (i.e., the ‘task list’) task list task struct task struct task struct task struct

  6. Several dozen fields • Dozens of separate items of information are kept in a Linux ‘task_struct’; e.g.: • pid; // process ID-number • state; // current task-state • priority; // current task-priority • start_time; // time when task was created • sleep_time; // time when task began sleeping • . . . // … many more items … • This info is used by the Linux ‘scheduler’

  7. The ‘states’ of a Linux process • A process can be in one of several states: • 0: TASK_RUNNING • 1: TASK_INTERRUPRIBLE • 2: TASK_UNINTERRUPTIBLE • 4: TASK_ZOMBIE • 8: TASK_STOPPED

  8. state-transition diagram UNINTERRUPTIBLE INTERRUPTIBLE RUNNING event create schedule wait signal preempt CPU signal terminate STOPPED ZOMBIE

  9. A process needs ‘resources’ • Each task may acquire ownership of some system resources while it is executing • For example, a task needs to use some system memory (code, data, heap, stack) • And a task typically needs to read or write to some disk-files or to peripheral devices • Usually a task will want to call subroutines which belong to a shared runtime library • Every task needs to use some CPU time

  10. The OS is a ‘resource manager’ • Sometimes a task is temporarily granted ‘exclusive’ access to a system resource (e.g., each task has its own private stack) • But often a task will be allowed to ‘share’ access to a system resource (e.g., many processes call the same runtime libraries) • Acquisition of resources by a task is kept track of within the task’s ‘task_struct’

  11. Examples: memory and files • The ‘task_struct’ record has fields (named ‘mm’ and ‘files’) which hold pointers to OS structures (‘mm_struct’ and ‘files_struct’) that describe the particular memory-areas and files which the task currently ‘owns’ task_struct mm_struct mm files_struct files

  12. Demo-module: ‘tasklist.c’ • This kernel module creates a pseudo-file (named ‘/proc/tasklist’) that will display a list of all the current processes • It traverses the linked-list of all the process control blocks (i.e., the ‘task_struct’ nodes) • It prints each task’s pid, state, and name • It’s a ‘big’ proc-file (over 3K), so it needs to utilize the ‘start’, ‘offset’, ‘count, and ‘eof’ parameters to the ‘proc_read’ function

  13. Demo: ‘sleep.c’ • For kernel version 2.4.26, Linux stores two timestamp-values in every ‘task_struct’: • start_time; // time when task was created • sleep_time; // time when task went to sleep • The ‘/proc/sleep’ file shows each process ‘state’ and ‘sleep_time’ • EXERCISE: Add the needed code to show each process’s ‘start_time’, too (‘big’ proc)

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