Operating System Chapter 3: Processes

Process Concept

• A process is a program in execution, which forms the basis of all computation
• A process consists of:
  • Program code (text section)
  • Current activity (program counter, processor registers)
  • Stack containing temporary data (function parameters, return addresses, local variables)
  • Data section containing global variables
  • Heap containing memory dynamically allocated during runtime

Process State

• A process can be in one of the following states:
  • New: The process is being created
  • Running: Instructions are being executed
  • Waiting: The process is waiting for some event to occur
  • Ready: The process is waiting to be assigned to a processor
  • Terminated: The process has finished execution

Process Control Block (PCB)

• A PCB is a data structure that contains information associated with each process
• PCB contains:
  • Process state
  • Program counter
  • CPU registers
  • CPU scheduling information (priorities, scheduling queue pointers)
  • Memory-management information (memory allocated to the process)
  • Accounting information (CPU used, clock time elapsed since start, time limits)
  • I/O status information (I/O devices allocated to process, list of open files)

Process Representation in Linux

• In Linux, a process is represented by a task_struct data structure
• The task_struct contains information such as:
  • Process ID (pid)
  • State (long state)
  • Time slice (unsigned int time_slice)
  • Parent process (struct task_struct *parent)
  • List of children (struct list_head children)
  • Files (struct files_struct *files)
  • Memory management (struct mm_struct *mm)

Process Scheduling

• The process scheduler selects among available processes for next execution on CPU
• The scheduler 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

Schedulers

• Short-term scheduler (or CPU scheduler): Selects which process should be executed next and allocates CPU
• Long-term scheduler (or job scheduler): Selects which processes should be brought into the ready queue

Interprocess Communication (IPC)

• Cooperating processes need IPC to share data and affect each other
• Two models of IPC:
  • Shared memory
  • Message passing

Message Passing

• Implementation of communication link:
  • Physical: Shared memory, hardware bus, network
  • Logical: Direct or indirect, synchronous or asynchronous, automatic or explicit buffering

Direct Communication

• Processes must name each other explicitly:
  • Send (P, message) – send a message to process P
  • Receive (Q, message) – receive a message from process Q

Indirect Communication

• Messages are directed and received from mailboxes (also referred to as ports)
• Each mailbox has a unique id
• Processes can communicate only if they share a mailbox
• Operations:
  • Create a new mailbox (port)
  • Send and receive messages through mailbox
  • Destroy a mailbox