Operating Systems: Introduction to Processes and Process Scheduling, Summaries of Process Control

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COP 4610: Introduction to Operating Systems (Fall 2016)

Chapter 3: Process

Zhi Wang

Florida State University

Contents

• Process concept

• Process scheduling

• Operations on processes

• Inter-process communication

• examples of IPC Systems

• Communication in client-server systems

Process Concept

• A process has multiple parts:

• the program code , also called text section

• runtime CPU states , including program counter, registers, etc

• various types of memory:

• stack : temporary data

• e.g., function parameters, local variables, and return addresses

• data section: global variables

• heap : memory dynamically allocated during runtime

Process in Memory

Diagram of Process State

Process Control Block (PCB)

• In the kernel, each process is associated with a process control block

• process number (pid)

• process state

• program counter

• CPU registers

• CPU scheduling information

• memory-management data

• accounting data

• I/O status

• Linux’s PCB is defined in struct task_struct: http://lxr.linux.no/linux+v3.2.35/

include/linux/sched.h#L

Process Control Block in Linux

• (^) Represented by the C structure task_struct pid_t pid; /* process identifier / long state; / state of the process / unsigned int time_slice / scheduling information */ struct task struct parent; / this process’s parent / struct list head children; / this process’s children */ struct files struct files; / list of open files / struct mm_struct mm; / address space of this process/ …

Process Scheduling

• (^) To maximize CPU utilization, kernel quickly switches processes onto CPU for time sharing
• (^) Process scheduler selects among available processes for next execution on CPU
• (^) Kernel maintains scheduling queues of processes:
  • (^) 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
• (^) Processes migrate among the various queues

Ready Queue And Device Queues

Schedulers

• (^) Long-term scheduler (or job scheduler )
  • (^) selects which processes should be brought into the ready queue
  • (^) long-term scheduler is invoked very infrequently
    • (^) usually in seconds or minutes: it may be slow
  • (^) long-term scheduler controls the degree of multiprogramming
• (^) Short-term scheduler (or CPU scheduler )
  • (^) selects which process should be executed next and allocates CPU
  • (^) short-term scheduler is invoked very frequently
    • (^) usually in milliseconds: it must be fast
  • (^) sometimes the only scheduler in a system
• (^) Mid-term scheduler
  • (^) swap in/out partially executed process to relieve memory pressure

Scheduler

• Scheduler needs to balance the needs of:

• 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

Context Switch

• Context switch : the kernel switches to another process for execution

• save the state of the old process

• load the saved state for the new process

• Context-switch is overhead ; CPU does no useful work while switching

• the more complex the OS and the PCB, longer the context switch

• Context-switch time depends on hardware support

• some hardware provides multiple sets of registers per CPU: multiple contexts

loaded at once

Process Creation

• Parent process creates children processes, which, in turn create other

processes, forming a tree of processes

• process identified and managed via a process identifier (pid)

• Design choices:

• three possible levels of resource sharing : all, subset, none

• parent and children’s address spaces

• child duplicates parent address space (e.g., Linux)

• child has a new program loaded into it (e.g., Windows)

• execution of parent and children

• parent and children execute concurrently

• parent waits until children terminate

Process Creation

• UNIX/Linux system calls for process creation

• fork creates a new process

• exec overwrites the process’ address space with a new program

• wait waits for the child(ren) to terminate