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OS - 02 - Processes

Oct 01, 20245 min read

Process Concept

An OS executes a variety of programs:

  • Batch system - jobs
  • Time-shared systems - user programs or tasks

Process - a program in execution, process execution must progress in sequential fashion.

It has multiple parts:

  • The Program Code - also called the text section
  • Current activity including program counter, processor registers
  • Stack
  • Data Section
  • Heap containing memory dynamically allocated during runtime

Program vs Process

A program is a passive entity, stored on disk in the form of an executable file. A process is an active entity. A program becomes a process when the executable file is loaded into the memory.

One program can be several processes. (Consider the case of multiple users executing the same program).

Process State

As a process executes, it changes its state. Below are some of the states:

  1. new - the process being created
  2. running - instructions are being executed
  3. waiting - the process is waiting for some event to occur
  4. ready - the process is waiting to be assigned to a processor
  5. terminated - the process has finished execution

Process Control Block (PCB)

Information associated with each process (also called task control block)

  • Process State
  • Process Number
  • Program Counter
  • CPU Registers
  • CPU Scheduling Information
  • Memory Management Information
  • Accouting Information
  • I/O Status Information

Schedulers

  1. Short-Term Scheduler - Selects which process should be executed next and allocates CPU. It is sometimes the only scheduler in a system.
  2. Long-Term Scheduler - Selects which processes should be brought into the ready queue. Long-term scheduler is invoked infrequently. It controls the degree of multiprogramming. Processes can be described as either:
  3. I/O-bound process - spends more time doing I/0 than computations, many short CPU bursts
  4. CPU-bound process - spends more time doign computations, few very long CPU bursts.

The long-term scheduler strives for a good process mix.

Medium Term Scheduler

A medium-term schedular can be added if degree of multiple programming needs to decrease. Remove process from memory, store on disk, bring back in from disk to continue execution.

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 via a context switch.

The context of a process is represented in the PCB. The context-switch time is an overhead, as the system does no useful work during switching.

Process Creation

Parent processes create children processes who have their own children and so on, until a tree of processes is formed. Generally, processes are identified and managed using a process identifier or pid.

Some options for resource sharing:

  1. Parent and children share all resources.
  2. Children share a subset of parents resources.
  3. Parent and children share no resources.

Some options for execution:

  1. Parent and chidlren execute concurrently.
  2. Parent waits until children terminate.

UNIX Examples:

  1. fork() - this system call creates a new process
  2. exec() - this system call is used after a fork() to replace the process’ memory space with a new program

Process Termination

Process executes the last statement and then asks the operation system to delete it using the exit() system call.

  1. Returns status data from child to parent (via wait()).
  2. Process’ resources are deallocated by the operating system.

Parents may terminate the execution of children processes using the abort() system call. Some reasons for doing so:

  1. Child has executed allocated resources.
  2. Task assigned to child is no longer required.
  3. The parent is exiting and the operating systems does not allow a child to continue if parent terminates.

Cascading Termination - If a parent process is terminated, all of its children and grandchildren etc processes are terminated as well.

The parent process may awit for termination of a child process by using the wait() system call. This call returns status information and the pid of the terminated process.

pid = wait(&status)
  • If no parent is waiting (did not invoke wait()), the process is a zombie.
  • If the parent terminated without invoking wait(), the process is an orphan.

Cooperating Processes - Advantages

  1. Information Sharing
  2. Computation speed-up
  3. Modularity
  4. Convenience

Message System

Mechanisms for processes to communicate with each other without resorting to shared variables.

The IPC facility provides two operations:

  1. send(message)
  2. receive(message)

We can use direct communication for this to handle these messages explicitly:

  1. send(p, message) - send a message to process P
  2. receive(q, message) - receive a message from process Q

Indirect Communication utilizes mailboxes. It sends a message to a mailbox, from which another process can collect/receive it. If mailboxes are being shared by more than process, how do we decide who gets the message?

  1. Allow a link to be associated with at most two processes
  2. Allow only one process at a time to execute a receive operation
  3. Allow the system to arbitrarily select the receiver. Sender is notified who the receiver was.

Synchronization:

Message passing may be either blocking or non-blocking. Blocking is considered synchronous. Non-Blocking is considered asynchronous.

If both send and receive are blocking, we have a rendezvous.

Buffering

A queue of messages attached to the link. Can be of zero, bounded or unbounded capacity

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