1. Independent process cannot affect or be affected by the execution of another process.
2. Cooperating process can affect or be affected by the execution of another process
3. Advantages of process cooperation

• Information sharing
• Computation speed-up
• Modularity
• Convenience

1. Message system – processes communicate with each other without resorting to shared variables.
2. IPC facility provides two operations:

• send(message) –message size fixed or variable
• receive(message)

3. If P and Q wish to communicate, they need to:

• establish a communication link between them
• exchange messages via send/receiven Implementation of communication link
• physical (e.g., shared memory, hardware bus) considered later
• logical (e.g., logical properties) now

• Batch system – jobs
  
• Time-shared systems – user programs or task

Textbook uses the terms job and process almost interchangeably.

Process – a program in execution; process execution must progress in sequential fashion.

A process includes:

• program counter
  
• stack


• data section
  
  

Process State

As a process executes, it changes state

• 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


Information associated with each process.


Process ID

Process staten Program counter

Threads


In computer science, a thread of execution results from a fork of a computer program into two or more concurrently running tasks. The implementation of threads and processes differs from one operating system to another, but in most cases, a thread is contained inside a process. Multiple threads can exist within the same process and share resources such as memory, while different processes do not share these resources.
On a single processor, multithreading generally occurs by time-division multiplexing (as in multitasking): the processor switches between different threads. This context switching generally happens frequently enough that the user perceives the threads or tasks as running at the same time. On a multiprocessor or multi-core system, the threads or tasks will generally run at the same time, with each processor or core running a particular thread or task. Support for threads in programming languages varies: a number of languages simply do not support having more than one execution context inside the same program executing at the same time.

1. Parent process creates children processes, which, in turn create other processes, forming a tree of processes.
2. Resource sharing

• Parent and children share all resources.
• Children share subset of parent’s resources.
• Parent and child share no resources.

3. Execution

• Parent and children execute concurrently.
• Parent waits until children terminate.

• Child duplicate of parent.
• Child has a program loaded into it.

• fork system call creates new process
• fork returns 0 to child , process id of child for parent
• exec system call used after a fork to replace the process’ memory space with a new program.

--Process Termination

Process executes last statement and asks the operating system to delete it (exit).

• Output data from child to parent (via wait).
• Process’ resources are deallocated by operating system.n Parent may terminate execution of children processes (abort).
• Child has exceeded allocated resources.
• Task assigned to child is no longer required.
• Parent is exiting.

----------- Operating system does not allow child to continue if its parent terminates.

----------- Cascading termination.

• In Unix, if parent exits children are assigned init as parent

• Process creation and deletion.
• Process suspension (process is in I/O wait queue, or “swapped” out to disk, …) and resumption (move to ready queue or execution) – manage the state of the process.

• Provision of mechanisms for:
• Process synchronization - concurrent processing is supported thus the need for synchronization of processes or threads.
• Process communication
• Deadlock handling