Processes and threads

• Basic concepts
• The UNIX model
• Implementation in the kernel

Process

Why process abstraction?

• Hide complexity
• Virtualize the physical CPU
• Time sharing
• Isolation

What is in a process?

Processes live in address spaces, which contains:

• Code: The code that your program executes
  • Program counter (PC)
  • Libraries that the program is linked against
• Data: static data
• Heap: Dynamically allocated memory
  • malloc/free, new/delete
• Stack
  • Stack Pointer(SP)

closer looks

/proc/$pid/maps

process control block (PCB)

To keep track of running processes, the kernel maintains a process table, which contains:

• Process state

Process states

• new
• ready
• running
• waiting
• terminated

Parents and children; PIDs

The first process (init/systemd) is created by the kernel during boot

Processes have a uniqe process ID

Creating a process: fork()

Terminating a process: exec()

The exec() system call replaecs the program that a process is running

Usually run after fork() 0< child exec a different program

Terminating a process: exit()

Waiting for termination: wait()

Zombie processes

• Once a process is terminated, the kernel releases any resource it has acquired
• However, the process control block is not released immediately
  • The PCB still contains information that the parent process may be interested in, e.. the exit code
  • During this time, the process is called a 'zombie'
• The PCB is freed once the parent call wait()

Interprocess communication IPC

• Pipes
• Signals
• Message queue

Pipes

One-way FIFO channel, has fixed size

How can we use this for IPC?

Signals

Async communication

Terminate a program

Message Queue

Richer messages & APIs

Associated priority

Context switching

CPU idle

Interleave

Preemption and scheduling

• Cooperative Multitasking: processes themselves yield the CPU periodically
• Preemption Multitasking: kernel preempts processes

When a switch happens

Thread

A process has two key elements

• A collection of resources
• A thread of execution

Threads share the same address space

• Advantage: Much lower overhead
  • Cheaper than creating an entire process
  • Context switching within a process is much cheaper
  • Threads within the same process can easily and cheaply communicate by accessing the same memory
• Disadvantage
  • Concurrency

Why multiple threads

A pool of threads

thread pool

• User-level thread
• Kernel-level thread

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pipe

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